I get asked "should you use a vapour barrier or a vapour control layer?" a lot, and truth be told there is a lot of confusion over what they are and what the differences between them are.
The biggest issue is that there are lots of different terms for products that stop or reduce the flow of water vapour through the various parts of the structure of your home. With lots of different terms for the same products, so it’s little wonder people get confused. So I’m going to trying explain what the difference is between a vapour barrier and a vapour control layer and I’ll try to make the explanation as simple as possible.
Firstly a vapour barrier is actually one form of vapour control layer and there are lots of different vapour control layers available.
As the name suggests a VCL controls the amount of water vapour that can pass through it. But people use the term VCL to describe everything from vapour barriers which prevent any vapour getting through (in theory) and airtightness layers etc. etc.
The biggest risk to your property is damp and one of the most common forms of damp today is condensation. Condensation forms when the water vapour within your home comes into contact with a cold surface. You’ll be well aware of seeing condensation on glass surfaces such as windows and bathroom mirrors but it also occurs on surfaces like walls, ceilings and even within the wall and ceiling structures.
When water vapour passes through the structure (and it will get through most materials) it travels from the warm side of the structure to the cold side and as this happens the temperature gradually drops. So for example, your internal room temperature may be 20 deg. C. and the temperature on the outside may be 6 deg.c. So as you go through the structure the temperature will gradually drop from 20 to 6. If (depending on the air pressure and relative humidity etc.) the dew point on that particular day is 11 deg.C. Then when the water vapour gets to the point within the structure that is at 11 deg.C, then that is where the condensation will form and this is called interstitial condensation.
This condensation can lead to:
You need to install a VCL on the warm side of the insulation (in the Uk this is the inside) and this will manage the movement of water vapour through the structure. Now this is where things have typically got confusing, as people throw about the term VCL when in truth there are lots of different types of VCL and each job will require a different type of VCL to perform a different task depending on the structure and the type of insulation used etc.
Some of the terms used under the banner of VCL include, but are not restricted to; vapour check, vapour permeable membrane, vapour retarder, vapour barrier, vapour diffusion retarder, variable diffusion membrane etc.
Typically we have relied on ventilation to take away water vapour, but air passing over the insulation has been shown to draw heat out of it and this reduces the efficiency of the insulation material and this is called “wind wash”.
You can certainly reduce it but you won’t be able to stop it. We all produce water vapour when we wash ourselves and our clothes, cook, breathe and dry clothes on radiators etc.. There is also always water vapour within the air. So we need to be able to reduce where possible and then manage the remaining water vapour inside our homes. We also need to control the moisture within the structure of our homes such as moisture from the construction process as the building dries out.
Traditionally we would have used a Polythene sheet on the warm side of the insulation in timber frame structures and the theory was that it would stop the water vapour getting to the insulation and/or timbers. But these need to be really well installed to prevent any tears, cut’s, nicks or scratches and the joints all need to be perfectly taped and this rarely happens rendering the vapour barrier ineffective. We also now know that water vapour can move from the outside towards the inside in the summer and the temperature graph in the table above is reversed. So when the water vapour is moving from the outside towards the inside it is trapped by the Polythene sheet and this traps the moisture within the wall where it can cause rot, damp and mould etc.
So should you use Polythene sheets as a VCL?
There are a few very specific uses that would require the use of a Polythene sheet but in most instances absolutely not.
The secret is to reduce the amount of water vapour that can get through the VCL but not stop it completely. That way a very small amount of water vapour can pass both ways through the VCL depending on the weather conditions as this will avoid trapping moisture within the structure. This is also the reason why I prefer to use natural insulation materials such as wood fibre as they easily absorb and release moisture without affecting their thermal performance. Whereas manmade insulants such as mineral wool or fibreglass wool do not readily absorb or release moisture.
That brings me on to the latest and I think best type of VCL which is an intelligent VCL. I’ve already discussed the fact that in the winter when the water vapour is diffusing towards the outside of the structure the holes in an intelligent vapour control layer are very small to let only a very small amount of vapour through. Then in the summer when the water vapour is diffusing towards the inside of the property, the structure of the intelligent VCL opens allowing a lot of vapour (up to 40 times more) through which allows the structure to stay dry.
So, in conclusion VCL’s should ALWAYS be used when you are installing insulation in your home, but not normally a vapour barrier. Otherwise you increase the risk of damp, mould, structural damage and a reduction in the efficiency of your insulation material.