Great idea that is easy to do and makes sense, is it ok to build single story structures on 24 inch walls vs 16”? Would that make it plenty strong yet less studs for thermal problems?
The theory behind this is that the wall can still dry to the exterior. That pretty much applies to all the conventional walls we build in Alaska that have the 6 mil poly vapor retarder in front of the studs. The key here is that 1-1/2" of foam board does not cool the surface of the vapor retarder enough that it will attract condensation. In climate zone 8 we keep the amount of insulation on the warm side of the vapor retarder to no more than 1/3 of total wall r-value. - and in cases like this I'd prefer closer to 25% of total wall r-value in the furring as with fiberglass batts, there can still be cold spots in the framing that will attract condensation otherwise. So for an R-21 insulated wall like this one - R-6 in foam board does well in zone 8 and lower.
@@IlyaBenesch Thanks for your reply. You should do a follow-up video on the drying potential . As to other conventional walls built in Alaska, why wouldn't they use something more vapor permeable for the vapor retarder, like Membrain, instead of poly? Also, I guess the interior rigid insulation method would only work well if they didn't use rigid insulation on the exterior of the sheathing, otherwise you'd create a foam "sandwich" that could trap moisture inside the wall cavity.
@@ryansoo4000 I think it is a matter of time before Membrain and other products make it to us here. It will need to get scrutinized by the various municipal code authorities and undergo a review process. Alaska does not have a unified state building code (yet) and the 6 mil poly vapor retarder is deeply entrenched in local codes. Because our heatflows and moisture movement are so dominantly from interior to exterior for much of the year, the poly works well here. It's not perfect....but it does work. All that said, I do think change is coming. Yes-the foam on the exterior with existing warm-side poly is a complex issue and requires me to be cautious.
The Raco 196 style is one used often here in Fairbanks. Most manufacturers have variations of this - Just make sure it has the raised boss in the box for the ground screw as that keeps the ground screw from puncturing the vapor retarder when it's tightened down. Usually they have tabs on the sides to ensure proper depth and support when mounting to furring. If the box needs more space then you can always go wider. For example a 2 gang or 4 gang box can still have a 1 gang mud ring for a single outlet and the rest of the space in the box provides space for additonal wiring junctions etc.
Great idea that is easy to do and makes sense, is it ok to build single story structures on 24 inch walls vs 16”? Would that make it plenty strong yet less studs for thermal problems?
Why don't they make foam backed drywall?
By adding rigid insulation to the interior do you run the risk of decreasing the walls drying potential?
The theory behind this is that the wall can still dry to the exterior. That pretty much applies to all the conventional walls we build in Alaska that have the 6 mil poly vapor retarder in front of the studs. The key here is that 1-1/2" of foam board does not cool the surface of the vapor retarder enough that it will attract condensation. In climate zone 8 we keep the amount of insulation on the warm side of the vapor retarder to no more than 1/3 of total wall r-value. - and in cases like this I'd prefer closer to 25% of total wall r-value in the furring as with fiberglass batts, there can still be cold spots in the framing that will attract condensation otherwise. So for an R-21 insulated wall like this one - R-6 in foam board does well in zone 8 and lower.
@@IlyaBenesch Thanks for your reply. You should do a follow-up video on the drying potential . As to other conventional walls built in Alaska, why wouldn't they use something more vapor permeable for the vapor retarder, like Membrain, instead of poly? Also, I guess the interior rigid insulation method would only work well if they didn't use rigid insulation on the exterior of the sheathing, otherwise you'd create a foam "sandwich" that could trap moisture inside the wall cavity.
@@ryansoo4000 I think it is a matter of time before Membrain and other products make it to us here. It will need to get scrutinized by the various municipal code authorities and undergo a review process. Alaska does not have a unified state building code (yet) and the 6 mil poly vapor retarder is deeply entrenched in local codes. Because our heatflows and moisture movement are so dominantly from interior to exterior for much of the year, the poly works well here. It's not perfect....but it does work. All that said, I do think change is coming. Yes-the foam on the exterior with existing warm-side poly is a complex issue and requires me to be cautious.
GREAT demonstration! Do you have recommendations for shallow electrical boxes designed to fit in the narrow furred-in service cavity?
The Raco 196 style is one used often here in Fairbanks. Most manufacturers have variations of this - Just make sure it has the raised boss in the box for the ground screw as that keeps the ground screw from puncturing the vapor retarder when it's tightened down. Usually they have tabs on the sides to ensure proper depth and support when mounting to furring. If the box needs more space then you can always go wider. For example a 2 gang or 4 gang box can still have a 1 gang mud ring for a single outlet and the rest of the space in the box provides space for additonal wiring junctions etc.
@@IlyaBenesch thanks for the tip!!!