Spray foam – rim joist and sill plate
Hello, I have a house in Zone 5 with a poured concrete foundation, at least 10 inches above ground, with slope towards back. The basement walls have 5” Type 2 EPS (unfaced) insulation installed. Regarding the rim joist and sill plate, some sections of the sill plate have pink foam and poly, I’m considering using P90 spray foam for the rim joist and sill plate, continuing the insulation down to meet the EPS. The P90 foam I’m planning to use is a low-density, hybrid product, but I consider it open cell.
Questions:
- Would fully encapsulating the sill plate with this foam create a moisture issue by preventing it from drying properly, given the potential for moisture wicking from the concrete foundation?, I’m worried about drying potential since I will encapsulate it
- Are there any concerns about long-term moisture management with this approach?
I will add that I do have high water table, but never had any water issues in basement always dry sill plate and rim joist are not pressure treated thank you
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Replies
So I have the above installed and now after reading … I feel like I should go scrape it out and just putt rockwool so there is drying potential indoor
So many horror stories on open cell and closed too encapsulating and rot
Max,
The sill-plate should have a good drying path to the exterior, and as long as it is protected from moisture in the concrete stem-walls with a sill-gasket it will be fine. If you are in climate zones 6 or 7, closed cell foam is safer.
https://www.greenbuildingadvisor.com/article/insulating-rim-joists
Thank you, since I’m in zone 5 … should I remove it :( and start again thank you !
Closed cell spray foam is safer here. My concern would be moisture accumulation IN the foam in a similar manner to what can happen with open cell insulation used under roof sheathing. If you have something on the exterior of the rim joist that is pretty vapor open (rain screen, house wrap, vinyl siding, etc.), then you should be pretty safe. If you have something NOT vapor open (foil faced polyiso, some kind of PVC trimboard tightly installed, etc.), then I'd be more concerned and would prefer to foam in EPS blocks to allow a limited amount of inward drying. Either way, a capillary break is very helpful to protect the rim joist and mudsill from moisture wicked up by the foundation. Basic sill seal gaskets can provide this (although I prefer EPDM or HDPE on new installs since both are far, far more durable materials), or HDPE in thin ~1/32"-1/16" or so thick sheets can be slid in place after temporarily jacking up the edge of the house slightly ("slightly" is about 1/8" or a bit over, just enough to be able to slide the HDPE sheet into place).
Bill
Thank you, I think outside is Tyvek and then Brick ..
Thank you, I’m on the fence about removing the foam and starting again…
I forgot to mention I added some fiberglass over foam as well
The fiberglass isn't a problem IF you have enough spray foam to avoid any condensation issues. I use the usual ratios for each climate zone to determine this. I prefer to use only spray foam and/or rigid foam though, I don't like to use any batt materials in this application.
Bill
If you did scrape it out, would simply using Rockwool work? This would allow inward drying.
Emil,
In heat dominated climates the vapour-drive is to the outside, so with permeable insulation like rockwool, you don't drying, but rather more wetting.
https://www.greenbuildingadvisor.com/article/insulating-rim-joists
Would it be opposite then in a cold climate? Our home is a different scenario where we are in a cold climate and our trusses are dropped down inside something like a brick ledge (not sure if that’s the official name for it). Because the trusses are essentially behind an insulated stem wall with a waterproof membrane in the outside, we opted to not do spray foam and go with mineral wool to allow drying. You have me wondering if that was the right choice now.
The "heat dominated climate" Malcolm is referring to is a "heating dominated" climate, basically somewhere where you spend more time heating your home than cooling it. That is the case in the more Northern climate zones where most of us on GBA live and work.
In cooling dominated climates, you are air conditioning more than you're heating, so vapor drive tends to be inwards, with moist outdoor air condensing on the cool interior surfaces of your walls because of the air conditioning. This is the opposite of heating dominated climates where moist indoor air gets into the wall and condenses on the cold exterior portion of the wall, since it's colder outside than inside. Note that by "interior" and "exterior" here, I mean the surfaces of the drywall on the "inside the wall" side, and the exterior sheathing on the "inside the wall" side, respectively. The condensing occurs INSIDE the wall, and that's why you can end up with moisture and mold problems INSIDE the wall assembly when you have issues with moisture drive and drying.
BTW, why are you going through all the effort of removing the spray foam? If it wasn't causing any problems, I would just leave it alone. Spray foam is either a problem right from the start, or it's fine forever. Spray foam does not "go bad" over time.
Bill
Thank you bill. So would the Rockwool we chose in our situation work ok?
Emil,
Sorry for the confusion. As Bill said, I meant "heating dominated" climates.
As long as there is sufficient exterior insulation to keep the inside face of the rim-joist above the dew-point you should be fine.