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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.

  • *Note: Since the creation of this post, Celotex GS5000 has been replaced with Celotex GD5000.

So a little while ago we featured an acronym blog, revealing the ins and outs of our industry acronyms. This time, we are talking about another vital construction component called a vapour control layer or a VCL.

At Celotex and within our bustling technical department, 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 featured a VCL Technical Twitter Takeover, which is a monthly opportunity for Tweeters to ask our Celotex Technical Centre a question or query using the hashtag #AskCelotex, hosted a VCL special.

This seems like a good time to crack on with a few questions.

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 rises and condenses into a liquid on contact with the colder surfaces above the insulation. The idea behind a vapour control layer is to install it on the room side of the insulation so it blocks the passage of warm moist air entering the structure.

Why is a VCL important?

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 a construction element. 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.

Think of it like this; a jumper will keep you lovely and warm but a jumper AND a waterproof coat will give you a greater chance of staying warm and dry in a storm.

A VCL prevents damp and mould which in turn, prevents major respiratory problems, according to the NHS. As well as significant health implications, condensation can also cause structural problems, weakening timber framed buildings and corroding other materials. Another significant advantage of the VCL is that it will prevent installed insulation getting damp and losing it’s thermal properties.

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

Yes you do. The VCL works in conjunction with the ventilated cavity to minimise the risk of harmful condensation.

Should I use in a warm flat roof construction?

BS6229:2003 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 positioned to the inside of the insulation in order to minimise the amount of warm moist air entering the construction. Breather membranes are positioned to the outside of the insulation acting as a weather barrier while still allowing moisture particles to escape from the inside.

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

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

Do Celotex make vapour control layers?

What is Vapour Control Layer? (VCL) Celotex Blog

What is Vapour Control Layer? (VCL) Celotex Blog

Although Celotex does not manufacture vapour control layer products, our premium performing plasterboard laminates Celotex GD5000 and GS5000 both have an integral 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. Celotex strongly recommend the use of a suitable VCL as they are vital to minimising the risk of harmful condensation.

Do I need a VCL in ground floors? #AskCelotex

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

Hopefully this short blog gives you an idea of the what, why and where a VCL is required.

If you have any questions for our technical team and you are on Twitter, send it over to #AskCelotex and we will pick it up. You can also catch us on other social media platforms including Facebook and LinkedIn.

Or find out how Celotex GD5000 with its build-in vapour control layer could be suitable for your project.