Insulated Stud Walls

[Rickstein]

Hi,

I am wondering what is the best way to model a wall which comprises metal or timber studs with insulation (mineral wool or rigid foam) between each stud?

So far I have effectively averaged the thermal conductivity and vapour resistance using an area weighted average, but is this the correct approach? I assume there is no way to create a stud wall within Wufi?

Regards,

Richard

[Manfred Kehrer]

Geometric averaging works from a thermal point of view not from a moisture perspective though.
I heating climate zones and no interior insulation you should be fine with ignoring the wooden studs, we cover this topic in a WUFI 2D workshop. For metal studs I am not sure. Actually a WUFI 2D approach would be the correct one.

[Rickstein]

Hi Manfred,

Thanks for the response. The insulated steel studs will typically form the internal part of the wall and would be filled with mineral wool. In this case I believe the thermal impact is large, but the vapour resistance will be dominated by the mineral wool properties. I understand that a 2D model would be better, but do you think my assumptions are sound?

The insulated timber studs on the other hand are outside of the metal framing and typically insulated with rigid foam. Again, here I'm reasonably comfortable with the thermal aspect, but as there are two relatively vapour closed items I'm not sure how to deal with it. In theory, if installed perfectly, little vapour will get past, and there may be a condensation risk, however in reality there will be gaps between the insulation and the studs and I believe this will disproportionately increase the vapour flow. How do you suggest I model this? I could introduce an air source perhaps, or simply calculate the likely effect of the gaps and modify the overall vapour resistance accordingly? Do you have any advice?

Richard

[Manfred Kehrer]

I presume you are doing the average according to ASHRAE Handbook of fundamentals' parallel heat flow method. This is only allowed if the thermal conductivities are in the same ball park, which for sure is not the case for metal vs. mineral wool.