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vapour control layer diagramA Vapour Control Layer (VCL) is a super-thin material, often made of thermoplastic polymer foil sheet, used throughout construction projects designed to stop water vapour from seeping into walls, ceilings, or floors when it's cold outside. It's typically placed between the plasterboard and framework, to keep buildings dry. VCL can take the form of a polythene membrane, plasterboard with foil backing, foil faced insulation or even OSB.

VCL’s guarantees proper drainage of moisture from the building and therefore it should always be used when you are installing insulation in your home. The most popular brands of VCL membrane are Novia, Visqueen, Proctor Group, Delta, Klober and DuPoint Tyvek. Most of these brands are for sale at well-known stores such as Screwfix, B&Q, Toolstation, Wickes or Travis Perkins.



The Vapour Control Layer (VCL) is usually placed behind or underneath insulation to control moisture levels. For instance, when insulating between rafters, the VCL goes over them and is left exposed or covered with plasterboard. In a suspended floor setup, the VCL goes between the insulation and the floorboards.
When insulating walls, like in a loft, the VCL goes between the insulation and the plasterboard.


Vapour is generated from daily activities like bathing, washing dishes, or cooking, spreading moisture throughout the house. In winter, indoor heat and moisture raise the indoor temperature and vapour pressure, causing vapour to move outward. Moisture from these sources moves through the structure due to humidity and temperature differences. While often harmless, it can lead to issues like rot or corrosion in sensitive materials such as insulation, affecting thermal insulation performance and potentially staining interior decoration. That's why it's necessary to use a vapour control layer to prevent these issues.
To determine if you really need a vapour control layer, the best way is to measure the humidity level in your home. Special weather stations can help with this. If your humidity level consistently exceeds 70%, it means there's excess moisture in the space, and that's when you need to use a VCL when installing insulation.
In summary, if your home has poor ventilation, you'll likely need a vapour control layer. However, if your house has efficient mechanical ventilation, there's no need for a VCL. In such cases, air exchange is mechanical and controlled by a controller installed in the house.
Existing properties might require a VCL if you are introducing internal stud walls and insulation, installing a new concrete floor with insulation, or modifying the roof space.



The vast majority of homes in the UK were built around 100 years ago. Back then, people and architects didn't prioritise insulation, let alone consider humidity levels in homes. These houses still serve us today, often being poorly insulated or not insulated at all. When attempting to insulate them, the question arises: do I need a VCL in the UK?
As we all know, the weather in the UK isn't always pleasant. We're a large island where most days of the year are rainy, and moisture is constantly generated by the surrounding water. Islands surrounded by seas and oceans are particularly prone to increased humidity levels, and the United Kingdom is no exception. The answer to whether you need a VCL in the UK is straightforward: yes, you do.



When picking a vapour control layer, you should consider both its thickness and SD value.
The SD value measures breathability, or how resistant the material is to water transmission, while thickness often indicates its strength during installation. A lower SD number means the VCL membrane disperses water vapour more quickly, and the thicker the vapour control layer, the stronger the foil during installation.

VCLs come in various thicknesses, ranging from simple single-layer polythene at 500 gauge (125 microns or 0.125mm thick) to reinforced ones at different thicknesses like 0.15mm, 0.29mm, 0.31mm, 0.37mm, 0.4mm, or 0.5mm. Rolls are available in different sizes, such as 4m x 25m, 1.5m x 50m, 1.6m x 50m, and 2.7m x 50m.

All vapour control materials should have an approval certificate or be specified by condensation risk analysis under EN13788. According to the National Construction Code, a vapour control layer should be at least 0.2mm thick, or 200 µm, to effectively create a moisture barrier.



Vapour control layers (VCLs) are necessary in specific new construction scenarios as per Building Regulations. Approved Document F outlines the minimum standards for ventilation in buildings. All VCLs must meet standards set by I.S.EN1931:200 and EN12572:2001, and should come with an approval certificate or be specified through condensation risk analysis under EN13788. A common type of VCL is 500 gauge polythene sheeting with a vapour resistance of 250MNs/g.
According to BSI Standards Publication ‘The selection of vapour control materials should be determined by:
a) the degree of vapour resistance required (determined by assessing the
likelihood of interstitial condensation using the method and guidance in
Annex D and the vapour resistance values of various materials given in
Annex E);
b) the practicability of installation, including the formation of sealed laps and
c) the anticipated life to renewal.
In order to form an effective VCL, side and end joints should be kept to a minimum, joints in flexible membranes should be formed over solid backing members or a rigid substrate, be lapped at least 50 mm and be sealed. Any damage should be repaired using matching material and jointing techniques. Unstabilized plastics-based sheeting should be protected from heat and sunlight to prevent degradation.’

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In ground floor setups like concrete slabs, it's crucial to have a VCL on the warm side of the insulation. This not only shields the insulation from moisture in the screed or concrete but also prevents concrete from seeping into any gaps between boards. When using PIR insulation such as Celotex on the floor, laying VCL foil is also essential to prevent the aluminium PIR face from reacting with the concrete.
Suspended timber floors require a different approach. There are debates regarding whether to introduce a VCL and its placement. Improper installation of the VCL can exacerbate condensation issues.
Ideally, the VCL on suspended timber floors should be placed on top of the joists before laying the boards, with insulation beneath it to keep it on the warm side. This arrangement prevents warm, moist air from penetrating structural elements.



When insulating a roof, whether it's flat or pitched, there's one fundamental rule to follow: you'll need to install a vapour control layer (VCL) on the warm side of the insulation and an airtightness layer on the cold side. This is crucial because the VCL limits the amount of water vapor entering the insulation.
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Both serve as physical barriers, possibly made of the same material, but they fulfil distinct roles in construction. While both are unidirectional, aiming to prevent moisture from entering the structure either from the outside or inside, their literal functions align with their names - a Damp Proof Membrane (DPM) halts dampness from the ground, whereas a vapour barrier obstructs vapour from internal air.

DPMs are typically positioned beneath floors to stop rising damp, whereas vapour control membranes hinder internal moisture from infiltrating walls and accumulating within insulation voids. Vapour control membranes are usually lighter and thinner than DPMs.

In cases where the DPM is situated on the cold side of the insulation, it's advisable to install a separate Vapour Control Layer (VCL) on the warm side of the thermal insulation. Given the minimal extra cost of additional polythene, this additional layer is certainly worth considering for enhanced protection.



Typically, VCL foils come at a reasonable price, often sold per square metre, with rolls commonly available in standard 50 metre lengths, offering 50 square metres of coverage. Some manufacturers offer wider options, like 1.5 metres, providing a total of 75 square metres.

If you need to purchase a VCL membrane, prices can vary, ranging from approximately £0.72 per square metre for basic options like Novia 500G Vapour Control Layer, to around £2.30 per square metre for specialised products such as Novia STRP Reinforced Vapour Control Layer. Generally, a standard polyethylene foil costs around £1-1.5 per square metre, while aluminium vapour foil may range from £3-5 per square metre on average.
Keep in mind that professional installation services for vapour control layers can add to the overall cost, typically around £5-6 per square metre in addition to material expenses.



When installing the VCL, avoid puncturing it. Any pipes, electrical fittings, etc. which pass through the vapour control layer should be kept to a minimum and taped and/or sealed.
The vapour check layer must be seamless, with careful attention given to sealing overlaps between strips. Inadequate finishing compromises the effectiveness of the vapour control layer. Therefore, for framed structures like timber or steel frames, the VCL should be placed on or near the warm side of the insulation, but only after the studwork's moisture content has dropped below 20%.

After securing the VCL with screws, the edges may require taping, but duct tape should never be substituted for proper vapour tape.
Vapour control layers are typically sealed using double-sided butyl tapes according to specific criteria:

  • Vertical seals require a minimum width of 150 mm.
  • Horizontal seals should overlap by at least 100 mm.
  • All laps should be taped with a minimum width of 50 mm.

Joints in vapour control membranes should align with studs or noggins. Special attention is needed around openings such as ceiling hatches and service penetrations like cables, ducts, sockets, and light fittings.
To ensure an effective seal, the vapour control layer should extend a minimum of 75 mm into door frames, window sills, and ceilings. Small tears can be patched, but significant damage warrants replacing the sheet and resealing the laps. It's advisable to inspect the vapour control layer post-installation.


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