Module 10: Use of vapour control membranes
If you’re working on a building project and need help specifying your materials, checkout the following.
In simple terms, a vapour control layer is a layer on the inner face of the building which is used to reduce moisture flow into and through the walls, roof or floor. This is to prevent damaging levels of condensation occurring in the fabric of the building which in turn would cause decay in timber, corrosion in metals or frost damage in masonry.
Vapour control layers differ from vapour barriers in that they are designed to slow the flow of moisture and not block it completely, the reason for which we explain below. Vapour control layers are also airtight and generally used as an airtight layer to make buildings airtight.
Understanding your walls, temperatures and condensation
With a plethora of membranes on the market, each designed to do slightly different jobs and behave in a subtly different way it is easy to be confused about which membranes are required to create a dry and airtight building structure.
Firstly, starting with the basics, when you insulate 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 from the inside to the outside surface of the wall.
Secondly, when you take warm, moist air and cool it you find that the moisture in it condenses, usually on to the surfaces of the container it is within. If you pass this same air through the walls of your building the same condensation process occurs but instead of being in a container, it accumulates inside the fabric of your walls, roof or floors or your insulation.
How does moisture affect buildings?
Different construction and insulation materials cope differently with this condensation. Some materials, such as masonry, can absorb and release it again once the weather warms without too much damage. However, when using vapour impervious insulations in timber frame construction, any condensation forming in the walls tends to be absorbed by the timber, a process that 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 rather than vapour block
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 an impervious full vapour barrier was used on the inner face of a timber frame and was thought to prevent condensation formation by simply blocking the flow of moisture-laden air through the wall. However, it has since been found that not only are vapour barriers regularly full of holes which let moisture through during the winter months, they also cause the accumulation of moisture inside the wall during the summer months by preventing moisture escaping towards the interior of the building.
The high humidity levels and warm temperatures found in these walls combined to form perfect conditions for mould and rot to thrive. This was problematic to both the timber structure, as it rots, 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) or vapour retarder. 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 barriers are still useful though. One of the few places above ground level where a complete vapour barrier should be used is in flat roofing when using PIR insulation or when insulating swimming pool buildings. In this case, you need to lay a full vapour barrier, such as Ampatex Sisalex 518, on top of your flat roof deck before you lay the insulation and your flat roof covering or on the inside of the walls of your swimming pool building.
Vapour control membranes
Vapour control can be performed by the many membranes available but it can also be performed by sheet products such as Smartply ProPassiv or Unilin Durelis VapourBlock, whose vapour resistance (or vapour permeability) is similar to that of manufactured membranes. The benefit of using a board 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.
This image shows a typical installation for the Ampatex Sinco VCL membrane on the inside of a roof:-
The latest type of VCL membrane is the 'intelligent' membrane, such as the Ampatex Variano membrane or Proclima’s Intello membranes. These are very useful products that remain 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, caused by heat from the sun, 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 possible but is usually only required in situations such as the flat roof construction below.
Typical pitched roof or timber-framed walls do not require intelligent membranes and where green roofs or metal-clad roofs or walls are being constructed, these membranes should not be used.
The below drawing shows the typical location for the variable vapour control layer within an un-vented flat roof construction.
Vapour control layers are always required whenever you insulate, irrespective of the insulation used. They should be used to form a continuous airtight layer and so all the joints and any penetrations must always be sealed with the appropriate airtightness tapes. Without good levels of airtightness, the VCL does not work and moisture levels cannot be controlled inside the walls, roof or floors of the building.
Products such as vapour-check, foil backed or insulated plasterboard tend to act as a vapour barrier but with none of the joints or penetrations sealed. This allows moisture to flow into the walls or roof but because the main bulk of the surface is impermeable, it does not readily allow moisture back towards the interior of the building. Consequently, these products may cause moisture to accumulate in the building fabric and should not be used instead of a VCL or where a VCL is used.