Module 10: Use of vapour control membranes


With a plethora of membranes on the market, each designed to do a different job and behave in subtly different way it is easy to be confused about which membranes are required to create a building structure.

Starting with the basics, when insulating walls you create a temperature gradient across them with the warmest being on the inside and coolest on the outside during the winter months. You can imagine a graph of the temperature showing a fairly steady decrease in temperature as you move closer to the outer surface of the wall.

When you take warm moist air and cool it you find that the moisture inside it condenses, usually on to the surfaces of the container it is inside. However, if you pass this same air through the walls of your building the same condensation occurs but instead of being in a container, it accumulates inside the fabric of your walls or your insulation.

Different construction and insulation materials cope differently with this condensation. Some, such as masonry, can absorb it and release it again once the weather warms with little effect. Some impervious insulations focus the condensation on to structural elements and when that is timber this process can cause rot. Additionally, during the winter months when this condensation tends to occur driving rain may also enter the fabric of the building, further increasing moisture levels in walls. It is therefore very important to prevent this condensation process occurring in walls for the longevity of the building.

Vapour control

One further complication to the above processes can be found in the summer months. The temperature gradient is often reversed and the higher temperature is found on the outside of the wall and cooler inside. This creates a situation where condensation can form close to the inner face of the wall instead.

In the UK until relatively recently a vapour barrier was used on the inner face of a timber frame and was thought to prevent condensation formation by simply completely blocking the flow of moisture laden air through the wall. However, it has since been found that this causes the accumulation of moisture inside the wall during the summer months. The high humidity levels and warm temperatures combine to form perfect conditions for mould and rot to thrive. This is problematic to both the timber structure but also to the inhabitants of the building as mould spores are well known to cause respiratory problems and ill health.

The answer is to use a layer to restrict the flow of moisture but not to try and stop it, known as a vapour control layer (VCL). A VCL (ALWAYS used on the inner face of a wall) reduces the amount of moisture passing through the layer to low levels but does still allow moisture to pass, ensuring only insignificant amounts of condensation occur within the structure. Additionally, they will allow moisture that is driven towards the interior in the summer months to slowly pass back inside the building. This prevents the conditions for mould forming and ensures the longevity of the structure.

Vapour control membranes

Vapour control can be performed by the many membranes available but it can also be performed by sheet products such as OSB, whose vapour resistance (or vapour permeability) is similar to that of manufactured membranes. The benefit of using OSB as a VCL is that it is far more robust than a 0.2mm membrane, is easier to tape and does not require the installation of another layer into your timber frame structure if used internally.

The latest type of VCL membrane is the 'intelligent' membrane. This is a very useful product that remains very vapour tight during the winter months where it is important to try and prevent moisture from entering your structure from the interior. As temperature and humidity in the walls rises the pores in the membrane open and allow moisture to migrate towards the interior of the building. This gives the best of both worlds and ensures your structures remain as dry as it is possible to be.

Vapour control layers are always required whenever you insulate, irrespective of the insulation used. They should be used to form an airtight layer and so all the joints and any penetrations must always be sealed with the appropriate tape. Without good levels of air tightness the VCL does not work and the above issues arise. Products such as vapour-check or insulated plasterboard act as a vapour barrier and so should not be used instead of a VCL or where a VCL is used.

The below drawing shows the typical location for the vapour control layer within a roof construction.