Roof vapor barrier: an overview of materials and recommendations for laying a vapor barrier film

As part of the insulated roof, there is such an element as a vapor barrier film. Underestimating its importance means condemning yourself soon to costly roof repairs. Misunderstanding of the difference between a vapor barrier film and a waterproofing membrane will also lead to unpleasant consequences, which even sales consultants in hardware stores often sin today. Let's consider what a vapor barrier is and how it is applied.

Purpose of vapor barrier

Under natural conditions, air always contains a certain amount of water vapor. The maximum possible concentration depends on the temperature. At 100% relative humidity, that is, at maximum saturation, a cubic meter of air contains:

  • at a temperature of +18 ° C - about 20 g of steam;
  • at 0 ° C - only 1 g.

If the maximum steam-saturated air with a temperature of +18 ° C is cooled to 0 ° C, then 19 g of steam in each m3 condense into liquid water. And water is not at all as harmless as steam:

  • it increases the thermal conductivity of materials;
  • makes possible the growth of mold and rotting fungi that destroy materials;
  • when freezing, it increases in volume, which also leads to the destruction of the materials impregnated with it.

    The temperature difference will lead to the conversion of steam to water

In winter, the air in the living quarters is much warmer than the outside, and therefore contains more steam. There is no shortage of sources for the latter: washing, cooking, hygiene procedures, breathing of people and animals, sweating, etc.

With a significant difference in the amount of steam inside and outside the room, the so-called partial pressure takes place - the steam tends to move from an area with a high concentration to an area with a low one. But in the course of diffusion through the enclosing structures - walls and roof - it, when it reaches colder outer layers, can turn into liquid water, as described above.

The vapor barrier is always mounted on the warmer side

Whether steam will condense and in what volumes depends on many factors:

  • outside temperature;
  • indoor air temperature and humidity;
  • the thickness of the enclosing structure, as well as the thermal conductivity and vapor permeability of the materials of which it consists.

In the case of walls, a combination of conditions may take place under which moisture in the outer layer does not form at all or is formed in scanty amounts. Then the wall is not sheathed with a vapor barrier. Partial removal of steam through walls is beneficial as it reduces ventilation performance, which in turn leads to less heat loss.

But the roof is a different matter: the warmest and dampest air rushes to it, which is due to convection. Therefore, building codes prescribe that the inside of the roofing cake should be protected with a vapor barrier under all conditions.

The vapor barrier is not the only element in the roof structure to prevent vapor condensation. Ventilated gaps are arranged under the roofing, and in some cases under the waterproofing membrane. The draft existing in them blows out the steam that has seeped into the roofing cake, preventing it from condensing.

A vapor barrier layer protects the insulation from vapors generated inside the building

Vapor barrier and gaps must be mandatory, since there are many materials vulnerable to moisture in the roof structure:

  • roofing - steel sheets are often used, which, in contact with moisture, rust in places;
  • rafters and crate - made of wood, which, with constant moisture, even after treatment with an antiseptic, begins to rot;
  • insulation - usually mineral wool is used, which absorbs water well and, when wet, loses its heat-insulating qualities.

Review of materials

The vapor permeability of a material is not an abstract characteristic. It has a numerical expression that denotes the amount of steam that penetrates through the material per hour at a certain pressure difference. The unit of measurement is mg / (m · h · Pa). Often the vapor permeability of films is indicated in units such as g / m2·day.

And, finally, they can indicate the inverse value - resistance to vapor permeation. The unit of measurement for the material as a whole is (m h Pa) / mg, for a film of a specific thickness - (m2H Pa) / mg. Films with a vapor permeation resistance of no more than 7 (m2H Pa) / mg.


This material is a cardboard impregnated with petroleum bitumen. It has the following advantages:

  • high resistance to vapor permeation;
  • strength;
  • low cost.

    Glassine is characterized by high vapor permeability, but by other properties it is inferior to modern films and membranes.

But today glassine is not used very willingly, since it has serious disadvantages:

  • significant weight;
  • the presence of an unpleasant odor, especially in the heat;
  • the inability to reliably glue the joints between the panels.

The latter circumstance is especially important, since the vapor barrier layer performs its function only under the condition of complete tightness.


Polyethylene film can be used as a vapor barrier if it has the following properties:

  1. Thickness - 200 microns or more. Thin films may break during installation, resulting in steam loopholes.
  2. The raw material is primary polyethylene. Recycled films are less elastic and therefore more easily damaged.
  3. Transparency. In the absence of transparency, it will not be clear whether the overlap between the panels is hermetically glued.

    Polyethylene film is a budgetary, but not very reliable material for vapor barrier

Polyethylene perfectly resists steam and at the same time has an affordable cost. You can use varieties with less than indicated thickness, but reinforced with polypropylene mesh or fabric.

Reinforcement allows the use of a film whose thickness is less than recommended by the designer


Polypropylene film is superior to polyethylene in a number of parameters:

  • strength;
  • elasticity;
  • heat resistance.

As for frost resistance, then polyethylene takes precedence: when the temperature drops, polypropylene will become brittle much earlier.

Polypropylene film is inferior to polyethylene in frost resistance

But polypropylene vapor barrier films have another important advantage: they can be equipped with an absorbent layer made of viscose and cellulose. If moisture condenses on such a film, it will not drain onto the floor, but will remain in the absorbent layer and, over time, when the humidity in the room returns to its normal value, it will evaporate again.

Films made of polypropylene are more expensive than polyethylene films.

Foil films

In terms of resistance to vapor permeation, foil surpasses all other materials. But in this case, the most important is its other property - the ability to reflect infrared (IR) radiation. Infrared waves carry heat, so the reflective film has an energy-saving effect. For the same purpose, the inner surface of the thermos flask is polished to a mirror finish.

Foil vapor barrier film contributes to energy saving at home

The use of foil vapor barrier films is most justified in baths and saunas.

Today on sale you can find both conventional vapor barrier films and branded ones:

  • Izospan;

    "Izospan" is laid with the smooth side out

  • Rockwool;

    Rockwool vapor barrier is used to protect wall and roof insulation

  • Dorken Delta-Dawi;

    Dorken Delta-Dawi universal vapor barrier film suitable for roofs and walls

  • "Ondulin Ondutis R70";

    Film "Ondulin Ondutis R70" is compatible with all types of thermal insulation

  • Technonikol, etc.

    Technonikol produces three-layer vapor barrier membranes

Usually the manufacturer offers a line in which the films differ in strength and price. For example, the Izospan line includes films:

  • Izospan A;
  • Izospan V;
  • Izospan C and D.

    Modifications of "Izospan" differ in the magnitude of the breaking load: their vapor permeability is the same

Branded materials are more expensive than conventional materials, which, according to marketers, is explained by their reliability and high quality. However, many experts consider the purchase of such vapor barriers unjustified. At least, it is known that in the developed countries of the West, in the overwhelming majority of cases, ordinary polyethylene film with a thickness of 200 microns (0.2 mm) is used as a vapor barrier.

Some finishes are vapor tight. These include, for example, vinyl wallpaper and plastic panels. In the case of their use, the installation of a vapor barrier film is not necessary.

Difference between vapor and waterproofing

It is clear that a vapor barrier film is at the same time a waterproofing film: if it does not let the air in with the vapor dissolved in it, then it certainly will not let the water through. Based on this, many citizens without construction experience and even sellers argue that steam and waterproofing films are one and the same.

And here you need to be very careful: there is a special type of waterproofing films that allow steam to pass through perfectly. Such material can be called differently:

  • windproof film;
  • diffusion or superdiffusion membrane;
  • waterproofing vapor-permeable membrane.

The film has microscopic perforations through which water does not seep, but air, and hence steam, passes freely. There are more holes in a superdiffusion membrane than in a simple diffusion membrane.

Membranes are different: some do not allow water to pass at all, others pass in one direction. The vapor permeability is the same in both directions.

Waterproofing membranes do not allow water to pass through, but are vapor permeable

Due to its vapor permeability, the membrane can be laid close to the insulation, while when using a conventional, vapor-tight waterproofing film, a ventilated gap must be left between it and the heat insulator. Thus, the membrane protects the insulation not only from getting wet, but also from blowing.

Varieties that let water pass in one direction are laid on the walls of frame houses. Those are known to be OSB-clad sandwich panels. If, due to errors in installation, water still leaks out to the sandwich panel, due to the one-sided hydropermeability of the membrane, it will be able to leak out from under it.

Obviously, a vapor barrier film and a waterproofing membrane are not the same thing anymore. If, instead of a vapor barrier, a membrane is laid inside the room, steam through it will freely enter the building structure, which will lead to the unpleasant consequences described above. On the other hand, if instead of a membrane close to the insulation, a vapor barrier film is laid outside, then steam - and it still enters the insulation in small quantities - will not be able to enter the under-roof space and, having accumulated over time, will condense in the heat insulator.

Another important difference: polymer waterproofing films and membranes usually contain additives that make the material resistant to ultraviolet (UV) radiation for a while. This is due to the fact that such films are laid on building structures from the outside and are in the open sun before installing the cladding or roofing. In the characteristics of the material, they write: "UV stability - 3 months."

Some of the waterproofing vapor-permeable membranes are also prescribed by the instruction to be laid on insulation with a gap, so when buying, you should carefully study the documentation.

Vapor barrier film laying technology

The vapor barrier is laid on top of the insulation necessarily from the side of the room. In this case, they act as follows:

  1. Having spread the roll, the film is shot to the rafters with a stapler or nailed with galvanized nails with wide heads. It is advisable to lay the panels horizontally, starting from the bottom. The foil side or absorbent layer, if present, should be facing the room.

    The vapor barrier film is nailed to the rafters with a construction stapler or galvanized nails

  2. Having removed the protective film from one side of the double-sided tape, it is glued to the edge of the laid strip. Butyl rubber tape should be used - ordinary tape can come off the vapor barrier film.
  3. The second section of the vapor barrier is laid on the first with an overlap equal to the width of the adhesive tape. During the installation process, you need to remove the second protective film from the adhesive tape and glue the edge of the vapor barrier to be laid to it, then roll the joint with a roller. Along the way, the film is attached to the rafters. It is necessary to ensure that no non-glued breathable gaps remain in the overlap - if they are present, the vapor barrier will not be effective.

    Film joints are glued with double-sided tape

  4. The rest of the panels are laid in the same order until the roofing cake is completely covered from the inside. The pipes passing through the roof and the dormer windows installed in it should be tightly wrapped with a vapor barrier film, after which it should be securely fixed with clamps or clamping strips.
  5. If the roof structure contains elements of concrete or brick, then the film must be attached to them using special adhesives.

    The vapor barrier film is attached to brick or concrete elements using adhesives

  6. After completing the installation of the vapor barrier, a crate made of rails or metal profiles is nailed to the rafters on top of it. These elements are positioned perpendicular to the rafters in increments of 500 mm. The lathing is needed so that the wall cladding is protected from the vapor barrier at a certain distance. Under this condition, in the event of moisture condensation on the film, the casing will remain dry.

    On top of the film, a crate is nailed to the rafters

Video: installation of vapor barrier

So, the main conclusion: it is extremely important not only to understand the purpose of the vapor barrier in the roof structure, but also to understand what materials can be used in this capacity. Well versed in these issues, the builder is guaranteed to be able to build a warm and durable roof.

Watch the video: Training Session - Vapor-Permeable Air Barrier Membrane REDZONE STICK VP

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