What’s the difference between a breather membrane & a vapour control layer?

Breather membrane v 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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Kingspan Insulation is a market leading manufacturer of optimum, premium and high performance rigid insulation products and insulated systems for building fabric and building services applications.

0 comments on “What’s the difference between a breather membrane & a vapour control layer?
  1. I want to insulate the external brick wall (without a cavity) of the front/rear of terraced property to improve the EPC rating.
    1. Do I use a vapour membrane or breathable membrane? 2. Do I have to use battens to provide air space between barrier and insulation board, or can I fix boards directly onto vapour/breathable membrane? Can boards be fitted BETWEEN batens, directly onto membrane?
    Many thanks for your helpful suggestions. Ivor Thomas

    • Hi Ivor

      1) When using Kingspan Kooltherm K118 Insulated Plasterboard to dry line additional membranes are not required as the insulated plasterboard has an integral vapour control layer. However we do recommend using a foil tape to tape the joints of the plasterboard and create a vapour seal.

      2) On a single skin of masonry, we do recommend that you use battens to provide a cavity between the face of the insulation and the external wall. This is to protect the insulation from any driving rain moisture that may penetrate the masonry. Strips of damp proof course should be used behind the battens only, to protect the timber from any moisture. We do not recommend running a vapour control layer or a damp proof course across the entire face of the masonry as this can cause water vapour to condense internally and become trapped in the cavity betweem the insulation and the wall.

      Hope this helps.

  2. I have looked at many websites and advice sites however I cant get a straight answer. All sites have different solutions. From increase ventilation, use breather, buy dehumidifiers, don’t put insulation in the eves, leave your loft hatch open et.al. My issue is the damp is under the loft insulation and is still a problem after doing all these things. (no its not a leak) any ideas???????

    • Hi Graeme

      Could you please let us know the full build-up of the roof from internal to external finishes? We will be able to advise a little better then. Without this information we would struggle to come up with an answer as it is dependent on what membranes have or haven’t been used, and whether or not there is ventilation already in the roof.

  3. Dear sir/Madam,

    I am refurbishing an old building with 2ft thick solid masonry walls. Because the original internal lime plaster layer has been hacked off, I am now required to provide internal insulation to meet current B Regs. requirements.
    The walls are too uneven to fix insulated plasterboard to the stonework and I propose erecting a partition inside using 100 X 50 mm studs backed with a breathing membrane. 100 mm thick quilting insulation will be inserted between the vertical studs and the ‘partition’ will then be faced with insulated plaster boards.
    What is the minimum thickness of Kingspan thermal plasterboard I would need to buy to satisfy current B Regs. standards? I look forward to receiving your advice.


    • Hi Dave

      We would recommend a thinner solution, which is to line the walls with a minimum 25mm thick timber battens, and then line these battens with insulated plasterboard. The thickness of insulated plasterboard required to achieve 0.30 W/m²·K (regulations for refurbishment walls) is 62.5mm Kooltherm K118.

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