What is vapor barrier for: analyzing the causes of steam formation and methods of protection against it

To preserve the integrity and lifespan of your roof, it is essential to comprehend the function of a vapor barrier in roofing. The purpose of a vapor barrier is to keep moisture out of the roofing structure, where it can cause mold growth and condensation that can be harmful. We can understand the importance of a vapor barrier by looking at the factors that lead to the formation of steam.

When warm, humid air inside a structure comes into contact with a cold surface—like the underside of a roof in the winter—condensation results. Water droplets may form as a result of this interaction and eventually seep into roofing materials, reducing their effectiveness. As a shield, a vapor barrier lessens the amount of moisture-laden air that enters the roofing system, hence decreasing the likelihood of condensation.

Choosing the right kind and positioning of vapor barriers is one way to prevent the formation of steam. To effectively manage moisture levels within the roofing structure, different materials and configurations might be needed, depending on the climate and building design. To guarantee that the vapor barrier performs at its best and offers long-term protection against condensation, proper installation is also essential.

The role of steam and the mechanism of its formation

Extensive steaming has a direct bearing on the details of the microclimate formation inside the buildings operating in our latitudes. The climate makes it necessary to keep indoor temperatures higher than outdoor ones. In our region, the heating season lasts for a portion of the year during which an increase in the thermostat is not necessary in homes.

A rise in the absolute humidity level is observed in addition to temperature indicators. This occurs as a result of warm air’s greater capacity to retain water vapor than does cold air. There may be less moisture in the air the lower the temperature.

Reasonable claims made by physicists state that a cubic meter of air with a temperature of +20 °C and 100% absolute humidity contains approximately 17.3 g of water in the form of steam. If the street thermometer, for instance, fixes T ° = -10 ° C, a similar 100% humidity is noted at the same moment, and the relative humidity is only 2.3 g.

In actuality, cold air has a far higher density than the same indicator at a higher temperature. It is evident that in the process of cooling the air mass, she must give up more steam than she can hold. Condensate, which is this water released, settles on building structures as it cools.

We are all familiar with the phenomenon of excess water being expelled from the cooling air mass. Let’s remember the fog that appears in the early morning hours during the sweltering summer months, following a cool night. It’s true that damp air doesn’t harm building materials and systems in the same way.

The majority of construction materials are unable to tolerate the effects of condensation on their surfaces:

  • A fungus is started on damp wood, which leads to the unsuitability of the details of the supporting structures.
  • The foci of rust arise on metal elements, even if they had inconspicuous microscopic scratches.
  • Raw heater loses insulation qualities, which is why heat is not held in the rooms, cold and unpleasant musty smell are felt.

Household fumes are a common source of condensation, which is caused by the difference in temperature between the inside and outside of a building. It also affects the materials and construction systems. Plants, animals, and owners set them apart during the breathing process. When you cook, wash, clean, and practice good hygiene, steam is produced.

The evaporation that is allotted during the critical evaporation activity rushes to the areas where it is less saturated. The steam is continuously moving through the air until there is no longer enough, as indicated by the thermometer readings below. This clarifies his wish to escape through the ventilation and enclosing systems.

We refer to the flowing process as diffolding. Evaporations are mostly different through the construction systems, not the air itself, which is more easily lost through gaps in windows that fit tightly to boxes, ventilation devices, open windows, etc.

Because warm air and the moisture it contains always move upward, the majority of the fumes escape through the ceiling, roofing structures, and upper portions of the walls. Since these are the parts of the building that are most exposed to moisture, they must be equipped with vapor barriers.

On the "All about the roof" website, the main focus of the article "What is vapor barrier for: analyzing the causes of steam formation and methods of protection against it" will be to explore the role of a vapor barrier in preventing condensation inside buildings. Condensation happens when warm, moist air inside meets cold surfaces like roofs, leading to moisture buildup that can damage insulation and structures. A vapor barrier acts as a protective layer, stopping moisture from penetrating into the roof assembly and causing problems like mold or rot. This article will delve into why vapor barriers are crucial, how they work, and methods to ensure effective installation to maintain a dry and healthy building environment.

Nuances of vapor barrier protection device

The steam creates a vapor barrier barrier to shield structures from damaging effects. It is referred to as either completely sealed to prevent steam from escaping through building systems or reduced to the least amount that this barrier could possibly surmount.

You must understand the nature of the vapor barrier and how it operates in order to handle the device of the designated protective system. Actually, this is a roll of water-repellent material that keeps thermal insulation and building systems from sinking into its thickness and landing on damp surfaces.


The first thing that is put in the way of the fume movement is the vapor barrier film. T.e. Initially, the two must encounter a designated barrier that stops the majority of the vapor-shaped moisture from penetrating. Although complete isolation should prevent evaporation, there are currently no perfect circumstances for safeguarding roofing systems.

It follows that some moisture will presumably still find its way through the insulation’s thickness. All of this should be visible through the ventilation system’s components since it was able to permeate even the tiniest groats, microcracks, and segments of the loose panel compound into a continuous insulating carpet. There is nothing left in the body of the system with a capable device for the roofing cake of water in any condition.

If you concentrate on the heated room, the first barrier against the effects of the steam is established:

  • When arranging the attic room, vapor barrier is attached from the inside of the rafter system, and the insulation is installed according to slopes or between rafters.
  • When arranging a house with an attic roof, vapor barrier is placed first after the ceiling sheathing. It is laid with a continuous carpet on the beams of wooden ceilings or on concrete slabs.

The vapor barrier is affixed to the black ceiling surface when performing maintenance without changing the components of the attic flooring. These days, materials with a self-adhesive basis are produced, making repairs and greatly enhancing the insulating qualities of buildings possible without any issues.

Accounting for the ability to pass steam

Vapor permeability is a crucial property of insulating materials that is considered when installing a roof pie. The capacity to conduct vapors through oneself at a volume determined by the technical characteristics is known as this. Its values range from 0 to 3000 and are expressed in mg/m² per day.

This indicates that, for a full day, the amount of vapor-like water specified in the material’s technical documentation will be able to pass through one square meter of vapor barrier material.

The materials are arranged in a specific order to prevent moisture from being trapped in a roofing membrane or the attic ceiling insulation system. It is predicated on his capacity to release steam and allow in his thickness:

  • The first on the side of the room is installed with the lowest vapor permeability.
  • The second layer is thermal insulation, with higher than the previous layer with steam permissive capabilities.
  • The third layer is waterproofing, characterized by the highest vapor permeability compared to layers installed in front of it.

The definition of simplified process mechanics is as follows: After vapor barrier protection, evaporated water falls into the insulation’s thickness, which partially separated from the first layer of water in the form of steam. The next step eliminates the steam even more than insulation does: waterproofing.

In addition to being placed on the enclosing structures and supporting walls, vapor barrier barriers are also placed between rooms that have different operating conditions. For instance, above the kitchen, bathroom, and inner pool ceilings, if they are situated beneath the residential floor or insulated attic.

Take note of the ventilation gap that is placed between the roofing and the waterproofing to facilitate the conversion of water from underneath the roof into steam. The only space left is between the water-repellent carpet and the roof if a polymer membrane is utilized in its construction. She allows moisture to freely exit the thermal insulation array.

When using a polyethylene or polypropylene film for waterproofing, two levels of undercut ventilation are constructed. The first one is placed between the waterproofing and coating, and the second one is placed between the coating and insulation. The truth is that since direct contact with the insulation is prohibited, regular polyethylene cannot pass moisture.

These days, though, these films are made with holes in them so that water can evaporate from the thermal insulation and prevent water droplets from penetrating the film. Using this option lowers the overall cost and makes the roofing system easier to install.

Materials for a vapor barrier barrier

A prime owner requires knowledge not only about the skillful installation of insulation systems but also about the kinds of vapor barriers that are appropriate for the design of the attic roof and layout of the cold attic. It has previously been determined that the material with the lowest capacity of the pair will be needed to protect thermal insulation.

This implies that the film’s vapor permeability should be estimated in increments of several hundredths of a unit to several dozen. There is a daily maximum of one hundred mg/m² that is acceptable. The greater the susceptibility to evaporation, the greater the responsibility in relation to ventilation system construction: to produce formation, aerator installation, and ventilation window device.

One method of applying a vapor barrier is by using persamin. The daily range of its vapor permeability is 70–95 mg/m². Although plastic structures have not been used in the building of homes, the material performed admirably in protective roles.

Once polymer windows, doors, and décor started to be actively used in home building, the materials’ vapor barrier qualities needed to be strengthened. They currently employ this as a barrier for vapors:

  • Polyethylene and polypropylene films. Reinforced options with increased strength and resistance to ultraviolet exposure. Their good plus lies in an affordable price.
  • Foil polymer membranes. Vapor barrier materials that have a foil coating on the one hand. In addition to protection against steam, vapor barrier with foil prevents heat leaks, it is extremely in demand when the saunas and Russian steamers are arranged.
  • Anticandenate vapor barrier membranes. Materials with smooth and rough sides. The rough surface is unfolded towards the stream of steam in order to eliminate the formation of dew, smooth prevents the possible opposite to sew condensate from thermal insulation.

Membranes against candida are ubiquitous. They have a unique structure that allows them to function as waterproofing and steam. It is crucial to bear in mind that the values of vapor permeability must be considered when selecting polymeric materials for the roof’s arrangement. The capacity of the waterproofing shell to conduct steam ought to be greater.

An anti-condensate membrane can be used as a hydraulic tank in roof slope arrangements with unexplained attics. The vapor barrier layer is positioned on the ceiling in these schemes, and there may be little to no variation in the vapor permeability parameters.

Even now, vapor barrier devices installed on the floor of unheated attics beneath backfill insulation use the morally repugnant chemical permamine. Polypropylene and polyethylene films will function similarly enough. It is thought that there won’t be any mechanical influences on the designated layer, so reinforced varieties are not required for this.

In the event that construction budget is constrained, polyethylene films—better yet, polypropylene species—are installed as a vapor barrier on attic roofs. They are overlapped, joined by glue and tape, and fastened to the rafters using rails or staplers.

It is not possible to claim that polymer membrane materials cost significantly more than polyethylene. It is best to spend the money on these specialized vapor barrier brands rather than trying to save money. One- or two-way tape is used to connect them. A proven benefit of membranes is their enhanced strength and their near-identical lifespan to roofing coatings.

Maintaining the longevity and effectiveness of your roof requires that you comprehend the role that a vapor barrier plays in protecting it. A vapor barrier keeps moisture from entering your roof structure, reducing the likelihood of condensation and the development of mold.

Warm air inside a building condenses when it comes into contact with a cold surface, like the wintertime underside of a roof. Water vapor is converted to liquid as a result of this interaction, which over time may erode insulation and other roofing materials.

Selecting the appropriate materials and making sure the installation procedures are followed are essential to a successful vapor barrier installation. Water vapor can’t enter the roof assembly as much thanks to barriers like polyethylene sheets or specialty membranes.

In order to maximize the effectiveness of the vapor barrier, proper ventilation is also necessary. Sufficient ventilation facilitates the escape of moisture from the building, thereby mitigating the probability of condensation accumulation and consequent harm to the roof structure.

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Gleb Zuev

Exterior designer, author of books about roofing materials. I will help you make your roof not only reliable, but also beautiful.

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