Permeance of Peel-and-Stick WRB
andyfrog
| Posted in General Questions on
German DIN 4108-3: interior materials should have a permeance 5x lower than exterior materials — is this realistic with self-adhered WRB if the interior material is taped plywood?
So let’s say you are using taped plywood on the interior of the framing as your air control layer and vapor throttle, at about 10 US perms (it changes with humidity but let’s run with that for this example).
Where are you going to find a self-adhered WRB with a permeance of 50?
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> So let’s say you are using taped plywood on the interior of the framing as your air control layer and vapor throttle, at about 10 US perms (it changes with humidity but let’s run with that for this example).
Have no idea about that German standard nor the climate zone equivalence, etc., but... why do you need to use taped plywood inside the building?
It seems to be a common method of achieving an air control layer in a lot of example assemblies:
https://www.youtube.com/watch?v=ow93BkBIF1I
https://youtu.be/tooEnC9EYak
Interior plywood is typically dry, which suggests dry perms, which means more like 2 perms. Obviously there are various ways to lower interior side perms to get a favorable ratio.
ah woops. okay, that makes sense, thanks.
If the exterior materials are not airtight e.g. untaped rigid insulation like gutex, foam, mineral wool -- does their permeance really matter?
IMO, it depends. Would be nice to see test data for various materials in various assemblies.
It's the dry cup permeance not the wet cup permeance that you should use to compare the materials. Dry plywood is around 1 or 2 perms as Jon R wrote.
I don't know about the DIN standard, but for cool weather perm ratios, use dry perms for the interior and wet perms for the exterior.