EcoRenovator - View Single Post - Remodeling the office (new floor, ceiling & adding insulation) passive house retrofit

S-F · 08-09-11, 09:24 PM

Can you get Polyiso which isn't foil faced? I'm all for not demanding that moisture stay in one place because you can end up with problems if there is too much of it. Talk to some local green builders to see if they can point you to recycled foam boards. Call all of 'em if you have to. This is going on all over the country.

About your idea, if you later decide to outsulate you might be in a bit of a pickle. It's not advisable to have two vapor barriers in a wall. The moisture that inevitably gets trapped in there has no avenue of escape and you get rot and mold. Personally I think it's nuts that people sandwich OSB/CDX sheathing between spray foam and foil faced polyiso but there haven't been any disasters yet that I've heard of. Maybe in 30 years we'll see the truth. But none the less, putting fiber glass or cellulose between them is a no no. Also it's best to have your air barrier (polyiso and 1 part foam sealant) on the outside to minimize unconditioned air entering the cavity and convecting. All your insulation is next to useless if you have cold air cycling through there. The big advantage of the foil on polyiso sheets is that you can quite easily seal them together with foil duct work tape. Otherwise you need to seal boards together with spray foam. Some people say you can use tyvek tape but in my experience that stuff doesn't last more than a decade.

Oh, one more thought. You could put the foam boards on the inside if you air sealed the sheathing properly. This is effectively no different than putting it on the outside of the sheathing as far as moisture is concerned. You just completely eliminate the wall's ability to dry to the inside. Just don't make a vapor barrier sandwich and make sure that outside air doesn't get in. That's why I originally recomended the boards and 1 part foam. It's a relatively cheap (compared to filling the whole wall with spray foam or even filling it 1" deep) way of creating a fantastic air barrier. You kill two birds with one stone there. Oh and if you do try to dense pack the wall yourself you will have no way of knowing if you did it right. At first dense packing can be a little tricky. Even today I still am uncertain when dense packing a cavity I can't test. Well, at least I can't test it until the cold weather hits and the thermal camera shows me what a stupid job I did. :P

And your price on the spray foam looks berserk. I think that's about what I paid for the 600' kit. I think even on the Tiger Foam page they list it at $250 for the 200'' kit.

08-09-11, 09:24 PM	#19
S-F You Ain't Me Join Date: Nov 2009 Location: Northampton MA Posts: 662 Thanks: 6 Thanked 71 Times in 58 Posts	Can you get Polyiso which isn't foil faced? I'm all for not demanding that moisture stay in one place because you can end up with problems if there is too much of it. Talk to some local green builders to see if they can point you to recycled foam boards. Call all of 'em if you have to. This is going on all over the country. About your idea, if you later decide to outsulate you might be in a bit of a pickle. It's not advisable to have two vapor barriers in a wall. The moisture that inevitably gets trapped in there has no avenue of escape and you get rot and mold. Personally I think it's nuts that people sandwich OSB/CDX sheathing between spray foam and foil faced polyiso but there haven't been any disasters yet that I've heard of. Maybe in 30 years we'll see the truth. But none the less, putting fiber glass or cellulose between them is a no no. Also it's best to have your air barrier (polyiso and 1 part foam sealant) on the outside to minimize unconditioned air entering the cavity and convecting. All your insulation is next to useless if you have cold air cycling through there. The big advantage of the foil on polyiso sheets is that you can quite easily seal them together with foil duct work tape. Otherwise you need to seal boards together with spray foam. Some people say you can use tyvek tape but in my experience that stuff doesn't last more than a decade. Oh, one more thought. You could put the foam boards on the inside if you air sealed the sheathing properly. This is effectively no different than putting it on the outside of the sheathing as far as moisture is concerned. You just completely eliminate the wall's ability to dry to the inside. Just don't make a vapor barrier sandwich and make sure that outside air doesn't get in. That's why I originally recomended the boards and 1 part foam. It's a relatively cheap (compared to filling the whole wall with spray foam or even filling it 1" deep) way of creating a fantastic air barrier. You kill two birds with one stone there. Oh and if you do try to dense pack the wall yourself you will have no way of knowing if you did it right. At first dense packing can be a little tricky. Even today I still am uncertain when dense packing a cavity I can't test. Well, at least I can't test it until the cold weather hits and the thermal camera shows me what a stupid job I did. :P And your price on the spray foam looks berserk. I think that's about what I paid for the 600' kit. I think even on the Tiger Foam page they list it at $250 for the 200'' kit. Last edited by S-F; 08-09-11 at 09:43 PM..