VAPOUR CONTROL LAYER

VCL stands for Vapour Control Layer and is designed to be installed on the warm side of the insulation to limit the transfer of moisture to the building fabric and to aid air tightness in roofs and walls. In fact, it controls the amount of water vapour that can pass through it and manage condensation risk. It means it allows only a small amount of water vapour through from either direction. That way a very small amount of water vapour can pass both ways through the VCL depending on the weather conditions as this will avoid trapping moisture within the structure. A VCL material is always used as close to the inner face of a wall as possible and reduces the amount of moisture passing through the layer to low levels, ensuring only insignificant amounts of condensation occurring within the structure. A VCL in conjunction with the correct use of ventilation and membranes will effectively eliminate the risk of interstitial condensation. An advantage of the VCL is that it will help prevent installed insulation getting damp and losing its thermal properties. A VCL can also help prevent the occurrence of damp and mould, which can also cause structural problems, weakening timber framed buildings and corroding other construction materials. Additionally, this 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. 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 structures. Some of the terms used under the banner of VCL include: vapour check, vapour permeable membrane, vapour retarder, vapour diffusion retarder and variable diffusion membrane. The thickness of VCLs ranges from a simple single layer polythene VCL at 500 gauge, which is 125 microns or 0.125mm thick. The latest type of VCL membrane is the 'intelligent' membrane that can vary their vapour resistance and adapt to changes in humidity conditions. When internal humidity levels are high, the membrane will be vapour closed. This prevents water vapour in the humid air passing through the membrane and condensing. When the humidity level in the building drops past a certain point, the membrane changes to a vapour open state, allowing the escape of any trapped water vapour. Even when the membrane is in a vapour open state, the membrane remains airtight. A quick online search for ‘Vapour Control Layer’ will turn up an extensive list of products, many with fancy technical names, however to make things a little simpler, we can put products into 6 main brands. The most popular brands of VCL membrane are Novia, Visqueen, Proctor Group, Delta, Klober and DuPoint Tyvek. All rolls come in multiple roll sizes such as 4m x 25m, 1.5m x 50m, 1.6m x 50m and 2.7m x 50m. 

VAPOUR CONTROL LAYER INSTALLATION

When fitting a VCL, you need to ensure that you don’t puncture the layer itself. You can use screws to fix your VCL to timber so long as they go through the joists and not the actual layer. Using staples to fix your VCL follows the same principles as using screws. After the VCL has been screwed, the edges may still need to be taped appropriately. Duct tape is not an appropriate substitute for vapour tape and should not be used when fixing a VCL. Vapour Control Layers are sealed using one of our double-sided butyl tapes, using a minimum overlap of 150mm.
 

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