How to Choose the Right Vapour Barrier

So, how do you select the right vapour barrier that suits your roof design, building’s environment and estimated moisture levels? There is a range of vapour barriers available in the market and opting for the right one ensures the roof performs properly over the span of its life. Making a poor choice can lead to hidden condensation issues that will be expensive to repair and resolve.

What Vapour Resistance Values Mean

Perfectly named “vapour barriers”, they do however differ in their ability to resist vapour and have different resistant values. These values indicate how resistant the material is to water vapour. The higher the value, the greater the material’s resistance against the rising vapours. Also worth noting, the lower the value, the more breathable the material is, meaning it allows more vapour to filter through.

How Much Vapour Resistance Does Your Roof Need? 

For many standard domestic applications, vapour control layers (VCLs) with resistance values in the hundreds of MNs/g are commonly used. However, the required resistance will depend on the roof build-up and expected internal moisture levels. Higher-humidity environments may require a VCL with greater vapour resistance. However, where internal humidity levels are greater, like bathrooms, kitchens and commercial settings, you will require a more resistant membrane that has been specifically engineered to handle the increased moisture load.

Types of Vapour Barrier for Flat Roofs 

It’s clear that choosing the right vapour barrier is important but on the same token, it is important how the vapour barrier fits within the roof build up. For a warm roof construction, the vapour control layer should be installed directly on top of the structural deck and beneath the insulation. The correct positioning of the vapour control layer guarantees that moisture is stopped early and prevents it filtering through to the other layers above.

Where the Vapour Barrier Sits in a Warm Roof 

In line with the moisture-management principles set out in BS 5250:2021, the VCL is generally positioned on the warm side of the insulation. The overall roof build-up should be designed so that moisture is controlled and does not become trapped within the construction.

Aside from the different values the vapour barriers offer, you also need to decide what type of vapour barrier you desire, or what is within your budget. You can choose from three main options:

• Reinforced polyethylene VCL - cost-effective standard option, loose laid + taped joints.

• Foil AVCL - when extra vapour resistance or airtightness is needed, loose laid + taped joints + higher vapour resistance.

• Self-adhesive bitumen AVCL - premium/commercial style build-up. Peel-and-stuck to a primed surface, bonded directly to deck, overlaps sealed by adhesion/pressure rolling.

For higher-performance applications, self-adhesive vapour barriers are often preferred because they provide a more robust, fully bonded installation. As it fully bonds to the deck, there is less chance of movement or gaps. Because of this factor, airtightness and vapour control are outstanding. Whereas the reinforced polyethylene vapour barrier like Damplas & foil-back vapour barrier like Tyvek AirGuard, both rely quite heavily on the quality of taping and detailing which is one reason many installers prefer self-adhesive.

Why Installation Matters as Much as Product Choice 

Installation experience and quality is just as important as product choice. The membrane must be continuous, with all laps properly sealed and all penetrations, such as pipes, rooflights, and upstands, carefully detailed. These areas are the most common points of failure, and even small gaps can allow significant amounts of moisture to pass through over time.

Getting Your Vapour Barrier Choice Right 

Browse our vapour barrier range to find the right option for your roof, or find an approved installer if you’d rather leave the detailing to a professional. Our team is also happy to advise on the best choice for your build-up.