Purpose and location of vapor barrier

It is known that the roofing pie consists of several layers (that’s why it is called that). It may include different quantity interlayers, but hydro-, heat- and vapor barrier remain unchanged. And if waterproofing is designed to prevent moisture from above from penetrating into the house, then vapor barrier prevents vapors from inside from entering the roofing pie.

It is extremely important to place the vapor barrier lower thermal insulation material, because otherwise it will absorb steam coming from below, swell and become damp. However, we all know that there are also roofs that are not insulated; is it really possible to make them without a vapor barrier? Unfortunately, no - vapor barrier not only preserves the insulation, but also maintains a comfortable indoor microclimate. If it is not there, the house will be very stuffy and humid, and the cozy home will turn into an ordinary greenhouse.

To fully appreciate and understand the structure of a roof vapor barrier, you should study the structure roofing pie:

1. The roof covering can be continuous or consist of segments (tiles, slate). It is laid on a wooden sheathing or monolithic base.
2. Waterproofing layer – protects subsequent layers from moisture penetration from the outside. Is waterproof non-woven material, which is attached to the sheathing, leaving a small gap between the next layer.
3. Insulation – roofing thermal insulation materials are slabs with minimal thermal conductivity. They can be made from compressed mineral or stone wool, polystyrene foam or polyurethane foam. Depending on the material, water absorption also differs, which also affects the choice of vapor barrier. Polyurethane foam and polystyrene are practically not afraid of moisture, but mineral and stone wool require the installation of high-quality vapor barrier.
4. Vapor barrier layer - films or membranes are installed on the sheathing immediately after the insulation and leave a gap between the sheathing ceiling material to ensure natural ventilation. When water vapor coming from below hits the membrane, condensation will form, which will flow down and dry due to ventilation of the under-roof part.

Even the highest quality and most modern vapor barrier will not “work” if you do not take care of roof ventilation. Ventilation is important at any time of the year - in winter it will help retain heat and prevent icing of the roof, and in summer it will provide more coolness.

If a vapor barrier is missing or incorrectly installed in the roof of a house, this will inevitably lead to the formation of unaired condensation, dampness, mold and fungi. Eventually, wood structures and roof elements will rot and collapse.

Vapor barrier materials

Today you can find a lot of roofing materials on the construction market. Most of these are polyethylene-based films. They are waterproofing, vapor barrier and antioxidant. The latter are special in that with inside covered with an absorbent fleecy fabric layer that actively collects condensation, preventing it from getting onto the next layer of roofing cake.

Previously, the main material for roof vapor barrier was glassine. Today it is used only by overly economical builders, but if you look at the performance characteristics of glassine and compare them with the qualities modern materials, then the benefit of the latter will be obvious.

Depending on the physical and technical parameters, three types of vapor barrier films can be distinguished:

All film materials used for vapor barriers must have good elasticity, be tear-resistant and as comfortable as possible to use. It is extremely important that the vapor barrier film does not tear during fixation, but wraps around fastener. Even if there is accidental mechanical damage to the roof, for example, due to a tree branch falling during a hurricane, the vapor barrier should maintain its integrity, minimizing losses. Ideally, the thickness of the roof vapor barrier should be such that the material can withstand the load of thermal and waterproofing if the sheathing and rafters fail.

Vapor barrier with isospan

One of the most popular materials on domestic market is the vapor barrier of the roof isospan. It is relatively inexpensive and has impeccable performance characteristics, durability and ease of installation. Izospan can be used for waterproofing and as a vapor barrier at the same time.

The manufacturer offers several different modifications of Izospan, each of which is intended for a specific purpose, but there are also universal films. Therefore, it is very important to clarify the characteristics of the selected material before purchasing, so as not to accidentally buy the wrong variation of Izospan.

Izospan B is widely used among professional builders and self-taught owners. It can be combined with any type of roofing. In addition to protecting the insulation from moisture, it does not allow microfibers from the cake to fall off inside the room, as is often the case with other materials.

Installation of vapor barrier

It doesn't matter if you do a vapor barrier for flat roof or work with gable roof– the film or membrane should be installed only after laying the insulation.

Features of vapor barrier installation:

1. The canvases can be installed both vertically and horizontally.
2. If you prefer horizontal installation, start installation from the top.
3. Lay the pieces of canvas with an overlap of at least 10 cm. Be sure to seal the seams with single-sided or double-sided adhesive tape.
4. Choose a wide ribbon at least 10 cm.
5. Double-sided tape is applied inside the overlap - first apply it to bottom layer, then remove protective coating and fix the top one with an overlap.
6. If you are laying a vapor barrier along the rafters without rough filing the insulation, overlap and fix the sheets directly on the rafters.
7. TO wooden sheathing or the vapor barrier is secured to the rafters with a construction stapler or galvanized nails (they do not rust).
8. It is recommended to strengthen the places where the canvases are glued with pressure strips. This advice is especially relevant for roofs with a slope of up to 30 degrees and with loose thermal insulation.
9. At the points of contact with passages, hatches or roof windows, a so-called attic apron should be installed through the roof, which is usually included in their package. If it is not there, glue it around the perimeter window frame or opening 2-sided butyl tape.
10. Where it passes through the roof water pipes fold the vapor barrier material downwards, wrap it around the pipe and seal it tightly with electrical tape.

When the vapor barrier is laid, attach bars treated with antiseptics on top of it at a distance of 50 cm from each other. This way, you will create an air corridor where condensation will quickly evaporate, and the film will not touch the ceiling cladding. If you are using a vapor barrier membrane, it is not necessary to install bars. But they will come in handy to occupy this space with various communications. If you are planning to decorate interior space at home with plasterboard, instead of wooden blocks, use a galvanized metal profile.

Errors when installing vapor barriers

Very often, when arranging a roofing pie, builders make mistakes that lead to fatal consequences. If the location of the insulating layers is mixed up, the roof may become unusable after just a year of operation.

We have prepared a list of the most common mistakes when installing vapor barriers:

1. The junction points of the film are not sealed tightly, as a result of which the effectiveness of the vapor barrier is reduced to a minimum, the insulation becomes damp, and the rafter structures begin to rot.
2. Using narrow adhesive tape to seal seams - during use, tape 5 cm wide or less will simply come off. The recommended tape width is 10 cm or more.
3. There is no deformation margin when working with window openings - any house shrinks, so you need to leave a margin in the form of a fold of 20-30 mm.
4. Film around skylights is not protected by finishing from the inside - if this is not done, after a few months of operation the vapor barrier will completely become unusable and collapse under the influence sun rays.
5. The film envelops the rafters - if the vapor barrier material goes around the rafter legs, moisture will penetrate to the wooden elements, as a result of which they will begin to rot.
6. Where the vapor barrier is adjacent to internal walls houses, mines or chimneys, it is necessary to take into account the specifics of the irregularities to which the material will be attached in order to ensure tightness of the fixation.
7. If you are attaching the film to a rough brick wall or unplaned wood surface, this can only be done using polyurethane, acrylic glue or synthetic rubber.
8. Do not use polyurethane adhesive tapes or butyl rubber tapes as they do not adhere well and may soon peel off the surface.

The penetrating ability of water vapor is extremely high, so you should approach the vapor barrier device extremely responsibly. In addition to the popular Izospan, TechnoNIKOL roof vapor barrier is widely used today. The most important thing is to choose the materials correctly and install them in the right sequence, then the roofing pie will last for decades without repair.

During the process of washing, cooking, and cleaning indoors, steam is generated, which rises upward in accordance with the laws of convection. Having passed through all layers of roofing materials, it cools and turns into condensation, which settles on the thermal insulation, as a result of which it gets wet and loses its effectiveness, as well as on the rafter system, contributing to its rapid rotting. To prevent this from happening, a roof vapor barrier is installed.

Why do you need a roof vapor barrier? This question is asked by many owners before building a roof. Often people simply do not understand the purpose of this material and they want to save money by completely abandoning vapor barrier, but in vain. It ensures effective ventilation of the roof both in summer and winter, while protecting the roof from excess moisture escaping from the room.

IN summer period correct design will save you from the heat, because hot air, when rising upward, will freely escape through the ventilated hole, taking with it the moisture contained in the insulation, as well as heat from the hot roof. This is especially important if your home is equipped with any of. Therefore, when the question arises whether to install a vapor barrier or not, it is worth knowing that in its absence there is a risk of mold, accumulation of moisture in the rafters, freezing of the roof due to moisture entering from inside the house, as well as damage to the ceiling finish.

There are several types of vapor barrier films:

• Foil. This film is used in buildings with increased temperature requirements.
• Vapor barrier reinforced membrane.
• Polyethylene. The advantage is the low price, but durability and strength leave much to be desired.
• Universal vapor barrier film. The most commonly used material, perfectly compatible with all types of insulation.
• Non-reinforced vapor barrier membranes with a reflective aluminum layer.

Features of vapor barrier installation

Before you begin installation, you should pay attention to which side to lay the vapor barrier for the roof, since it has a one-way direction, namely, it allows steam to pass in only one direction, like a membrane. Fastening the film should ensure its maximum safety, for which the most suitable would be to use double-sided tape. Fasteners to wooden surfaces possible construction staples or self-tapping screws with large heads.

Basic rules for installing vapor barrier film:

• Laying a vapor barrier on the roof should be done with the smooth side in relation to the insulation. If condensation forms, it will fall on the vapor barrier film from the rough side, which will not give it the opportunity to roll off, but will hold it on its surface until it dries completely.
• The vapor barrier must be laid with an overlap, and the joints must be covered with special adhesive tape.
• Due to the fact that steam has a high penetrating ability, during the installation of the roof, special attention must be paid to the places where the film adheres to pipes, rafters, and walls.
• Steam diffusion is much more intense with a sharp temperature difference, because steam moves from a heated room to a cold one, therefore, when asking the question “Do you need a vapor barrier under cold roof?”, the answer is obvious – it is absolutely necessary.

Instructions for installing vapor barrier on the roof

The installation of a roof vapor barrier is quite simple, so experience is not necessary to carry out such work. In case of ignorance of how to properly make a vapor barrier, or uncertainty in own strength, you can always turn to specialists for help. Below is a step-by-step sequence of doing the work yourself:

1. Vapor barrier film can be mounted both horizontally and vertically. If selected horizontal installation, it should start from the bottom, gradually rising to the top of the roof roof.
2. Each subsequent strip is carefully laid on the previous one, ensuring an overlap of at least 10 cm. In this case, the seams must be carefully taped to avoid the appearance of gaps in the insulation, allowing steam to penetrate through it. When using double-sided adhesive tape, it will be convenient to apply it first to the bottom layer, then remove the protective coating and fix it with the top layer.
3. Fastening the film to the rafters is most comfortable using construction stapler. In case of nailing, you need to use galvanized ones, which do not rust. If the roof slope is less than 30 degrees, then it would not be superfluous to additionally strengthen the junction of the panels with planks.
4. If there are pipes in the roof, you should wrap them in film, first turning it down, and then carefully seal it with adhesive tape.

Before laying a vapor barrier on the roof, it is necessary to prepare and apply an antiseptic coating to the bars, which, upon completion of installation, will need to be used to secure the film, maintaining a distance of 0.5 m. This is necessary to form a ventilation corridor that promotes the rapid evaporation of emerging condensate.

Vapor barrier is not a material that is worth saving on, because it is what ensures the safety and long service life of the roof, thereby guaranteeing residents a pleasant and comfortable existence.

An important element in the roofing pie is the vapor barrier of the roof. In short, layer vapor barrier material protects the insulation from oversaturation with moisture and, as a result, loss of its thermal insulation properties. To avoid this situation, it is necessary to perform both hydro and vapor barrier of the roof.

Well, if it’s completely to the point, then the answer is already clear from the subtitle. Installing a vapor barrier on the roof protects or insulates the insulation from vapor.

It's that simple. But where does the steam come from? There is such a thing as moisture transfer. The room inside becomes damp for several reasons:

• Cooking;
• Human breath;
• Taking a shower;

It's no secret that warm air tends to rise up. In addition, its ability to transfer moisture (steam) is much higher than that of cold water. This means that it also carries moisture with it. What obstacles does this couple encounter on its way? These could be:

• Concrete and wooden floors;
• Internal ceiling and roof lining.

However, some of the steam still reaches the very top of the roof - the place where the roofing pie is located. And if there were no vapor barrier for the roof, all this steam would directly get into the insulation.

Considering that foam plastic (penoplex) is not insulated inside, insulation means mineral wool, which is used most often.

For example, here is a statement that can be found on the official website of the TechnoNikol company - major manufacturer stone wool:

“Rock wool should not be directly soaked, as it loses its properties once wet.”

Total: why do you need a vapor barrier? We found out that the inside of the insulation needs to be covered with vapor film to prevent steam from getting into it. The roof must first be insulated, and only then covered with film from the inside.

Protective membrane on the outside

A waterproofing coating is used under the main roof covering (for example, for a metal tile roof). Since from the outside the insulation will be affected not only by steam, but also by water in two other states, for example in the form of rain, waterproofing is needed there. Using a vapor barrier membrane like Izospan B for outdoor use would be a big mistake.

By the way, the video below tells you very interestingly and scientifically about whether a vapor barrier is needed.

What to use

As in many cases, there are different materials, used to solve one problem. Roof insulation is no exception.

Polyethylene films

To install a roof vapor barrier, high strength of the material is sometimes necessary, for example, if we are talking about insulation attic floor. To increase the strength of polyethylene, they began to reinforce it. There are two types of such films:

• Perforated;
• Non-perforated.

Separately, it is worth mentioning the films on one side, covered with aluminum foil. Their ability to resist steam is much higher than that required for ordinary premises. Most often, such films are used for saunas and baths, so there is no point in choosing them for the roof.

However, there are films that have two significant advantages over polyethylene films.

Polypropylene films

The notorious advantages of such films are:

• Higher strength;
• Resistant to ultraviolet rays.

For example, such films can protect exposed rafters from rain and snow throughout the year.

There is a particularly attractive type of film made from this material. Films with antioxidant coating. On one side, such films are covered with a layer of viscose fiber with cellulose. This coating is capable of absorbing moisture (condensed steam) and storing it for a certain time. After the cause of condensation disappears, the layer dries quickly, preventing moisture from penetrating into the thermal insulation. It is important to install this film correctly. The antioxidant layer should face down from the insulation.

The most commonly purchased films are the following brands:

• TechnoNIKOL,
• Izospan;
• Klober;
• Tyvek;
• Juta;
• Nikofol;
• Everlasting.

Did you know?

Glassine was previously used to insulate the roof. Now this material is practically not used. There are much more advanced, and relatively inexpensive materials. Also previously used were roofing felt, roofing felt, or ordinary polyethylene film.

Which vapor barrier to choose for the roof? As we have seen, this mainly depends on two factors:

• Places where insulating material is used;
• Owner's financial capabilities.

Having noticed possible types vapor barrier, you can more clearly understand which vapor barrier is better for the roof. We settled on polypropylene films with an antioxidant coating.

Principles of vapor barrier installation

Installing a roof vapor barrier is not a complicated process in itself, however, there are some nuances here too. So, we can highlight the following:

• You need to lay a vapor barrier on top of the thermal insulation. Both horizontal and vertical film laying is possible. You need to lay the film horizontally (transversely to the rafters) starting from the top row and moving down. It is definitely important to make an overlap of 10, and preferably 15 centimeters. The films are nailed to the sheathing with a construction stapler. To fix joints, two types of tapes are used: double-sided and single-sided. Double-sided tape is glued to the inner strip of film and covered with the next one. Single-sided tape is used to secure the seams from the outside and is glued directly at the joint.
• If you need to lay the insulation vertically, make sure that the joint is on the sheathing, otherwise the film may tear, even if it is glued with tape. (Besides, it is very uncomfortable when weighed).
• In places where the film adjoins the windows (if the roof is attic), use a special apron. If such an element is not included with the film, apply special tape to the joint.
• An additional beam is attached on top of the laid insulation as a sheathing. This is necessary so that the vapor barrier for the roof and the internal cladding have a ventilation gap between them and air circulation is possible. The minimum thickness of the timber is 25 mm, preferably 50 mm.
• Avoid sagging foam. Mount it tensioned.

What is neglected

As already mentioned, installing a vapor barrier on a roof is not a difficult task, but here there are several common flaws. For example, good, expensive European films are purchased, and, say, connecting tapes are ordered from China. Doing this is extremely wrong. Or the material is not used at all for its intended purpose. For example, Izospan V is used as waterproofing.

Also, puncture sites with self-tapping screws should always be fixed with tape and aluminum coating. Unfortunately, you can often find torn insulation on the roof. Holes greatly reduce protection and steam quickly penetrates the mineral wool, forming frost and damaging the heat-saving layer.

Cost calculation

How much will it cost to create a vapor barrier for a roof? Price depends on density (measured in grams per square meter) and manufacturer's brands. Rolls come in widths from 1 to 3 meters (rolls wider are rare).

For example, vapor barrier membrane Tyvek Airguard SD5, a roll 1.5 m wide and 50 m long, costs 9.5 thousand rubles. ($164). This means that 1 square meter of such insulation will cost 126 rubles or $2.17.

A roll of Izovek V Eco costs only 540 rubles or 9.3 dollars. (Size 1.6 by 44 m). Thus, 1 square meter of vapor barrier will cost 7.67 rubles or $0.13. In our example, the last option will be 17 times cheaper.

Izospan RS of approximately the same size will cost about 1,800 rubles ($30).

Don't forget to include joining strips, tape, and stacked staples in your calculations. When ordering a complex roof, laying a vapor barrier (the work itself) will cost 40-80 rubles per square meter.

Roof vapor barrier is the most expensive, but important element of the roofing pie. Vapor and waterproofing reliably protects rafter system from rotting, and the insulation performs its functions.

Real mistakes on real objects 3 videos

See how an incorrectly installed layer of insulation can damage the sheathing.

Every person dreams of his home being cozy and warm. To achieve this goal it is not enough to choose suitable project, we also need to make sure that everything construction work, including the installation of the roof, were carried out in strict accordance with technology. Everyone knows that roofing materials must protect the house from penetration of external moisture. However, we should not forget about the destructive properties of steam, which is formed indoors during cooking, cleaning, washing, etc. In order for the house to have a normal microclimate and retain heat, a roof vapor barrier is required.

According to the laws of physics, warm air saturated with vapor rises and accumulates near the ceiling. It tends to go beyond the living space, into the under-roof part, where the humidity and temperature are lower. If there is unprotected thermal insulation in the way, the steam will penetrate into the insulation and begin to destroy it.

With the onset of winter, the following will happen: the low temperature on the outside of the insulation will stop the steam, it will first turn into frost, and a little later into an ice crust that will freeze the insulation, filling all its cracks. With the onset of spring, the ice will melt and flow, eroding the interior decoration. The thermal insulation material will lose its properties and become unusable. Mineral wool will deteriorate in one winter, polystyrene foam will last a little longer.

To avoid such consequences, it is necessary to install a vapor-tight barrier, the main task of which is to prevent condensation from entering the thermal insulation layer. This will protect the insulation from moisture, thereby preventing the formation of mold, heat loss from the room and rotting wooden structures.

Vapor barrier is used for both pitched and flat roofs, regardless of the type of coating

What materials are used to protect against internal moisture?

For a long time, the main material for vapor barriers was glassine. Particularly economical builders even now use it. However, if you consider modern requirements to strength, service life, environmental friendliness and insulation characteristics - this is yesterday.

Recently, film materials with a complex multilayer structure have begun to be used as vapor barriers. They have a wide temperature conditions applications and high vapor permeability. Thus, the vapor permeability of simple polyethylene is 13-20 g/m2 per day, and that of a more modern vapor barrier material is about 0.4 g/m2.

Film materials are elastic, tear-resistant and easy to use. This is very important because:

1. when fastening with dowels and self-tapping screws, the material does not tear, but tightly grips the fastening itself;
2. durable film will maintain its integrity even if mechanical damage in roof construction;
3. the film must withstand the load that will fall on it if the insulation loses its elasticity and cannot stay on the rafters.

Vapor barrier films are of two types:

• polyethylene;
• polypropylene.

Film-type vapor barrier materials are produced in rolls, which greatly facilitates their installation.

Polyethylene films are not very durable, so they are reinforced with special fabric or reinforcement mesh. They can be perforated or non-perforated.

Important! For roof vapor barrier, non-perforated polyethylene films should be used; perforated ones are more suitable for waterproofing.

In addition to conventional reinforced polyethylene films Films laminated on one side with reflective aluminum foil are commercially available. The vapor barrier properties of such films are very high, so they are used primarily for vapor barrier in hot and wet areas– saunas, baths, swimming pools, etc.

Polypropylene films are a woven material laminated on both sides with the finest polypropylene. They are characterized by higher strength and resistance to ultraviolet radiation. In addition, polypropylene films can have a special antioxidant layer that prevents the formation of condensation. It easily absorbs and retains all resulting moisture, and after condensation conditions disappear, it dries quickly due to ventilation. Obviously, the antioxidant film has a one-sided application - the antioxidant rough layer is down, and the smooth surface is up.

It is very important that the vapor barrier material is high quality, because it is located at the very base of the roof structure and is difficult to access for repairs.

Three-layer polypropylene hydrophobic membrane

How to install vapor barrier

How a roof vapor barrier is performed + video and photos of installation can be found on the Internet. Let's take a closer look at how this process should work:

• Installation of vapor barrier should be carried out from inside the room after installing thermal insulation.
• Vertical and horizontal laying of canvases is allowed.
• Horizontal laying should start from the top. Each subsequent sheet is laid on top of the previous one with an overlap of at least 10 cm, with the seams necessarily sealed using single-sided or double-sided adhesive tape. Single-sided tape is glued on the outside of the overlap, and double-sided tape on the inside.
• If installation is carried out along rafter legs, and there is no rough filing of the insulation, then it is recommended to overlap the panels on wooden rafters.
• The vapor barrier material is attached to the wooden structural elements with staples or galvanized nails.
• All joints must be tightly connected. It is advisable to reinforce the gluing areas with pressure strips. It is especially important to use such planks on roofs with a slope of up to 30 degrees and with low insulation density.
• In areas adjacent to skylights, hatches and other passages through the roof, it is necessary to use a vapor barrier apron included with them. If there is none, then double-sided butyl tape must be glued around the perimeter of the frame.
• In those places where they pass through the roof ventilation pipes, the film must be folded down, wrapped around the pipe and sealed tightly with one-sided adhesive tape.
• After laying the vapor barrier film, it is necessary to attach antiseptic-treated bars in increments of 500 mm so that the inner ceiling lining does not come into contact with the film. The bars perform several functions at once: they fix the thermal insulation layer and form an air gap between the inner lining and the vapor barrier. Various communications can be installed in the resulting space. When finishing a room with plasterboard, galvanized profiles should be used instead of bars.

Important! The vapor barrier film should be installed without sagging, with slight interference.

The vapor barrier film is laid along the lower plane of the rafters and fixed with staples or nails

Common installation errors

If the installation of vapor barrier on the roof is carried out without the help of professionals, then mistakes may be made:

• The most common vapor barrier defects are considered to be unsealed connections of the film to purlins, crossbars, ridge, valley and ridge beams. The more difficult truss structure and, accordingly, the shape of the roof, the more expensive and difficult the task of high-quality insulation becomes.
• To seal seams, you cannot use narrow (up to 50 mm) tapes. The width of the adhesive tape must be at least 100 mm.
• With vapor barrier window openings very often they forget to leave a deformation reserve of the film (a fold of 20-30 mm for the draft).
• If the plastic film around the roof windows is not protected interior decoration, then after a few months it will completely collapse under the influence of sunlight.
• The film should not be allowed to go around the rafters. In this case, moisture will enter the gap between the vapor barrier and the rafters, and when the “dew point” is reached, condensation will form on cold surfaces.
• In places where the vapor barrier adjoins the internal walls, chimneys, shafts and other elements should take into account the nature of the surface to which the film will be attached. If the surface is rough ( brick wall, unplaned wood), then the film can only be glued using special adhesives from polyurethane or acrylic mixtures and synthetic rubber. Widely used butyl rubber and polyurethane adhesive tapes, as well as adhesive tapes, cannot be used, as there may be a loss of tightness due to poor adhesion.

To seal the joints of foiled polyethylene films, it is recommended to use aluminum-coated adhesive tapes

Water vapor has a very high penetrating ability, so installing a vapor barrier is a responsible process. Specialists who know all the subtleties and nuances will help you complete all the work efficiently. The main thing is to choose the right materials and do everything in the right sequence. Only a properly laid roofing pie will provide the residents of the house with a carefree and pleasant existence for a long time.

It seems to people ignorant of construction that installing a vapor barrier on the roof of a residential building is excessive caution and overinsurance, which only results in additional costs. However, every experienced roofing specialist will confirm that a correctly selected waterproofing film significantly increases service life roofing, and also improves the microclimate in the attic or attic. In this article we will tell you how to choose your own hydro-vapor barrier for the roof, as well as how the process of laying the material on the rafter frame occurs.

Vapor barrier film is a special material that is necessarily included in the roofing pie, which has a pronounced vapor blocking effect. According to the well-known law of convection, warmer and more humid air, abundantly released during human activity, rises up into the under-roof space, then it settles in the form of condensation on the roof rafter frame, leading to dampening of the insulation and rotting wooden elements designs. The vapor barrier film as part of the roof performs the following functions:

• Preservation of the performance qualities of the insulation. Installing a vapor barrier prevents the accumulation of condensation inside the thermal insulation material, which significantly reduces the effectiveness of insulation.
• Extending the service life of roofing coverings. Roofing materials are much better protected from moisture and corrosion with outside. Installing a vapor barrier solves the problem of condensation affecting the lower surface of the roof covering.
• Protection wooden frame roofs from rotting. A properly installed vapor barrier film prevents the roof rafters from getting wet due to condensation, which results in wood rotting.

Pay attention! Vapor blocking film “works” effectively only if it is correctly selected for climatic conditions, roof structure, and type of roofing material. An unsuitable vapor barrier will not only be of no benefit, but will also intensify the process of condensation.

Material requirements

The most primitive and cheap material for roof vapor barrier is glassine. But in modern realities, the installation of this outdated fabric is practically not carried out, since there are more effective and durable analogues. Current vapor barriers are made from high-strength polymers with improved performance characteristics. A properly selected vapor blocking film has the following qualities:

1. Wide temperature range. To do possible installation in any climatic conditions, the film must withstand high, low temperatures and even them sharp changes. The optimal temperature range for products of this type is considered to be between -70 and +100 degrees.
2. High degree of vapor permeability. The lower this indicator is for the vapor barrier, the better. Previously, polyethylene films with a vapor permeability of 13-20 g/m2 were laid, but now polypropylene analogues with a permeability of no more than 0.4 g/m2 are used for roofing.
3. Long service life. Installing a vapor barrier whose service life does not exceed 10 years is impractical, since modern roofing materials last more than 20-25 years, and it is almost impossible to replace the film without disassembling the structure.
4. Elasticity. Vapor blocking film based on resistant polymers is highly elastic, which simplifies the installation of the material and also reduces the amount of damage during installation.

Important! Experienced craftsmen They believe that the main indicator of the quality of a vapor barrier is the high strength properties of the material. A properly installed vapor blocking sheet can even withstand the weight of snow, rain or melt water in the event of extensive damage to the roof.

Types of materials

Vapor-blocking film is a special material; it does not allow moisture-saturated, heated air to pass through, which leads to the formation of condensation, wetting of the insulation, and destruction of the roof frame. It is a thin, lightweight, but very durable and UV-resistant fabric based on thermoplastic polymers. There are the following types of materials for vapor barrier:

• Polyethylene. Polyethylene is a fairly thin, lightweight and at the same time cheap polymer that is used to make vapor-blocking membranes. However, the strength qualities of such a film are low, so it is reinforced with a special mesh or fabric. Installation of laminated polyethylene vapor barrier is carried out in saunas, baths and other rooms where it is necessary to retain heat.
• Propylene. Propylene-based vapor blocking membranes differ increased strength, UV resistance, low vapor permeability. They are a woven material with an anti-condensation layer.

Please note that the use of perforated materials to organize a vapor barrier for the roof is not allowed. It is important to choose the right vapor blocking membrane to protect roofs from negative influence condensate

Technology of use

Laying vapor barrier material is important technological operation, performed during the construction of the roof, the quality of which determines its reliability, service life and strength. Installation of vapor barrier is carried out from the inside attic room after the roof truss frame is ready in accordance with the following rules:

1. The vapor-blocking fabric can be laid along or across the rafter legs of the frame.
2. Be sure to start installation from the roof ridge, placing strips of material overlapping with an overlap of 10-15 cm.
3. The material will be fixed using construction staplers or galvanized nails.
4. The joints between the strips are hermetically connected using special adhesive strips located along the edges of the canvas.
5. After installing the waterproofing, a glazing bead treated with an antiseptic is nailed along the rafters so that the sheathing does not touch the membrane.

Important! The vapor blocking film has two sides, which differ in the nature of the surface and properties. When installing, you need to place the canvas with the rough anti-condensation side down.

Video instructions