I would love to install rockwool comfortboard on my knee wall but I got a devils triangle (osb wall, fiberglass batts in floor,rafters)and can not get there. Any ideas?
I wouldn't "air seal" it. In my opinion it's better to leave 1/2 inch gap on top and bottom and also don't bother with spray foam at all. Well, spray foam all the cuts through the drywall is a good idea, like that power outlet. But your insulation layers outward from the drywall should "breathe"
Umm, you have kraft faced fiberglass insulation against the interior wall. In this case the vapor barrier(kraft paper) is as it should be; facing the heat source. But when you add foam based insulation to the outside of the insulation, you're probably adding another vapor barrier. This is known as a vapor barrier sandwich and any moisture that gets in the wall can't easily get out, leading to declining performance of the fiberglass and mold/mildew problems. Not all foamboard is a vapor barrier, in particular in thinner sheets. Polyiso is a vapor retarder in most typical selections. But 2" of XPS is a vapor barrier by contrast. I'd suggest you check the ratings of your foamboard which is usually printed on the board. If your thickness/foam type is a vapor retarder, you might be OK. If it's a vapor barrier you'll have wall problems, all the more so since you thoroughly foamed the seams. The foil placing towards the interior wall further prevents moisture transmission. One other issue is leaving foamboard exposed. That's against all US building codes as it will aggressively burn with toxic fumes. This will shorten the time you and your family have to escape. You might think I'll take the chance, but if your house burns the insurance company could deny payment because you violated building code. And if people are hurt there are other problems with law enforcement. And this video is further proof! In this scenario I have had good luck using rockwool comfortboard as a secondary layer. It's essentially fireproof, allows moisture movement and is a great sound barrier. Good luck.
If I have kraft faced fiber glass against my interior wall. What do you recommend to stop air from passing through it if not foam board? This for a bonus room/knee wall.
@@jordancavins4267 I have a knee wall in a house in northern NH. Very cold! I use an infrared thermometer (cheap ~ $20) to look for cold spots. I found very cold areas at the bottom of the wall. So... from the non conditioned side of the wall, I sprayed foam along the floor plate and that made the bottom of the wall significantly warmer by stoppign the cold air penetrating along the base of the wall. Then when I was looking for a project :), I decided to put another layer of insulation. I used Roxul Comfortboard in this case. It doesn't air seal greatly like foam but you don't have moisture issues in the wall if you already have the vapor barrier in place. This made the wall warmer still and at this point I was satisfied.
@@BobPritchard Amazing info here. I have two large knee wall areas. It has a plywood floor but I would like to seal everything up not only for energy purposes, but to keep out bugs as I want to store some nice items in there. Would you still recommend a foamboard and then comfortboard? I was just going to add better insulation and then hang OSB plywood in there and seal the gaps but might be overkill?
@@WayneStrattonWayne if you have an existing knee wall with insulation already you might consider sealing the edges with a foam can and if you want more insulation you could use Comfortboard as one additional layer. It's vapor open and won't impact any vapor barrier in place. It also adds soundproofing. Unless it's going to be a finished room I don't think you need to finish off the surface with plywood or sheetrock.
Bob, Your point about a “vapor barrier sandwich” only applies to non-insulating vapor barriers. If plastic sheeting was applied over the batts, there’d be a risk of condensation on the interior side. But, thick enough rigid foam will keep the interior face warm enough that water will not condense. Any moisture in the wall cavity will remain as vapor. Polyiso usually has a foil face which acts as a complete vapor barrier. 2” of XPS will also effectively become a vapor barrier.
I would love to install rockwool comfortboard on my knee wall but I got a devils triangle (osb wall, fiberglass batts in floor,rafters)and can not get there. Any ideas?
I wouldn't "air seal" it. In my opinion it's better to leave 1/2 inch gap on top and bottom and also don't bother with spray foam at all. Well, spray foam all the cuts through the drywall is a good idea, like that power outlet. But your insulation layers outward from the drywall should "breathe"
Umm, you have kraft faced fiberglass insulation against the interior wall. In this case the vapor barrier(kraft paper) is as it should be; facing the heat source. But when you add foam based insulation to the outside of the insulation, you're probably adding another vapor barrier. This is known as a vapor barrier sandwich and any moisture that gets in the wall can't easily get out, leading to declining performance of the fiberglass and mold/mildew problems. Not all foamboard is a vapor barrier, in particular in thinner sheets. Polyiso is a vapor retarder in most typical selections. But 2" of XPS is a vapor barrier by contrast. I'd suggest you check the ratings of your foamboard which is usually printed on the board. If your thickness/foam type is a vapor retarder, you might be OK. If it's a vapor barrier you'll have wall problems, all the more so since you thoroughly foamed the seams. The foil placing towards the interior wall further prevents moisture transmission. One other issue is leaving foamboard exposed. That's against all US building codes as it will aggressively burn with toxic fumes. This will shorten the time you and your family have to escape. You might think I'll take the chance, but if your house burns the insurance company could deny payment because you violated building code. And if people are hurt there are other problems with law enforcement. And this video is further proof! In this scenario I have had good luck using rockwool comfortboard as a secondary layer. It's essentially fireproof, allows moisture movement and is a great sound barrier. Good luck.
If I have kraft faced fiber glass against my interior wall. What do you recommend to stop air from passing through it if not foam board? This for a bonus room/knee wall.
@@jordancavins4267 I have a knee wall in a house in northern NH. Very cold! I use an infrared thermometer (cheap ~ $20) to look for cold spots. I found very cold areas at the bottom of the wall. So... from the non conditioned side of the wall, I sprayed foam along the floor plate and that made the bottom of the wall significantly warmer by stoppign the cold air penetrating along the base of the wall. Then when I was looking for a project :), I decided to put another layer of insulation. I used Roxul Comfortboard in this case. It doesn't air seal greatly like foam but you don't have moisture issues in the wall if you already have the vapor barrier in place. This made the wall warmer still and at this point I was satisfied.
@@BobPritchard Amazing info here. I have two large knee wall areas. It has a plywood floor but I would like to seal everything up not only for energy purposes, but to keep out bugs as I want to store some nice items in there. Would you still recommend a foamboard and then comfortboard? I was just going to add better insulation and then hang OSB plywood in there and seal the gaps but might be overkill?
@@WayneStrattonWayne if you have an existing knee wall with insulation already you might consider sealing the edges with a foam can and if you want more insulation you could use Comfortboard as one additional layer. It's vapor open and won't impact any vapor barrier in place. It also adds soundproofing. Unless it's going to be a finished room I don't think you need to finish off the surface with plywood or sheetrock.
Bob,
Your point about a “vapor barrier sandwich” only applies to non-insulating vapor barriers. If plastic sheeting was applied over the batts, there’d be a risk of condensation on the interior side. But, thick enough rigid foam will keep the interior face warm enough that water will not condense. Any moisture in the wall cavity will remain as vapor. Polyiso usually has a foil face which acts as a complete vapor barrier. 2” of XPS will also effectively become a vapor barrier.
How has this performed since installing? Looking to do something similar to a few knee walls
yeah.........this looks wrong based on all the other videos i've seen Good luck with moisture.