How to reduce condensation in your building

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

Don’t know? I’m not surprised, but I am still surprised at how many building designers, guys on site and building contractors don’t understand the importance and difference of these two very different components. 

A vapour control layer stops internal moisture laden air [vapour] entering the building fabric and cooling down enough to create condensation inside the building fabric. This is called interstitial condensation and can create water droplets that drip back into the building and create havoc with damp spots, mould, black spot, poor internal air quality etc. 

A breather membrane stops external water [like rain or snow] entering the building but also lets internal moisture that may build up inside the building fabric be released into the atmosphere. Almost like Goretex for a building. 

Usually used on framed buildings, such as timber framed or metal framed. There are also different types of vapour control layer that are ‘intelligent’ and release more vapour in the summertime as it dries out quicker and less vapour in the winter time as it won’t dry out as much. Usually formed from sheets and rolls these need to be sealed and bonded together to create a homogenous sealed layer as part of a buildings air tightness strategy. You can also get spray on products as well, which are good for tricky junctions. 

There is a specific British Standard to refer to and also specialist condensation caclulations that should be done for any project, often using specialist building physics software. 

Don’t get caught out, make sure these are designed, installed and function properly. Some examples below of how I try to show this on our drawings with colour coded lines. However, despite showing on the drawings we always need to ensure this is communicated several times on site as well so it’s not missed off. 

Credit – 1st image obtained from Vulcan Cladding Systems website.