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Do you need a vapour control layer in your roof if all of the insulation is breathable?

 
Chris Brookman
(@chris)
Member Admin

Yes. Always.

Whenever you insulate you need to restrict the flow of moisture so that you do not create a situation where the levels of moisture within either the insulation or the timber rafters reaches damaging levels. Winter is when most of this condensation would occur but also when it will dry out the slowest and with a well ventilated structure you can get damaging levels of moisture accumulating, even in wood fibre insulation.

Vapour control layers do not have to be membranes, they can be plasters such as UdiMULTIGRUND or OSB3, and are also distinct from vapour barriers, which are effectively a complete vapour block. Vapour control layers slow the flow of moisture, not block it and in certain circumstances vapour barriers may actually cause a build-up of moisture, rather than prevent it.

You can read more about vapour control layers here - https://www.backtoearth.co.uk/what-is-a-vapour-control-layer-vcl/

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Topic starter Posted : 09/03/2021 5:32 pm
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