Ceiling vapor barrier in a cold attic

Vapor barrier is a practical solution to the problem of ceiling condensation in homes with a cold attic. The material prevents the penetration of moisture, protects the floors from the formation of condensation and further destruction, and also prevents the appearance of mold and mildew in the insulation plates.

Features of the design device

The cold attic includes a gable roof surface with roofing material and an insulated ceiling that separates the living space from the attic. To ensure ventilation, the attic is equipped with dormer windows, without which the air exchange in the attic will be disrupted, which will lead to the formation of condensation on the roof..

The floor of the attic is a ceiling, which simultaneously serves as the ceiling of the living room. With a decrease in outdoor temperatures, the overlap becomes susceptible to the appearance of condensation, the formation of which is due to the temperature difference between the lower and upper parts of it. In order for the condensate droplets that have appeared not to penetrate into the ceiling, it is covered from above with special materials that do not allow water to pass through..

In addition to protecting the base from moisture, the material performs an important thermal insulation function, preventing warm humid air from rising up. This technology significantly reduces the heat loss of the living space and allows you to significantly save on heating.. Vapor barrier should be performed on all types of substrates, including concrete and wooden floors.. Isover slabs, glass wool or bulk materials can be used as a heater..

Types of materials

Before the advent of modern polymers, high-fat clay was used to seal the ceiling in a cold attic. Its disadvantage was a rather large weight and labor costs during installation. Today the construction market offers a large selection of vapor barrier products that differ in the form of release, the method of installation, properties and cost..

Film vapor barrier

Film vapor barrier is the most popular and demanded type, which is represented by polyethylene and polypropylene films and membranes:

  • The most widespread is polyethylene. This inexpensive and practical material reliably prevents steam penetration, but has restrictions on its use. It is recommended to use this type of film only in warm climates, subject to moderate temperatures: under the influence of extreme influences, it quickly loses its operational properties and collapses. The disadvantages include the low strength of polyethylene, which can lead to material ruptures even at the laying stage. Glassine, often used as a vapor barrier, is very similar in its properties to polyethylene: it also retains moisture well, but at the same time does not allow air to pass through at all.

  • A more practical type of vapor barrier is polypropylene.. Such a film tolerates thermal surges well and is highly resistant to ultraviolet radiation. The service life of this material is several times higher than that of polyethylene. Modern technologies make it possible to produce films with the addition of viscose and cellulose. This significantly increases the strength and hygroscopicity of the material. A prerequisite for the use of such a vapor barrier should be the presence of good ventilation..

The water accumulated and held by the pores of the material must evaporate freely, otherwise the properties of the material will be violated, which will lead to moistening of the floor.

  • The most modern and practical type of vapor barrier material is membranes. The isolant is designed in such a way that the ability to pass steam is possible only in one direction. Thanks to this property, moisture is quickly removed, as well as air exchange is carried out between the ceiling and the attic room. In the domestic construction market, the Izospan model is considered the most famous, the fleecy structure of which is able to hold condensate droplets and quickly evaporate them. Installation of a membrane covering involves the mandatory formation of a ventilation space, designed so that liquids have a place to evaporate.

Foil materials

This type is intended for operation at high temperatures and is used in the construction of baths as a vapor barrier for wooden floor beams. An insulator is a film covered with a thin foil layer on one side. Thanks to this structure, the material is able to reflect thermal radiation and retain steam well.. There are several varieties of it:

  • The most budgetary option is foil kraft paper. The material fits well, but in the process of long-term use it is susceptible to the appearance of mold and mildew. The disadvantages include its low hygroscopic properties..

  • Lavsan coated kraft paper able to withstand temperatures up to 140 degrees. This allows it to be used as a vapor barrier material in the construction of baths. The disadvantages include low resistance to chemical compositions of detergents.
  • Foil fiberglass is considered the highest quality vapor barrier, and is characterized by increased strength and long service life. The downside is the high cost of the material.

Liquid products

Liquid means for ensuring the vapor barrier of the ceiling are represented by varnishes and mastics. The compositions are applied to the floor surface and, after complete drying, form a thin film capable of retaining moisture and allowing air to pass through. This contributes to good ventilation of the floors and significantly reduces the likelihood of mold and mildew..

Some liquid products are intended only for use in non-residential areas and may be harmful to health upon contact with them, therefore, personal protective measures should be observed when using them.


Compliance with simple installation rules and operating standards will help to significantly extend the service life of the vapor barrier.

  • Fasten the film material with staples or small nails, placing a thin wooden strip under the fasteners. This will better press the material against the substrate and reduce the chance of accidental damage to the film..
  • The size of the overlap during the installation of any materials should not be less than 15 cm.The joints should be glued using a wide tape, and when installing foil materials, foil tape should be used.

  • Laying of vapor barrier materials should only be carried out with thermal insulation..
  • In the process of laying, it is recommended to carefully control the tension of the material: the film should be in a free position. This will avoid tearing of the material when it is stretched and compressed under the influence of thermal changes..
  • To form the insulation of the ceiling of a residential building, it will be enough to use a membrane or reinforced polypropylene film, and during the construction of baths, only foil vapor barrier should be used.

Vapor barrier is one of the prerequisites for the construction of houses and baths with a cold attic.

Correctly selected material and competent installation will extend the service life of the ceiling, significantly reduce heat loss and make the stay in the room pleasant and comfortable.

For more information on vapor barrier ceilings, see below..

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