What is the difference between a vapour-tight and an airtight layer?Why should a vapor barrier be installed with a timber frame wall, but not with a cavity wall?And why are the vapor barrier requirements stricter for a flat roof than for a pitched roof?You will discover it in this article published in WTCB-Contact n° 80.Formation of condensation There is always a certain amount of moisture in the air.Cold air can store much less moisture than warm air.Therefore, condensation easily forms on cold surfaces, such as single-pane glass or thermal bridges.Condensation can also form in the construction.In that case we speak of internal condensation.This moisture accumulation usually does not come to light or only shows up very late, but in some cases it can have serious consequences, such as significant heat losses, mold or even rotting of wooden elements in the wall.Therefore, the necessary precautions must be taken during design and construction to avoid this type of condensation.Causes of internal condensation There are two possible causes of internal condensation: air convection or water vapor diffusion.With air convection, the air moves through the wall, after which the indoor air (which is often warmer and more humid than the outdoor air) condenses on a cold surface.This can occur if the wall itself is insufficiently airtight (eg with an unplastered masonry wall) or if there is a defect in the air screen (eg a membrane perforation).Major moisture problems can arise through these air leaks and a lot of heat can be lost.It is therefore recommended to provide the building parts with an air screen on the inside, for example via plaster or membrane.The continuity of this air screen is critical.Any mistake can cause problems.Consequently, the number of holes in the air screen (eg through a pipe cavity in a timber frame construction) must be limited, the necessary holes must be correctly finished and a good connection must be provided between the air screen and the other building parts.Would you like more information about the airtightness of buildings?Then be sure to read TV 255. An air screen is not always vapour-tight.This is the case, for example, for plastering or a vapor-permeable roof underlayment (conversely, a vapor barrier is always airtight).This means that moisture can still migrate through a perfectly airtight wall via vapor diffusion (ie the transport of water vapor molecules through the wall).Unlike serious air leaks that can quickly introduce large amounts of moisture into the structure, vapor diffusion is a slow process where smaller amounts of moisture enter the structure.Nevertheless, this form of moisture transport also requires the necessary attention because in some cases serious moisture accumulation and damage can occur over time.To avoid condensation via vapor diffusion, a vapor barrier must be installed on certain walls, while no measures are required for other walls.This depends on: • the wall structure.Ideally, the vapor diffusion resistance (µd value) of the different layers decreases from the inside out.Vapor barrier layers should therefore preferably be on the inside • the moisture sensitivity of the materials.Any form of condensation must be avoided with moisture-sensitive materials (eg plaster or wood).Other materials (eg masonry) can collect a small amount of condensate • the humidity of the interior.In a damp room, more moisture can migrate into the wall.Some examples With a timber frame wall (see figure 1 on the previous page), a vapor-permeable rain screen is usually placed on the outside.However, because very vapour-permeable insulation materials are often used (such as mineral wool), there is a risk of condensation between the insulation and the rainscreen without additional measures.Since there are wooden elements in the wall construction, the water vapor must be stopped on the inside.If the rain screen is vapour-permeable, a light vapor barrier material, such as an OSB panel or a vapor barrier of class E1 (µd value > 2 m), is generally sufficient.This vapor barrier can also serve as an air barrier.More information about this wall type can be found in WTCB-Contact 2013/1, which is devoted to timber construction.The second example is a warm flat roof with a wooden support structure insulated with mineral wool (see image 2).Also with this construction, the water vapor on the inside will have to be stopped.The vapor barrier will then have to have a greater vapor diffusion resistance than that in the first example, since the roof sealing on the outside is vapor barrier (see TV 280 and WTCB-Dossier 2019/2.3).With this wall type, the vapor barrier ensures the airtightness of the wall.With a traditional cavity wall (see figure 3), no vapor barrier is installed.If the cavity insulation is made of mineral wool, in some cases a small amount of condensation may form, but this will not cause any damage.After all, in that case the moisture is absorbed by the outer cavity leaf or discharged to the outside of the moisture-resistant cavity insulation.With this wall, the airtightness is in most cases guaranteed by an interior plaster.Is a vapor barrier necessary for my wall construction?There are various methods to check this and to determine the minimum µd value of the vapor barrier.For standard situations, the design rules can often be found in the BBRI publications.In the case of more complex situations, numerical simulations can offer a solution.Source: WTCB-Contact 2022-2, p.20-21.Would you like to be immediately informed of the latest architectural news?The Architectura newsletter offers a fascinating mix of current events, project descriptions, opinion pieces, product innovations, info about events, but videos, podcasts and so much more.Sign up now and receive our weekly newsletters in your mailbox.Would you like to be immediately informed of the latest architectural news?The Architectura newsletter offers a fascinating mix of current events, project descriptions, opinion pieces, product innovations, info about events, but videos, podcasts and so much more.Sign up now and receive our weekly newsletters in your mailbox.Lazarijstraat 168 3500 Hasselt info@architectura.be