BLOG: Don't get caught out by ignoring the Vapour Control Layer

By Alan Boyt: 

The Vapour Control Layer (VCL) - often overlooked, underrated, punctured adlib and sometimes even completely omitted, mainly because we’re just not sure if it really is all it’s cracked up to be…after all, even if we leave it out altogether the roof shouldn’t leak, right?! 
...Wrong! sadly, it’s often the way – out of sight, out of mind, but the humble VCL is actually a very important part of the roof build-up. Be it a cold or warm roof construction, the primary function of a VCL is to keep warm moist air from inside the building – inside the building. It’s the first line of defence against high vapour pressure air entering the roof structure and causing interstitial condensation. Because of this, the VCL always sits at the warm side of the insulation.

 

The VCL in a cold roof construction

In a cold roof build-up, the VCL normally ends up on the underside of the joists or rafters and is punctured many times when covered over by the internal finishes. Should the ceiling below also have downlighters, the VCL could be seriously damaged and therefore virtually useless. 
For this reason, it’s very important that the cold roof has the correct ventilation to ensure that any potential condensation that forms can dry out without causing damage to the roof structure or covering materials. It should be noted that some metal roofs are seriously affected by the presence of moisture to the underside without free air movement. This can lead to underside corrosion.

The VCL in a warm roof construction

The VCL is easier to maintain in a warm roof construction as it’s generally out the way of the dangers of plasterboard fixings and downlighters. Often the VCL in a warm roof is bonded to a timber substrate which helps the integrity, however in a warm roof the VCL has two important functions;
 
1. Much like the cold roof, it stops the transfer of warm moist air from inside the building into the warm roof layers.
 
2. Whilst carrying out its first function, it is also keeping the air from inside the warm roof layers – inside the warm roof. This is important – Why? Well, once a warm roof is completed it’s effectively a sealed capsule. When the sun shines, the air inside the capsule expands, which may cause the membrane to balloon. When the roof cools, the air contracts. If when the temperature rises the air expands and escapes out of a defective VCL, there will be a negative pressure caused within the layers when the roof cools. This will mean that air from the building is drawn back in through the defective VCL layer. This air will likely be warm and of a high moisture content. Once inside, it will become trapped, naturally migrating to the underside of the top waterproofing membrane layer where it will cause mould growth, reduce the effectiveness of the insulation and – if there is any timber in the warm roof layers – will cause this to rot this very quickly.
 
So, all in all the VCL is very important for the long-term effectiveness of any roof structure and should be given the necessary care and consideration that it deserves. 
 
To find out more about the importance of ventilation in lead roofing specifically why not get us in to run a CPD. Find out more