How to properly install a vapor barrier film on the roof. Which side should the vapor barrier be laid on? Requirements for roof vapor barrier

When arranging a roof, several layers of different materials are needed. That is why this design is called a roofing pie. If you want your roof to serve you for a long time, you need to use materials for heat, hydro and vapor barrier.

Why do we need a vapor barrier, what is the point of protecting the roof from steam?

Waterproofing and vapor barrier layers consist of completely different materials.

Their differences are that waterproofing protects the house from moisture from above, and vapor barrier prevents vapors from entering the roof structure.

The need for vapor barrier is due to the fact that steam rises from below, from the living space, which enters the “roofing pie”. Thermal insulation material absorbs moist air, swells, becomes damp and becomes unusable. Wooden roof structures also begin to rot over time, fungus and mold form on them, which can lead to complete destruction of the roof.

Vapor barrier is also important for maintaining a favorable microclimate in the interior. If the steam rising from the living rooms does not leave the house, then the inside will always be humid and stuffy. Remember how you feel in a greenhouse - your home can turn into something like a greenhouse without a well-equipped vapor barrier layer.

In winter, a vapor barrier layer prevents ice from forming on the roof. On hot summer days it will help keep the house cool. However, it should be taken into account that for the vapor barrier to function properly, it is necessary to have a ventilated roof.

Do you need a vapor barrier under the corrugated roof of a cold roof?

Many people believe that installing a roofing pie in general and a vapor barrier in particular should only be done for warm roofs where the attic or attic will be located. But modern cold roofs made of corrugated sheets also need vapor barrier.

The fact is that the temperature inside the room, even if it is not heated, differs from the outside air temperature, so condensation will still accumulate on the inside of the corrugated sheets.

The vapor barrier layer will protect against unwanted moisture from entering roofing structures that are susceptible to rotting and deterioration. They fasten the film with lathing, and only then install the roof from profiled sheets.

What materials to use

Roofing materials can be divided into hydro- and vapor barrier materials, and antioxidant ones are also distinguished. Basically, these are film materials based on polyethylene. Recently, a variety of building materials have appeared for all layers of the roofing pie. And if previously only glassine could be used for vapor barrier, now the choice in construction stores is very large. Glassine is practically not used; although it costs less, it is inferior in performance to modern analogues.

There are several main groups of modern vapor barriers:

Based on polyethylene. The film can be perforated or non-perforated, the latter is used for the waterproofing layer. The perforated material, despite the presence of holes in it, reliably protects against unwanted moisture. The holes are microscopic, so drops simply cannot leak through them. But steam passes through the perforation perfectly. Another option for polyethylene is foil film. It not only provides steam removal, but also retains heat. If you lay it with the foil side down, it will reflect the heat back into the house. When installing, it is advisable to reinforce any film with mesh.
Based on polypropylene. Compared to the first option, polypropylene film is much stronger. Due to the presence of a layer of special fabric, polypropylene film has antioxidant properties.
Membrane. This is a relatively new material that is ideal for roofing. The breathable properties of the membrane allow you to save on ventilation. The material does not allow moisture to pass through, but steam penetrates inside, settles on a special fabric layer and dries there. Membranes, or diffuse films, can be single- or double-sided. The method of laying them on the insulation depends on this.

When choosing a material for vapor barrier, you should pay attention to the following properties:

High strength. Even with significant mechanical impact, the vapor barrier should not be damaged. For example, the material must withstand the weight of vapor and waterproofing if the integrity of the sheathing is damaged.
Easy to install. This is especially true for those who perform installation work on their own. If the film breaks during the process, is difficult to lay and is inconvenient to unwind from a roll, then it is unlikely that you will be able to make a good vapor barrier.
Good elasticity. This quality is important when fixing the film. When using fasteners, the film should not tear, but should stretch and tightly grip the screw.

How to install a vapor barrier on a roof. Step-by-step instruction

The vapor barrier layer is part of the roof, an important element of its arrangement. The first stage of installing a vapor barrier is preparation. The under-roof part should be thoroughly cleaned of dust and dirt; it is especially important to dry it thoroughly. Then you should lay the insulation, which always precedes the vapor barrier layer. The installation itself is carried out in several stages:

A roll with a vapor barrier film or membrane is unrolled directly at the installation site. If the roof is gable, installation begins from the bottom up, and then, after passing the ridge, goes down again. The film must be overlapped to prevent the appearance of cracks and gaps. Secure the material with tape around the perimeter and at the joints. (Read also: Do-it-yourself installation of a gable roof rafter system)
Next, you need to more carefully secure the vapor barrier material under the roof. To do this, use nails or screws with wide heads. Another option is a construction stapler. If you choose good material, it will not tear. The elastic film will close around the fastener. (How to choose the right self-tapping screw, read this article)
If you are installing a rough version of the roof, then before installing the vapor barrier layer you should make a sheathing. You can use wooden or plastic slats or a metal profile for it. The film is carefully stretched and attached to the sheathing using self-tapping screws or a construction stapler.

When installing a vapor barrier layer, it is important to follow the manufacturer's recommendations. Many materials have special perforations, so it is important to lay them on the correct side.

If you are installing a roof for the first time or have never worked with modern vapor barrier materials, professional advice will help you avoid the most common mistakes.

Regardless of what type of roof you are installing, installation of a vapor barrier is carried out only after laying the insulation.
When installing overlapping film sheets, they must cover each other by at least 10 cm.
It is imperative to glue the joints with adhesive tape/adhesive tape; moreover, it is better to use a wide tape, at least 10 cm wide. The best option is to use double-sided tape, which is glued inside the overlap. First glue it onto the bottom layer of film, then glue the top layer on top.
When using nails or self-tapping screws as fasteners, galvanized products are preferred. They are not subject to corrosion, which will extend the life of the vapor barrier.
If there is a window or hatch in the roof, then it is necessary to use a special apron, which is sold complete with vapor barrier materials. If there is none, then the perimeter of the window or hatch should be covered with butyl tape.
Also, when working with windows, you should leave a deformation margin around them. Over time, any house, even one built a long time ago, shrinks. This can damage the vapor barrier, which is why a fold of at least 20 cm is necessary.
If installation is carried out near heating pipes or water pipes, then the vapor barrier material should be carefully wrapped around the pipe and sealed tightly.

The main rule when installing a vapor barrier is to ensure tightness. If the tightness is broken, then the effectiveness of the vapor barrier layer will be minimal.

The main errors in installing roof vapor barriers are noted in the video

Thus, a vapor barrier device is a necessary condition for the long-term operation of the roof of a house. it is necessary both for a warm roof with an attic or attic, and for a cold one.

The roof structure is a complex system. It consists of several layers of dissimilar materials, each of which performs its own unique function.

To ensure comprehensive protection of the building from environmental influences, it is necessary to carry out all installation work with high quality, as well as the correct choice of basic and insulating materials. This whole process is briefly called.

The standard composition of the roofing cake includes:

Internal finishing layer;
Lathing design;
Counter-breach;
(does not apply to a cold attic);
(about);
Ventilation shafts or gaps;
Roofing covering.

Why do you need a roof vapor barrier? Vapor barrier layer protects the roof from the penetration of water vapor into the thermal insulation. The fact is that insulating materials for the most part have porous structure, since the air contained in them performs the function heat insulator. On contact with a colder environment, steam turns into condensate, which lingers in the voids.

This can lead to disruption of the functionality of the heat-insulating material, as well as the occurrence of decomposition and rotting processes in the summer. In winter, freezing water expands, thereby destroying the connections between cellular elements.

The main characteristic of vapor barrier material is vapor permeability, depending on the density of the material. The values of this indicator can be found in the passport of the packaged building material.

Form of release of vapor barrier films - roll. Vapor barrier can be made from various materials. Some of them have additional positive properties that increase the efficiency of operation of the building’s interior. Installing a vapor barrier on a roof is not as difficult as it seems at first glance, but it requires a careful approach and accuracy.

Types of vapor barrier

Single layer polyethylene films

Made from low-density polyethylene, which provides an increased degree of steam penetration. This material often has defects due to the ingress of foreign particles during manufacturing. The cheapest type of insulating material.

Polyethylene films with reinforcement

Compared to single-layer polyethylene, reinforced vapor barrier has little greater thickness, since it includes a reinforcing frame. It consists of polymer twisted threads located in mutually perpendicular directions. The mesh is attached to the polyethylene base by warm pressing.

This technique does not affect the waterproofing and mechanical properties of the material, but may reduce the degree of vapor barrier.

Anti-condensation film

Made from two layers: smooth and fleecy. The smooth layer is attached to the insulated surface. The fleecy layer is created from cellulose fibers. When steam hits such a surface, it clings to the fibrous structure, thereby preventing moisture from flowing onto the underlying structures of pitched roofs. Excess water is removed along with the air through the ventilated space.

Polymers with foil layer

They are made of penofol or foamed polypropylene, coated with a thin layer of metal. Due to the reflective properties of metal surfaces, such a vapor barrier provides an additional function heat loss protection.

Membranes (film for roof vapor barrier)

Membrane vapor barrier materials are divided into five main classes:

Type A. Has increased resistance to moisture and wind. It is laid between the roofing covering and the heat-insulating layer. Manufacturing technology: spunbond. This vapor barrier does not allow moisture to flow through the cracks of the roofing material, as well as from the condensation that forms. The absence of a laminating layer allows the use of such membranes in roof structures with a slope more than 35 degrees. To remove excess moisture, it is necessary to install ventilation holes from a double sheathing located between the membrane and the insulation.
Type AM. Unlike Type A has a multilayer structure. It is placed between the insulation and the roof to provide adequate protection from negative atmospheric influences. The most common is a three-layer construction, consisting of two layers of spunbond with a diffuse film located between them. It acts as a waterproofing agent, as it allows steam to pass through but retains water. Laying is done directly on the heat-insulating layer, which reduces labor costs for creating a ventilation gap.
Type B. Double layer material. Consists of a layer of vapor barrier film and spunbond. It is used to protect insulation from internal building vapors. Applicable only in insulated roof structures.
Type C. It is manufactured similarly to type B membranes. It has a stronger and thicker layer of spunbond, which provides better protection of the insulation from moisture. Used in insulated and non-insulated, pitched and flat roofs.
Type D. Consists of polypropylene fabric, protected on one side with a laminating polymer coating. The structure of the material allows it withstand significant mechanical stress. This dictates its installation between the insulation and the attic floor screed, as well as in non-insulated roof structures.

Which side is the vapor barrier for the roof laid on?

The principle of installation of vapor barrier materials depends on which side the layer responsible for repelling water vapor is located:

We have figured out which side to lay the vapor barrier on the roof, now we will look in detail at how to properly lay the vapor barrier on the roof.

Vapor barrier for the roof: how to install it correctly using special technology

Laying a vapor barrier on the roof is carried out as follows:

Laying can be done vertically and horizontally. In the second option, installation of roof vapor barrier is carried out starting from ridge part roofs. Each subsequent stripe is superimposed on the previous layer with an overlap, the size of which should not be more than 10 cm.

CAREFULLY!

When joining elements, mandatory seam sealing, providing optimal protection against moisture ingress into the gap between them. Most often, sizing is done with single- or double-sided adhesive tape or tape. On roofs with a slope of up to 3 degrees additional fixation using wooden planks.

The vapor barrier is attached to the wooden rafter elements using galvanized nails or a construction stapler.
When installing films and membranes near roof hatches, you need to use a special vapor barrier apron, which is included as standard.
At the junctions with ventilation pipes, film and membrane structures are folded down, wrapped around the pipe and tightly fixed with construction tape.
After laying the vapor barrier layer, it is necessary to nail down wooden blocks. The rules for laying vapor barrier on the roof dictate the following rule - the pitch between the sheathing bars should be 500 mm. In this case, a ventilation gap is formed, and additional fastening of the vapor barrier layer is carried out. Now you know how to lay a vapor barrier on a roof, you can move on to the issue of waterproofing.
If the vapor barrier for the roof was damaged, then small cuts or holes can be seal with special tape for vapor barrier membranes.

Roofing pie diagram

Sealing joints with tape

Final vapor barrier option

NOTE!

A tightly stretched film is the key to a good vapor barrier.

Hydro and vapor barrier of the roof - what is the difference?

Useful video

And now we invite you to familiarize yourself with the video instructions on roof vapor barrier:

Conclusion

Vapor barrier is the lightest and thinnest layer of the roof. But neglecting its structure can negate all material and physical efforts to create a high-quality roof. Therefore, you should not neglect this important element; it will significantly increase the service life of the entire building.

In contact with

When insulating a roof or room with a heat insulator (mineral wool, glass wool, loose polystyrene) capable of transmitting and accumulating moisture, it is necessary to install a vapor barrier layer. Moisture that gets into the thermal insulation significantly worsens its performance properties, increasing thermal conductivity. In addition, due to the humid environment, wooden structures with which the insulation comes into contact quickly begin to rot and collapse. At the stage of preparation for insulating a roof or house, it is important to figure out which side to lay the vapor barrier against the insulation - a violation of the technology will lead to steam from the room penetrating into the thermal insulation layer.

What kind of vapor protection is needed?

Types of vapor barrier materials

The primary issue is the choice of material that will protect the insulation from moisture. Classic roofing felt and glassine, which were used for water vapor barrier, have given way to modern polymer films with different performance parameters. Membranes used in construction are divided according to vapor permeability into completely impermeable films and partially permeable (diffusion) films.

Manufacturers offer the following types of vapor barrier membranes::

polyethylene film (can be used to insulate a roof or floor, the barrier is vapor and waterproof);
reinforced polyethylene film (higher strength);
aluminum foil film (used for internal insulation, mounted with the reflective side facing the room - additionally helps to retain heat, primarily designed for use in saunas and baths);
film with anti-condensation coating (prevents moisture condensation, intended for installation in structures with metal elements prone to corrosion - corrugated sheets, metal tiles, etc., the film is mounted with the treated side facing the heat insulator).

Reinforced polyethylene film If, when insulating rooms in a house, a completely impermeable film is used to create a vapor barrier, it is necessary to provide effective ventilation of the rooms that can remove excess moisture outside.

Vapor barrier films with a porous structure differ in their ability to diffuse. Due to the pores in the barrier, moisture escapes from the insulation to the outside, due to which the heat insulator retains its operational properties, and metal structures in contact with it do not rust, and wooden structures do not rot. Among vapor-permeable membranes there are differences:

Pseudo-diffusion. During the day, up to 300 g/m2 of vapor is passed through.
Diffusion. The amount of evaporation passed per day ranges from 300 to 1000 g/m2.
Superdiffusive. The evaporation rate exceeds 1000 g/m2.

The first type of vapor barrier film is considered an effective protection against moisture and is used for internal insulation of structures (from the room side). If a pseudo-diffusion membrane is mounted on top of a fiber insulation on an external wall, the vapor barrier will trap moisture in the insulation. Diffusion and superdiffusion membranes, which simultaneously serve as a wind barrier, are suitable for façade insulation.

Superdiffusion membrane

Principles for installing a vapor barrier

Laying a vapor barrier is an important stage in the work of insulating structures with fibrous materials that can accumulate moisture. The work is carried out as part of the repair or reconstruction of a house or at the stage of preparation for finishing a new building. You need to know how to properly fasten membrane sheets together to ensure a continuous layer that protects against moisture penetration, and how to attach the film to structures. Before laying the vapor barrier film, it is also necessary to determine which side it needs to be placed towards the insulating material.

Preparatory stage

To insulate a block or wooden house or arrange a bathhouse, thermal insulation materials are used, which must be protected from moisture accumulation. For this purpose, a material that does not allow evaporation is installed on the walls inside the house, on the ceiling or floor, on the inside of the roofing pie. Or a diffusion membrane for facade insulation.

At the preparatory stage, you should choose a vapor barrier option, taking into account the installation features and the requirements for the characteristics of the film. Popular options include Izospan (and its analogue Megaizol), a membrane material with high performance parameters. The manufacturer offers a line of membranes with various technical indicators, which allows you to choose a material depending on its purpose - vapor barrier for roofs, ceilings, wall structures made of wood or concrete.

Those who plan to build a bathhouse, not without reason, believe that foil film will reliably protect the mineral wool insulation from getting wet and will help maintain a high temperature in the room by reflecting thermal radiation. Along with the classic “insulation + vapor barrier” scheme, today ready-made non-combustible heat-insulating mats with a foil-coated vapor-proof surface are used.

Correctly attaching reflective vapor barrier

Before laying a vapor barrier, it is necessary to properly prepare the surfaces of the structures. The preparation technology depends on the material from which the walls, floor, ceiling or roof are made. It also takes into account what kind of work is being carried out at the site - construction or repair:

When building a wooden house, all wooden structural elements must be treated with compounds against rotting, pest damage and fire.
During the repair work, the finishing is first dismantled, the surfaces are cleaned, while:
- wooden structures are treated with an antiseptic and fire retardant.
- Concrete and block structures are treated with an antiseptic if they are damp and there is a risk of mold, as well as in wet rooms.

Due to improper preparation of structures, walls, ceilings or rafter systems can become unusable over time or become a source of fungal spores that can provoke allergies, asthma attacks, and exacerbation of respiratory diseases.

How to install a vapor barrier on a ceiling

Installation of a vapor barrier layer on the ceiling is required when insulating a flat or pitched roof in a house without an attic, when insulating a basement, as well as residential premises above which a cold attic is located. The ceiling in the bathhouse is also insulated and vapor-insulated. Before laying a vapor barrier on a roof made of a concrete slab, on a reinforced concrete or wooden floor from the inside, the surface of the structure should be prepared.

The fabric made of film or pseudo-diffusion membrane must be solid so that there are no joints through which moisture can penetrate into the insulation. If the width of the roll material is not enough, the strips will have to be joined together. The recommended overlap of the canvases is from 10 to 20 cm, while the joints on both sides are carefully taped with reinforced construction tape.

Fastening the vapor barrier membrane The foil film sheets are laid without overlap - end-to-end, and the seam is taped with aluminum tape.

If the base of the roof or ceiling is a wooden structure, you first need to lay a waterproofing membrane (solid sheet) and attach it to the base (a vapor barrier material can be used).

Then, in the spaces between the floor joists or rafters, a heat insulator is placed in the form of mats or rolled material made of mineral (basalt) wool. After this, you can lay a vapor barrier on the ceiling. If the thickness of the heat insulator corresponds to the thickness of the logs, you should nail a counter-lattice made of slats to create a ventilation gap.

The vapor barrier should be placed on the ceiling in such a way that the fabric extends along the entire perimeter of the walls and that all corners are closed. The joints of the canvases should be on the floor joists - this will allow them to be securely fixed. To properly install a vapor barrier on the ceiling, monitor the tension of the canvas, it should not sag.

Ceiling installation

We will also consider how to install a vapor barrier on a concrete floor. To insulate a ceiling or flat roof made of a concrete slab from the inside, you need to attach a waterproofing coating (vapor barrier film) to it using self-adhesive tape, and then install a sheathing made of bars or a metal profile.

The correct height of the sheathing should be selected taking into account the thickness of the insulation and the ventilation gap, the installation step should be 1-2 cm less than the width of the insulation, so that mats made of insulating materials fit into the cells at odds. How to attach a vapor barrier to the sheathing will be described in detail below.

How to lay a vapor barrier on the floor

Installation of a vapor barrier membrane on the floor is similar in technology to how vapor barriers are installed for walls and ceilings. After preparing the base of a wooden floor when insulating along the joists, before laying the vapor barrier on the floor, a waterproofing carpet is installed, which should go around the joists. Then a thermal insulator made of mineral wool is inserted between the joists. After which the vapor barrier is laid, it is important to know how to lay the film correctly.

The overlap of the sheets of rolled material should be at least 10 cm, with the joint taped on each side. The resulting canvas is secured in such a way that the overlaps lie on the floor joists, and along the entire perimeter the evenly stretched canvas extends 5-10 cm onto the walls.

Insulated floor with air gap

Before laying a vapor barrier on a concrete floor, it is necessary to install a sheathing, between the elements of which the waterproofing and insulation will be placed. Further work is carried out according to the standard scheme.

Fastening principles

If concrete structures or wooden walls are insulated, it is necessary to install sheathing made of bars. It is convenient to attach the film to the resulting sheathing, to the ceiling or rafter system using staples and a construction stapler. You can also secure the vapor barrier material with nails with wide heads or pads under the heads. It is advisable to use galvanized nails - they do not rust. Films and membranes are placed on concrete structures using a special connecting tape.

Overlap when installing vapor barrier

In order to properly fasten the vapor barrier, the canvas should be carefully pulled, and the fastening elements should be placed in small increments - no more than 30 cm. Installation rules require careful attention to the fastening of the canvas around the perimeter - it is spread and fixed in such a way as to exclude the possibility of moisture penetrating into the insulation.

Before attaching the vapor barrier, make sure that the sheet is positioned with the correct side to the thermal insulation.

Which side to install the vapor barrier material

Let's consider which side of the film or membrane is laid to the insulation:

polyethylene film (simple or reinforced) can be attached to either side - this does not affect the functionality of the barrier;
the foil film is placed with the shiny side facing the room so that the barrier reflects heat;
the anti-condensation film is attached with the treated side to the structures, the fabric side to the room;
the membrane should be facing the smooth side towards the heat-insulating material, and the rough side towards the room.

Rule for laying vapor barrier to insulation If the front side of the membrane looks similar to the back side, and it is difficult to determine how to properly lay the material, you can conduct an experiment. Cover the bowl of boiling water with a small piece of membrane - whichever side the condensation appears on is the side that is waterproof; it should be facing the insulation.

It is important to know which side to lay the vapor barrier against the insulation if the membrane is used to install a waterproofing barrier - front or back. The thermal insulation “pie” for internal insulation is mounted in such a way that the smooth side of the membrane faces the insulation on both sides. That is, the rough layer of vapor barrier should be facing towards the room, and when installing a waterproofing carpet - towards the structure that is being insulated.

Installation features

It is important not only to install the vapor barrier correctly, but also to provide a ventilation gap between the vapor-proof layer and the cladding of the structure for finishing, for which purpose counter-battens are placed along the sheathing. Moisture settling on the rough side of the laid vapor barrier sheets will evaporate naturally without damaging the finish.

Diagram of an insulated roof with a ventilation gap

If the vapor barrier is installed correctly, the insulation is reliably protected from moisture. Almost half of the problems associated with freezing and damage to structures are associated with defects in the installation of the vapor barrier.

Which side to lay the vapor barrier to the insulation, how to attach it correctly

Which side is the correct way to lay the thermal insulation material towards the insulation? Varieties of vapor barrier films with stages of installation on the roof, ceiling and floor.

Roof vapor barrier: which side to lay correctly

Roofing work is a critical stage in the construction of a building. Particular care must be taken not only in the construction of the rafter system, but also in the creation of the roofing “pie”. One of the protective layers of the structure is a vapor barrier. Just as the roof covering is a barrier to precipitation, laying a layer of vapor barrier on the roof prevents condensation from falling during the cold season and protects the insulation and structural elements from water vapor rising from inside the building. For installation, a large selection of materials is offered, differing in reliability, cost, and installation features. How to protect the roof from steam, and which side to turn the product towards the insulation, we will look in more detail.

Why is steam protection necessary?

During human life and the operation of various devices, the air becomes saturated with moisture. As the steam pressure increases, it rushes from the room to the outside, simultaneously settling on the insulation and roof material. A humid environment promotes the development of mold and mildew and reduces the insulating properties of insulation. In winter, when the temperature drops to negative levels, the liquid in the insulation freezes and thaws in the spring. One such cycle will seriously deteriorate the thermal insulation properties of mineral wool, and several will completely damage the insulation. Extruded polystyrene foam will last several years longer, but it will not withstand such loads. Wooden roof structures are also sensitive to moisture, so they need to be protected with a layer of vapor barrier.

What materials to choose for roof protection

Glassine is a budget option for insulation. The use of cardboard impregnated with bitumen was popular until recently, until new materials appeared that were significantly superior to glassine in quality and durability.

The second popular option is roofing felt laid on wooden flooring. The disadvantage of this method is the high cost of the wood needed to assemble the deck. Modern film materials are easier to install, stable and elastic. In the process of arranging a roof, you have to choose which type of vapor barrier is best to install for your home. Among the best options:

Requirements for roof vapor barrier

When choosing a material for vapor barrier, you should not focus only on its cost. Inexpensive glassine, roofing felt, thin polyethylene film are not able to withstand long-term loads. The following criteria will help you correctly evaluate the material:

tensile strength;
density;
vapor permeability coefficient;
UV resistance;
operating temperature.

How to install a vapor barrier yourself

Regardless of the type of roof, it needs protection from steam. The insulating sheet is part of the roofing “pie” of flat and pitched structures. Typically, the vapor barrier layer is installed on the inside of the roof, but two layers may be required.

Horizontal or vertical laying technology depends on what material is chosen. In the first case, the process starts from the top, each subsequent sheet is placed on top of the previous one with an overlap of 10 cm. The joints are sealed using adhesive tape. The joints are taped on the inside with double-sided tape, and on the outside with single-sided tape.

The strips must be laid with minimal tension, but without sagging. The material is made in rolls, from which parts of the required length are cut and attached to a wooden sheathing located on the inside of the roof using staples or nails with a wide head. We must not forget that the product is facing the insulation with the smooth side.

Steam has the ability to penetrate into the slightest crack; in order to ensure a sufficient degree of tightness, it is necessary to glue with double-sided tape all places where the vapor barrier adjoins the roof structural elements, utility openings and niches. If the roof slope is less than 30 degrees, it is advisable to use thin pressure strips.

After installing the film, it is necessary to fill the frame of the sheathing, creating an air gap of at least 5 cm. This gap can be used for two purposes - to provide ventilation and hide the wiring. For the lathing, bars are used that have been thoroughly treated with an antiseptic. This will protect the wood from rotting. The lathing is filled in increments of 50 cm. When using plasterboard to hem the ceiling, the wooden blocks are replaced with a galvanized profile.

Places of hatches, roof windows and other passages must be equipped with a vapor barrier apron. This element is included in the window components. You can replace it with double-sided butyl tape.
The passages of ventilation pipes are processed as follows: the film is wrapped inside and attached to the pipe using adhesive tape.
In areas adjacent to a rough wall, it is recommended to use a polyurethane or acrylic mixture to fix the film. Regular adhesive tape will not provide sufficient adhesion to the surface.
It is important to properly fasten the film in areas: valley, crossbars, rafter legs. These are complex structures, the insulation of which takes more time.
It is better to join the vapor barrier sheets using adhesive tape 10 cm wide.
Decorative trim must be installed at window openings. The film, left without protection, will quickly deteriorate under the influence of ultraviolet radiation.

The correct choice of vapor barrier film and its high-quality installation will significantly affect the microclimate in the house.

Laying a vapor barrier for a roof: which side should be laid correctly?

A vapor barrier on the roof will protect the insulation and wooden structures from condensation during the cold season. Selection of vapor barrier material.

Which side to lay the vapor barrier on: we resolve all controversial issues

Until recently, the only type of vapor barrier was glassine. We cut it, attached it, secured it – that’s all! It was only a few decades ago that a more convenient polyethylene film appeared, and more complex and reliable materials began to be made on its basis. Yes, modern options please not only with their strength characteristics, but also with their resistance to temperature changes and ultraviolet radiation, and with their versatility. But, at the same time, they are upset by the complicated instructions for their use: they should be connected along a clearly defined line, and only special tape should be used, and - most importantly! – the installation side must be chosen correctly.

Therefore, it is not surprising how often you can find panicky questions on the Internet about how and which side to lay the vapor barrier to the insulation, and what to do if the sides are mixed up? Will it really be necessary to disassemble the entire structure? We can assure you: you won’t have to. Let’s take a closer look at determining which side is “correct” - you will be very surprised!

What is the essence of roof vapor barrier?

Protecting insulation from moisture is one of the most important problems of thermal insulation, and we will now tell you why.

Water itself is an excellent conductor of heat, because it is used in heating and cooling systems for a reason. And, if the roof insulation is not sufficiently protected from steam from the room, then this will not end well. Even in the warm season, you will not know about the presence of a problem, because... such steam will easily disappear due to heat and good ventilation. And in hot countries, where there is no sub-zero temperature, they don’t think about vapor barrier of insulation at all, because the problem is quietly solved on its own. But in Russian latitudes, due to temperature differences in the cold season, steam rises and penetrates the insulation, concentrating in the form of water when it meets the so-called “dew point”.

At the same time, the top layer of insulation in the roofing pie freezes and creates yet another condition for getting wet from the inside. The effectiveness of the insulation itself is significantly reduced, and the changed structure contributes to the development of fungus and corrosion. Moreover, with a large amount of moisture, it can even seep back into the room and thereby damage the interior decoration. This is exactly what a vapor barrier is for.

And in order to understand how to properly install a vapor barrier, you first need to understand the structure itself. Thus, the insulation is protected on both sides by completely different films that perform opposite tasks. Below, on the side of the living room, a vapor barrier is installed that will not allow steam to pass through, and on top - a vapor-permeable membrane, which, on the contrary, will release excess steam from the insulation, if it is “wadded”, and will protect it from roof leaks:

But where is the logic, you ask? How can steam get into the insulation if there is a vapor barrier in front of it? In fact, no film or membrane protects 100%, and there are still poorly glued joints and other construction errors. Therefore, some minimal amount of steam will still be in the insulation, and it is important to properly remove the steam outside without harm:

Look carefully at the diagram: do you see where condensation appears in a well-designed roof? That's right, not from the side of the room, but quite a bit from the side of the roof, on that side of the insulation, and it is easily removed by a windproof anti-condensation film or membrane. But condensation should not appear on the vapor barrier, and no rough side of it can cope with it, because... it has a different structure, and we will now prove it to you.

Types of vapor barriers: A, B, C and D

To understand which side the vapor barrier should be laid and why, for example, both sides suddenly turned out to be smooth, you must first determine its type. After all, not every species has two different sides!

Type A insulation: only for steam outlet on the other side

For example, type A cannot be used as a roof vapor barrier because eventually all the vapors will end up in the insulation. After all, the main task of such insulation is precisely to provide them with unhindered passage, but not to allow rainwater to pass through from the other side.

Such insulation is used in roofs with an inclination angle of 35° or more, so that water droplets can easily roll off and evaporate (and the ventilation gap between such insulation and the insulation helps them evaporate).

Vapor barrier B: classic double-sided installation

But B is a real vapor barrier material. Vapor barrier B has a two-layer structure that allows you to avoid condensation, due to the fact that moisture is absorbed into its fibers in the morning and disappears during the day.

That is why type B vapor barriers are always placed with the smooth side facing the insulation (film side), and the rough side facing outwards. Vapor barrier B is used only in insulated roofs, because For an uninsulated one, its strength is too low.

Type C membrane: for enhanced protection against water vapor

Type C vapor barrier is a two-layer membrane of high density. It differs significantly from type B in the thickness of the vapor barrier film layer. It is used in the same place as type B vapor barrier, but in itself is more durable.

Additionally, such a vapor barrier is used in non-insulated roofs to protect the wooden elements of the attic floor and in flat roofs to enhance the protection of thermal insulation. Vapor barrier C should also be installed with the rough side facing the inside of the room.

Polypropylene insulation D: for heavy loads

The new-fashioned vapor barrier type D is an especially durable polypropylene fabric, one of which has a laminate coating on one side. This can withstand significant mechanical loads. It is used not only for insulating attic floors as a waterproofing layer, but also in insulated roofing to protect it from leaks. Moreover, type D vapor barrier is indispensable for rooms with particularly high humidity.

Here are the cases and where all these types of insulation are needed:

Does vapor permeability change when changing sides?

All of the above modern barriers are divided into the following types:

for one-sided installation, which need to be rolled out only on a certain side, and it is recommended not to confuse them;
and for double-sided use, usually with membranes that can be laid on either side.

You will be interested to know that for the first time, membranes that already had such properties as modern roofing membranes were used in astronautics! And from there they began to be used in construction and in many areas of the national economy. And until recently, there were not as many problems with their installation as there are today.

But now there is a strong opinion among ordinary people: if you lay the vapor barrier to the roof insulation “on the wrong side,” then the entire structure will not last long. In fact, the correct choice of side affects exclusively the service life of the interior finishing of the roofing pie, because the rough side has the same abilities as the smooth side and has absolutely the same vapor permeability. But how much it will retain droplets of condensate there is a little-studied question.

The right side of vapor barrier: myth or reality?

Let's understand such concepts as condensation - this is important. There is a catch here: for some reason, most ordinary people are sure that if high-quality vapor barrier is used, then there will be no condensation at all. Or, on the contrary, it will quickly evaporate on its own. In fact, condensation is formed from moisture that rises upward in a vaporous state.

There is such a thing as a “temperature limit”, i.e. that specific condition under which the air temperature and humidity are sufficient for steam to appear in the form of droplets. For example, at a temperature of 15°C and air humidity of about 65%, condensation will already begin to form. But if the air humidity reaches 80%, then condensation will appear at a temperature of 17°C.

In other words, the entire process of water vapor formation occurs as a result of the difference in the so-called “partial pressure”. All the water vapor contained in the air tries to escape out into the colder street through the roof enclosing structures, but on its way they encounter a barrier in the form of a vapor barrier. If the air in the house warms up faster than the surface of the vapor barrier, then moisture from the air will fall on it in the form of condensation. Here the difference between an insulated roof and a non-insulated one is clearly visible: any vapor barrier that is laid on the insulation will warm up much faster than something that is in direct contact with the cold elements of the roof.

If there is no vapor barrier layer at all, or there is not enough of it, then water vapor penetrates inside the roofing pie and encounters a “cold front” there, which turns the vapor into condensate, and under special circumstances, also into ice. And all this happens inside the roof! This ice will not bother you until spring comes and the outside air warms up, thereby warming the roofing elements. Then the accumulated ice will melt and form entire smudges on the slopes inside the house.

But with a properly equipped roof, condensation should not appear at all, and therefore, in fact, the difference between the smooth and rough side is not significant at least in this aspect.

What is the difference between anti-condensation film and “anti-condensation side”?

As we have already said, most modern manufacturers emphasize that their vapor barrier films have a so-called “anti-condensation side”:

The “anti-condensation” side differs from the usual one in the presence of a fleecy layer that absorbs a small amount of condensation and holds it until it evaporates.

Thanks to this, the risk of the film surface getting wet is much lower, which extends the service life of the interior finishing of the roofing cake. That is why the rough side should always be directed inside the living room or attic, and the smooth side should be leaned against the insulation. But is this really so?

Practice shows that if condensation forms inside the roofing pie, then the fleecy side of the film cannot help in this regard, and there is not much difference whether these drops adhere to the film or flow down. The fact that they exist at all is bad in itself. The anti-condensation side of the vapor barrier and the anti-condensation waterproof film on the other side of the insulation are completely two different things!

Therefore, let's summarize: the “correct” side of the vapor barrier is not equivalent in terms of the properties of the anti-condensation film: it does not remove water vapor, does not destroy drops of moisture and does not solve the problem with condensation.

But, if you are still in the process of building a roof, then for the sake of peace of mind, do as the manufacturer instructed in the attached instructions. If you have already installed a vapor barrier and are in doubt whether it is correct, forget it and don’t worry anymore. But if you hope that the “right” side of the vapor barrier will take care of all the future shortcomings of the roofing pie, don’t believe it.

Experienced roofers often declare that they consider the whole epic about which side to attach the vapor barrier to be a kind of shamanism. Allegedly, by complicating the product, they increase its positioning in the market. But in fact, as we have already said, with a well-equipped vapor barrier, there shouldn’t be any droplets on the walls, otherwise even the lining on the walls will swell and the wallpaper will fall off, since everything is so serious.

After all, this only happens when there are serious mistakes during roof construction. In addition, if the vapor barrier itself is located between the drywall and mineral wool, then there is no point in messing around with such a complex structure at all. Drywall itself absorbs moisture well, and steam will practically not be able to reach the internal vapor barrier. In this design, even simple glassine is quite acceptable!

For example, some curious roofers even conduct their own vapor barrier tests to determine whether the “wrong” side is working or not:

And those who are especially quick-witted even say that polyethylene vapor barrier with a rough side is obtained simply in the factory, when polyethylene is combined with a non-woven material: the film is glued to the rough layer, and the finished product actually has two different sides. And there is no point in modifying the second side so that it also becomes smooth by connecting it to another layer of polyethylene: the vapor barrier properties will not change, and the manufacturing process will become more expensive.

And therefore it is easier to give this meaning to the product itself. And in fact, quite a lot of people have already become convinced that, even if they mix up the sides of the vapor barrier, nothing like that happens, and the film works the same on both sides, fully performing its functions.

Therefore, in any case, just strive to implement roof steam protection correctly, think through all the necessary details and not skimp on quality!

Which side to lay the vapor barrier on: technical nuances for all cases

We reveal the whole truth about the “right” and “wrong” side of installing modern vapor barriers. We understand the issue down to the smallest detail!

The humidity inside a building is always higher than outside. In the space limited by building structures, the air is regularly saturated with vapors released during breathing, cooking, washing, cleaning and other everyday household activities.

The water suspension rushing upward has a negative impact on the wooden elements of the rafter system, the thickness of the insulation and the roof itself. In order to eliminate it, you need to know how to properly make a roof vapor barrier, how to protect it from fumes harmful to building materials.

Vapors suspended in the air constantly rush to occupy “free positions”, i.e. move to where their percentage is lower. Since the relative humidity inside the house is always higher than outside it, it is easy to guess in which direction the water suspended in the air regularly moves.

A ventilation system is installed to remove water-saturated air, but it is unable to collect and remove all the evaporation generated inside the premises. It is especially difficult to remove from rooms with characteristic high humidity: bathrooms, showers, steam rooms, kitchens, swimming pools, etc.

Steam that is not removed by ventilation “attacks” building structures, tends to penetrate through the fences to the outside, and when cooled, settles inside them or on the surface. Moreover, the predominant part of the steam flow, amounting to 30 to 40%, is directed towards the roof. After all, it is picked up by warm air, which, according to physical instructions, moves upward.

A certain proportion of vapors must penetrate through the roofing system to the outside, as well as through the walls and basement. However, if not properly installed, it settles on building structures or lingers in the roofing pie.

The phenomenon of steam penetration into building structures and subsequent release into the atmosphere is called diffusion. If the roof is constructed correctly, it is not dangerous. But if there are disturbances, the steam turns into condensate, which promotes the settlement of fungal colonies that actively begin to destroy the wood. In addition, moisture retained in the thickness of the insulation significantly reduces the thermal insulation properties.

In order to prevent the impact of steam on the system materials, reliable protection is needed - a vapor barrier. It is made in the form of an insulating shell, which either does not allow steam to pass through at all, or allows it to pass through in minimal quantities. Moisture that penetrates through this protection does not accumulate in the pie, but is removed through the elements of under-roof ventilation: vents, pitched and ridge aerators.

Schematic diagram of a vapor barrier

We have already said that steam spontaneously moves to where the air is less saturated with moisture and, as a rule, has a lower temperature. In accordance with our climatic realities, the flow of evaporation passes through building structures and is directed into the environment. This is exactly what happens for most of the year. Only on hot summer days, which, unfortunately, are few and far between, can vapors from outside rush indoors.

The vapor barrier is arranged in full accordance with the prevailing direction of the diffusing steam flow. It is installed very first on the inside of pies of all types of enclosing structures, including roofs. Those. First, on the side of the premises there is a film that protects the insulation and the roofing pie as a whole from steam, then other components come.

All materials used in roof construction have some degree of vapor permeability. It can vary from zero or thousandths of one to 3000 mg/m². This characteristic tells about the ability of the material to pass a specific volume of moisture per day. To form a barrier against moisture suspended in the air, options with the lowest vapor permeability are chosen.

The main principle of constructing a roofing pie is that on the side of the room there is material with the lowest vapor permeability:

With zero vapor permeability values, the film will not allow household fumes to penetrate into the thickness of the roofing pie at all. Condensation formed there due to the difference in temperature on the outer and inner sides of the roof is evaporated in such cases through vents or accumulates on the waterproofing, after which it flows into the gutter.
With permeability values other than zero, a certain amount of vapor penetrates into the roofing pie. In this case, there is a need for effective moisture removal. To remove steam, an under-roof ventilation system is installed, and dormer windows are constructed to ventilate non-residential attics and attics.

Any of the steam protection options installed on the side of the premises must make it difficult for water suspended in the air to seep into the roofing pie. However, if it does penetrate, the structure should be arranged in such a way that complete moisture removal is guaranteed. This means that the layers following the vapor barrier should allow it to pass through freely.

Therefore, when constructing roofs, the order of the layers, determined by the vapor permeability indicators, must be observed. The first of them should allow the least amount of water to pass through, followed by those with the highest ability. The specified arrangement of protective layers at the same time prevents the passage of cold air flows from the outside into the building.

How to choose a vapor barrier material

Before laying a vapor barrier on the roof, it is worth understanding the types of materials used and the difference in technology of use. They are now produced in a huge assortment, which is easy to get confused. The “pioneer” in the vapor barrier business was glassine, which is still used as a budget option. Later, polyethylene film joined the fight for protection against evaporation, and then polypropylene roll types appeared.

Common vapor barrier options

The development and implementation of new types is based on already well-known and tested vapor barrier barriers. It is associated with the desire to increase strength indicators, increase resistance to temperature fluctuations and resistance to ultraviolet radiation.

A wide range of vapor barrier membranes are produced based on polymer compounds. These include types with and without foil casing. If the foil material is installed so that its metal shell is turned inside the room, then in addition to protecting against the penetration of vapors, it functions as a reflection of heat waves. This is a priority quality when arranging saunas and steam rooms.

Among polymer membranes there are anti-condensation materials, one of the sides of which has a rough surface. This type of insulating film unfolds towards vapors with its rough side, which prevents dew from falling on the material. The second, smooth side prevents water from leaking from the outside, therefore anti-condensation membranes are used both to form protection against steam and for waterproofing.

The use of vapor barrier material depends on the presence of insulation in the roofing pie:

Glassine and vapor barrier membranes with a vapor permeability not exceeding 100 mg/m² per day are laid on the inside of cold roofs. Both types can be used as the bottom layer of insulated flooring in unheated attics.
Polyethylene and polymer single-layer membranes are used as inexpensive vapor barriers for attic structures with a small construction budget.
Polymer two- and three-layer superdiffusion membranes have been used most often recently. Most of these materials are universal, used both as a vapor barrier and a water barrier.

Among two- and three-layer polymer membranes, there are products that do not differ too much from polyethylene in terms of price. Almost all two-layer films are budget types. In terms of strength properties and wear resistance, they are inferior to their three-layer counterparts, the service life of which is almost equal to the similar characteristics of the roof itself.

Insulating materials with a steam transmission capacity of more than 100 mg/m² per day are not used in the steam protection device. They are intended for laying waterproofing that protects the insulation from the outside from atmospheric water and from the wind carrying away heat from its thickness.

Criteria for selecting a vapor barrier film

In addition to the price, the size of which corresponds to the construction budget, and vapor permeability, which determines the scope of application of the material, the choice is influenced by a lot of significant criteria, these are:

Strength. High quality insulating materials cannot be damaged by dropping a tool or stepping, and cannot be torn when applying force and attaching them to the elements of the rafter frame.
Low temperature resistance. Superdiffusive species can be left for the winter as a “cover” of an unfinished building without a laid roof. It will easily withstand both frost and snow deposits. Plastic film cannot be used like this; after a week of exposure to low temperatures, it will crack and crumble.
Resistance to water pressure. During the construction of the roofing pie, it may rain before the roof is laid. When using a membrane, there is no fear that atmospheric water will penetrate into the room and delay further stages of work. If the device is planned for the spring or autumn, it is better to prefer polymer roll material.
UV resistance. Polymer varieties can be safely used as temporary roofing. Unlike polyethylene and glassine, these materials will not lose their original properties under direct sunlight. This characteristic is important if the roof installation will be carried out in the summer.
Fastening. Before making a purchase, be sure to familiarize yourself in detail with the specifics of fixing the material. There is a vapor barrier, the fastening of which is carried out only with the help of nails with a wide head or only with the help of wooden slats. There are options that are simply attached with a stapler.

All details about installation, the formation of overlaps, the need to connect the panels with double- or single-sided tape must be carefully clarified before purchasing. This information is needed for correct calculations of material footage, as well as the consumption of fastening and connecting means. When using vapor barrier membranes in cold structures, for example, gluing them into a single sheet is not required, because The overlap is quite enough.

Some insulating materials do not necessarily require the individual strips to be joined together by gluing. If you need to glue it, then before you buy and lay a vapor barrier on the roof, you should find out whether adhesive tape from the same company as the protective film is available for sale. The use of consumables similar in purpose, but made differently, may not provide the required gluing effect, because it may differ in chemical composition.

Rules for installing a vapor barrier layer

Depending on the quality of the film, it can be laid before installing the roofing pie or after performing the specified work. Naturally, the choice of the optimal time for installation depends not only on the preferences of the roofers, but also on the ability of the material to resist weather influences, as well as on its strength properties.

The main rule that should be remembered and strictly followed by those who are going to lay a vapor barrier on the roof with their own hands is: the roll must be rolled out the way it was wound by the manufacturer. It is necessary to strictly follow all installation instructions and follow the directions indicated on the material.

There is no need to rewind the roll, trying to turn over the side whose function the manufacturer has included direct contact with steam. If the film is laid on the opposite side, it will not retain evaporation and will allow more water into the insulation than is allowed according to the technical parameters of the thermal insulation.

The second mandatory postulate of installing a vapor barrier is that the film cannot be attached “with tension.” The material should be fixed so that it sag slightly between adjacent rafters. The recommended sag amount is approximately 2 cm. A system made of wood will always “play” slightly: swell and expand during the rainy season, shrink and contract in hot weather. To prevent the vapor barrier from breaking through when the wood moves, this reserve is created.

Most manufacturers of vapor barrier materials allow both horizontal and vertical spreading patterns. But you should not unconditionally rely on the opinion of the majority; you must carefully study the instructions attached to the material, which necessarily indicate possible layout options and the nuances of their application.

Usually on the vapor barrier film along the edge there is also a distance for overlap. The instructions for use indicate the width of the overlap depending on the steepness of the roof being installed.

In addition to the width of the overlap, the slope of the slopes also affects the direction of the stripes. On flat structures, roll insulation is most often rolled out perpendicular to the rafters. To attach steep structures to frame systems, membranes are often spread along the rafters. A longitudinal arrangement is preferable, especially if it is possible to cover the slope with one sheet without forming transverse seams - the most common cause of leaks.

According to roofers with extensive experience in arranging roofs, vapor flows move upward and sideways. This direction must be taken into account when arranging the film panels. When the strips are located across the rafters, laying the vapor barrier begins from the ridge and moves towards the edge of the slopes.

To prevent condensation, when it is likely to form, from flowing into the roofing pie of the attic roof and from soaking the insulation, the vapor barrier strips are glued together with tape. The material is laid so that each lower strip overlaps the already secured upper strip by the amount of overlap specified by the manufacturer. This is the main difference between the installation technology and the waterproofing device, the upper strips of which overlap the underlying ones.

When using slats to fix a vapor barrier layer, they must first be antiseptic, because the wood will have to come into contact with the material of other heating equipment, which is fraught with the formation of condensation. Note that attaching the film using a block has a significant advantage - it can simultaneously serve as the basis for installing the inner lining, as well as for forming ventilation ducts.

Video about the specifics of roof vapor barrier installation

A step-by-step description of laying a vapor barrier film will help you understand the essence of the process:

Common mistakes made by self-builders:

Video explaining the principle of operation of a vapor barrier membrane:

Compliance with the technological rules for installing a vapor barrier layer guarantees a long service life of the roof, ensures the thermal properties specified by the design, and eliminates the likelihood of leaks and the need for unplanned repairs.

Modern roofing materials are an almost absolute guarantee that moisture will not penetrate into the roof, regardless of external conditions. However, from inside the premises, the roofing pie is not protected from this in any way. First of all, the thermal insulation material, which loses its performance characteristics when wet, is at risk. As a result, the house becomes cold and damp. A properly installed vapor barrier will help prevent this process.

The need for vapor barrier

The warm air in the room is saturated with vapor and rises, trying to escape through any openings. But since the air temperature and humidity in the under-roof space are much lower, condensation forms, which can be localized on the insulation, which can lead to its destruction.

Vapor barrier material protects insulation from the destructive effects of moisture

A more severe situation is observed in the cold season. The steam escaping into the under-roof space stops, but first turns into frost, after which it forms an ice crust that will freeze absolutely any insulation in a short period of time.

To preserve the insulation, a vapor barrier is needed

With the onset of spring, the ice in all pores begins to melt and erode the interior finish. This becomes the reason that the insulation ceases to perform its functions, and mineral wool will become unusable after just a year of such use, while polystyrene foam will last a little longer.

Without a vapor barrier, the insulation will last much less

That is why a vapor barrier layer is required. Its main task is to prevent condensation from reaching the insulation. This will warn:

the appearance of mold;
heat leakage from the premises of a private house;
rotting of all wooden structures.

Materials used

There is a wide variety of modern building materials that have vapor barrier properties. Conventionally, they are divided into three groups:

Polyethylene film. This type of material is universal because it can perform several functions at once. The film is covered with a special reinforced fabric. There are two types of this vapor barrier material: with and without perforation. But the holes are so small that they cannot be seen with the naked eye. For installation, it is necessary to use seals and connecting tapes, since the film is produced in rolls. The strips must be attached as tightly as possible, otherwise the functionality of the layer will be impaired. There are also polyethylene films with a foil layer that retains heat indoors.
Vapor barrier polyethylene film reinforced with fabric
Polypropylene film. Used as a hydro- and vapor barrier material. On one side, the film is covered with an anti-condensation layer consisting of viscose-cellulose fiber. The principle of operation of this material is quite simple: condensate protruding from the side of the film that faces the insulation is absorbed into it and therefore does not fall on the insulation. Since the installation of the roof implies the presence of a ventilation gap, the moisture absorbed into the vapor barrier dries out.
Polypropylene film has a smooth and rough side
"Breathable" film. The most modern vapor barrier material. Combines vapor and water resistance properties. The main advantage is that there is no need to arrange a ventilation gap, and this allows you to significantly save under-roof space. This membrane allows moisture to pass through itself, which subsequently settles on the rough layer of material, after which it is absorbed into it and dries. This allows the insulation to remain dry. There are different types of vapor barrier membranes available, which can be single- or double-sided.
Single-sided membranes conduct steam in only one direction; double-sided membranes can be laid on either side

For vapor barrier, it is better to use non-perforated film, since perforated film is more suitable for waterproofing.

A very important stage in arranging a roof vapor barrier can be considered not only the process of its installation, but also the choice. The quality of the roofing as a whole, as well as its service life, depends on this. When choosing, remember that the more weight the material has, the stronger it is. Typically it ranges from 60 to 270 g/m2. As for the vapor permeability parameter, you need to choose a material with a minimum value. The recommended parameter is less than 1 g/m2 per day.

How to properly install roof vapor barrier

Before installation, it is recommended to once again make sure that you have purchased exactly the vapor barrier material. You should also make sure that the vapor barrier you purchase is compatible with your roofing material. In this case, “Izospan B” is universal, which can be used regardless of the type of roofing pie.

Which side to put the vapor barrier on the roof

There are certain rules for laying vapor barrier material. Regardless of the type, the film has two surfaces: one smooth, the other rough. You need to lay it with the smooth side on top of the insulation. At the same time, carefully ensure that the material fits as tightly as possible; gaps are not allowed.

The smooth surface of the vapor barrier must be in contact with the insulation

When laying a vapor barrier membrane, it is necessary to provide a ventilation gap between the vapor barrier and the decorative roof sheathing. It should be at least 5 cm, and the type of roofing material in this case does not matter. A counter-lattice, which is mounted directly on the membrane, can provide this ventilation gap.

How to attach a vapor barrier to rafters

The material must be attached to the rafters from the inside of the insulation. For fastening, you can use different parts, for example, a construction stapler or galvanized nails. Try to minimize the number of punctures in the material, as this may cause condensation to penetrate through these holes during the operation of the roof.

You can use a stapler or nails to attach the vapor barrier.

The option of attaching a vapor barrier material to the rough lining of the attic is also allowed.

The material can be laid in horizontal or vertical strips, rolling out the entire coating without breaks. Additionally, it is necessary to process the joints, both vertical and horizontal. To do this, you need to lay the vapor barrier overlapping, the size of the overlap should be at least 15 cm. Next, seal the joints with a special tape, but you can only choose one that is designed to work with vapor barrier film.

All joints must be sealed with wide tape

It is also necessary to seal the joints of the material with a wooden or concrete base, even if a stapler or nails were used for fastening.

The vapor barrier film must be laid without sagging; it must be well stretched.

Video: rules for installing vapor barrier

Installation errors

Despite the fact that the process of installing a vapor barrier film is quite simple, sometimes mistakes cannot be avoided, but they are typical and can be prevented:

loose connection of the film to the purlins, valley and ridge beams, especially for complex roof structures;
the use of a narrow connecting tape, because of this, the sealing of the seam may be broken during the operation of the roof;
lack of deformation reserve when installing a roof with windows;
lack of internal protection of a vapor barrier film around the roof windows, due to which ultraviolet rays gain open access to this material and destroy it;
the film bends around the rafter legs, which creates a gap through which moisture can pass and be absorbed into the insulation.

Video: consequences of improper vapor barrier

Steam is a substance that is quite difficult to block, so vapor barrier of the roof of a private house is a responsible mission. The safety of the roofing pie and wooden structures and the long service life of the roof can only be guaranteed by proper installation of vapor-proof material.