What is the purpose of a Vapour Control Layer (VCL)?
When condensation occurs within a property’s structure, it may cause significant issues that are costly to rectify. VCLs play a key role in limiting the passage of moisture-rich air into the building fabric, safeguarding the structure.
What are VCLs?
VCLs are membranes that may be fitted separately or form part of composite insulation boards. They are installed on the warm side of any insulation measures which are fitted inside a building and are designed to control the amount of moist air which is able to pass through into the building fabric.
Why are VCLs needed?
When you fit insulation on the inner face of construction, whether it be a wall, floor or roof, it means less heat will be able to reach the outer fabric. As a result, the structure will remain cold. Warm, moisture-rich air from inside the building which is allowed to penetrate the building fabric may cool and condense inside the construction. This is known as interstitial condensation. If this is not addressed, it can cause damp issues and significant damage to the building structure. For this reason, VCLs must be fitted on the warm side of the insulation and properly sealed at all joints with tape.
VCLs can also be used to protect the insulation from wet screeds in a concrete floor build-up. Screed is a thin layer of material laid over concrete. Like concrete, it is usually made of cement, and its purpose is to provide a smooth, flat surface – levelling out the floor. When too much water gets into the screed, there will be an increased shrinkage and risk of cracking. Additionally, unless it is protected by a VCL, this could damage the floor insulation.
When is a VCL needed?
For new builds – VCLs are a requirement for certain new builds where insulation is being installed on the inner face of a construction.
For existing builds – certain installations and modifications to existing properties will require a VCL. If you are introducing internal stud walls and insulation, installing a new concrete floor with insulation or in any way modifying the roof space, a VCL might be required. This ensures current housing stock is kept in a liveable condition for longer and falls in line with the bid to make buildings more carbon neutral.
In any property, areas of high humidity (such as kitchens or bathrooms would benefit from a VCL).
What is important to consider when installing a VCL?
When making plans to install insulation and a VCL, it is important to address thermal bridging. Junctions around windows and doors and around joists or rafters need to be correctly insulated, otherwise warm, moist air will escape through the gaps and the VCL and insulation will be rendered ineffective, increasing the risk of surface condensation and mould growth.
For this reason, VCLs should be cut and fit accurately, with no gaps or variations, consistently across the whole wall, roof or floor. The care and attention that goes into fitting these components will help with the long term prevention of damp and energy loss, and generally, improve the health and safety of occupants.
It is also essential to correctly seal around any penetrations made in the VCL to ensure these do not act as a route for moisture to pass into the construction.
What are the options for a VCL?
There are four common options that can help control vapour:
- Polythene – 300-500 gauge polythene should be fitted to the warm inner side of the insulation. This option is mostly used on timber or steel-framed walls where foil-backed insulated plasterboard is not used
- Foil-backed plasterboard – utilises a thin foil backing to increase vapour resistance. However, it is not an effective a solution to damp. This option should be used in conjunction with other vapour control methods
- Foil-faced insulation – foil-faced insulations may be used as a VCL provided all joints are properly taped with a suitable foil tape. Standard duct tape will not do the job
- Composite products – Composite products such as Kooltherm K18 Insulated Plasterboard (pictured below) or Kooltherm K17 Insulated Plasterboard have integral moisture vapour control. When installed correctly, these can provide effective insulation and vapour control in a single layer - speeding up installations.
