Moisture and Vapour barriers

 

What is a vapour control layer? 

A vapour control layer is a vapour tight membrane that is installed on the warm side of the roof or on the exterior house wall. If a vapour control layer is installed in a wall, it must be a light construction, meaning that the inside wall of the house is made from wood, gypsum or similar lightweight building materials for wall. 

The function of a  vapour control layer is to avoid that the moisture which is build up inside a house, will find its way to the insulation material, which would then absorb the moisture, making the insulation material lose its ability to insulate.

Why use a vapour control layer?

Not many are aware of why the vapour control layer (VCL) is so essential, but consider what happens when warm and cold air meets, and what this will mean for the building physics in winter and summer climates. When warm air meets a cold surface condensation will occur and consequently water/moisture will start forming.

The following factors may lead to problems with moisture/vapour in a construction:

- Vapour from an indoor climate

- Building moisture in construction materials

- Ground moisture

The main reason for problems with moisture in buildings is vapour that comes from the indoor climate, from the habitat area. To further understand the importance of having a VCL we can look at the indoor air. The indoor air needs to absorb the moisture that builds up in a standard house with two adults and two children living under normal conditions which can be as much as 10 litres of water per 24 hours. All this water vapour naturally needs to go somewhere, and since it cannot all be ventilated out by opening a window, a need for a VCL is obvious. Without the VCL this water vapour would go into the insulation material as well as the wooden construction.