This post aims to cover what a vapour control layer is, why it’s important and when it’s applicable to roof and floor applications.

Today, we are talking about another vital construction component called a vapour control layer, or a VCL.

At Saint-Gobain Insulation UK and within our bustling Technical Support Centre, our advisors are regularly asked questions relating to the VCL. What is it? What does it do? Am I using it the right way? With this in mind, our technical advisors have put together a VCL focussed FAQ post.

What is a vapour control layer (VCL)?

A vapour control layer helps you protect your building from the consequences of condensation. Condensation is formed when warm moist air penetrates the building fabric and then condenses into a liquid on contact with the colder surfaces outside the insulation layer. The idea behind a vapour control layer is to install it on the room (i.e. warm) side of the insulation so it blocks the passage of warm moist air entering the structure.

Why is a VCL important?

Saint-Gobain Insulation UK strongly recommend the use of a suitable VCL in certain constructions, as they are vital to minimising the risk of harmful condensation.

A VCL is a vitally important component in your build up acting as one of the protective layers that minimises the amount of warm moist air that enters the building fabric. A VCL in conjunction with the correct use of ventilation and membranes will effectively eliminate the risk of interstitial condensation. It’s the two design principles working together that minimise the damaging effects of condensation on the structure.

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.

Do I need a vapour control layer if I have a ventilated cavity in my pitched roof?

Yes, as the VCL works in conjunction with the ventilated cavity to minimise the risk of harmful condensation.

How should I use a VCL in a warm flat roof construction?

BS6229:2018 gives advice on vapour control layers in flat roofs and the control of condensation within the building generally.

What is the difference between a vapour control layer and a breather membrane?

Vapour control layers are typically positioned to the inside of the insulation in order to minimise the amount of warm moist air entering the building fabric from inside the building. Breather membranes are typically positioned to the outside of the insulation acting as a weather barrier while still allowing moisture to escape from the inside.

Do Celotex make vapour control layers?

Although Insulation UK don’t manufacture vapour control layer products, Celotex PL4000 features a vapour control layer built in. It is positioned between the plasterboard and Celotex foam insulation. When the boards are tightly butted together, the tapered edges of the plasterboard are sealed with scrim tape and jointing compound to form an effective vapour control layer with a high vapour resistance.

How do I maintain my vapour control layer when installing services in a wall with Celotex PL4000?

Services should be kept to a minimum. Where necessary, a continuous bead of adhesive should be applied around these services.

Do I need a VCL in solid ground floors?

In solid ground floor applications, such as concrete slab and beam and block, a VCL is required to the warm side of the insulation. This membrane also serves as a protective barrier for the insulation, avoiding liquid screed migration and damage to the facer from wet screed or concrete.

Hopefully this short FAQ post will give an idea of the what, why, and where a VCL is required.

Find out how Celotex PL4000 with its built-in vapour control layer could be suitable for your project. If you have any questions for our Technical Support Team, get in touch.