How to Select the Best Breathable Roof Membrane
In this article we are going to outline why you want to use breather membrane in your build, and how to select the best one for purpose.
What is a breather membrane?
A breather membrane helps the building to breathe and release moisture trapped inside, while protecting it from the elements.
Moisture and air passes through into your structure, this needs an escape route via evaporation. While allowing this to happen, we also need to ensure that moisture coming from outside can’t get in.
A breather membrane is a special fabric which allows this process to take place. This material is micro-porous and allows water vapour to escape, but stops larger water droplets from getting in. The material is usually made from polypropylene, the fibres are matted together closely, it works similar to a fabric like Goretex (you may have one of these jackets). You’ll notice if you have one of these jackets that water simply falls off it, a breather membrane works the same.
Breather membrane is applied to the outside sheathing and taped to provide a waterproof but breathable membrane.
There are three factors you want to consider when choosing your breathable roof membrane;
- How waterproof is it? You need your membrane to be completely waterproof. Look out for BBA (British Board Agreement) certification.
- Resistance to water vapour. A low resistant membrane will have a resistance of 0.25MNs/g or less. The lower the number the better the membrane will perform in this area.
- How quickly will water vapour be released? The higher the value the better for this one, you want water vapour to exit the building in as large a quantity as possible over every 24hr period.
As you can see from above, breathable membranes are tested against three main factors, and these factors will help you to select the right membrane for your product. For example, if your membrane’s main job will be to keep water out as it’s in a damp area, this will be the most important factor and you should choose your membrane based on this. However an unventilated roof, where water vapour must be released rapidly will call for membrane that is strong in the water vapour permeability category.
If you’re not sure, remember we can always advise you on the best product for the job.
Breathable membrane is generally priced by the square meter, and comes in rolls of 50m (giving you 50 square metres per roll). You should use a roofing calculator to work out the quantity of membrane needed. Price of membrane varies significantly, so choosing the right quality and performance is essential.