What’s the difference between a breather membrane & a vapour control layer?
Yes, that’s right – they’re different things!
We often notice people using the terms ‘breather membrane’ and ‘vapour control layer’ interchangeably, particularly with regards to pitched roofs. Whilst they have a similar purpose, there are a couple of important differences between the two.
Firstly, why do we need either? Quite simply – to prevent condensation, which can cause a number of problems, including:
- Structural damage due to rotting timber, whether this be a timber frame, joists or rafters
- Insulation losing its thermal performance due to having absorbed the moisture
- Mould, which not only looks unsightly but can also lead to respiratory problems
To prevent condensation, a method by which to release water vapour from the inside of a building is needed. Traditionally this has been done through ventilation; for example, by ventilating the space between the insulation and the slate or tile on a pitched roof. However, studies have shown that ventilation directly above an insulation layer can reduce its thermal efficiency, which means more and more people are opting for an unventilated roof.
This is where a breather, or breathable, membrane, such as Kingspan nilvent, comes in.
Breather membranes are installed to the outer side of the insulation – for example, either over or under the counter-battens on a pitched roof – and allow water vapour to escape from inside a building without the need for ventilation. They also repel any water, most commonly rain, that tries to enter the building. To ensure maximum efficiency, all joints in the membrane should be properly sealed with tape so as to prevent accidental air-leakage.
So, what about any water vapour that tries to come into a building – can this be prevented?
The job of a vapour control layer, or VCL, is to minimise the amount moist outside air that enters a building. This is done by installing it on the inner side of the insulation – for example, in a pitched roof this would sit beneath the layer of insulation that has been installed between the rafters. It is essential that the VCL is continuous and sealed at all laps in order for it to perform correctly.
While often also a membrane – in the form of a polythene sheeting – VCLs can come in a number of other forms. For example, the Kingspan Kooltherm K18 Insulated Plasterboard contains an integral VCL, which allows for an increased thermal efficiency – helping you reach those U-values! – as well as minimising the amount of outside air coming in.
To summarise, there are two important differences between a breather membrane and a vapour control layer:
- A breather membrane is positioned on the outer side of the insulation, allowing vapour to escape from inside while repelling any water that tries to enter
- A vapour control layer, positioned on the inner side of the insulation, prevents water – in any form – from entering at all
However, both products – so long as they are properly installed and sealed – help to minimise the risk of condensation and, in turn, any damage to your home.
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