water vapour diffusion resistance factor

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Just trying to figure out what products are breathable and which are not.


Came across the vapour diffusion resistance factor when looking at the properies of a straw based board.It shows 10/10 for a istraw board and 2/5 for a woodwool board( like celenit)


Another thing is that When i introduce a breather membrane on top of the insulation ( roof with counter batens ) i get condensation,if i don't put breather membrane into build up no condensation 



Edited by passivhybrid

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I think the simple answer is that all materials are probably water vapour permeable, but there is a massive variation in that vapour permeability, from materials that can be considered, to all intents and purposes, to be water vapour impermeable, to those that could be considered to be highly water vapour permeable.


Perhaps worth just creating a list of likely materials, ranked in order of water vapour permeability, and using that, in conjunction with material thickness, to do a basic design that maintains a safe vapour permeability gradient.  Also worth trying to model the worst case transient/dynamic  conditions for the outer part of the structure, too, something that many models don't deal with that well.  It is possible to have a situation, for example, where after a cold night, that cools the outer part of the building fabric, you may then get a warm, wet morning, that drives vapour inwards, towards the cool area.  This may then condense, and if that part of the structure is then in shadow, it may not receive enough sensible heat in the area where moisture has condensed to be able to turn that moisture back into vapour, so it can move out of the structure.

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