Outbuilding insulation - breather membrane?

9 Oct 2014
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United Kingdom
Have posted on here about this project before and got some good advice. Now just need to check something.

I am insulating a garden building, for all year round use as a studio.

Currently the walls comprise (from outside in) - T&G timber cladding, a black membrane, some thin polystyrene sheets, plasterboard. The wall is a timber stud wall.

I am making the assumption that the polystyrene sheets do not offer much insulation, so...

My intention (having taken quite a bit of advice from here) is to remove the plasterboard and the polystyrene, and put in Kingspan (or similar) insulation boards (between and over the timber frame), then a vapour barrier, before re-plasterboarding.

What I want to know is - the black membrane that is already there, on the inside of the T&G cladding - is it likely to be a breather membrane? It's quite stiff, a little bit like roofing felt but not that stiff, possibly a kind of fabric (with possibly a bitumen type coating?)

I am attaching some photos, where I have knocked through the plasterboard and broken off a bit of polystyrene, to reveal the membrane in question.

If it is breathable, then I think I can fill the depth of the studs with insulation, with no need for an air gap on the inside of the membrane & timber cladding, as air will be able to pass through both membrane & cladding ?

If not breathable, then I guess I need to leave a 50mm air gap behind insulation. Which would be a pain, and possibly make it not worth doing as having to leave an air gap will leave me little room for the thickness of insulation boards that I need.

So the question is - is that black membrane breathable?

The building was constructed at least 17 years ago, if that helps...


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