Will closed cell spray foam at the rim joists increase rot potential?
I am working on a 2 story 100 year old decorative (rock face) concrete block house in Climate Zone 4. It is a modified balloon frame with 16oc floor joists that actually rest inside partially hollowed-out blocks of the exterior walls. After consultation with building engineers, we have just about decided on closed cell foam insulation (since our main objective is to keep out moisture that’s always present in the concrete block. There is no capillary break between the foundation and the above-grade walls and no way to fix that.)
Then I thought of these floor joists. There’s never been insulation in this house and the wood probably got wet and dried out over and over again without problem. But if we essentially encapsulate the wood joists inside the concrete block with closed cell foam, will they start to rot?
Should I use Rockwool or friction fit XPS instead? Or will that simply make the water condense in those areas?
Can I spray a wood hardener or water repellent on the tips of the floor joists that are inside the blocks before sealing them off? Any suggestions would be very welcome.
Replies
You do not want to insulate your rim joist with just mineral wool. If you do, the rim joist will be kept at close to exterior temperature but humid interior air will still get to it and condense. I've seen this happen when mineral wool was left in wall cavities with no vapor barrier in the winter. The interior side of the sheathing was soaked.
Do you mean EPS? XPS is vapor impermeable. If you used EPS, it'd be the same story as the wool. If you use XPS, it's similar to the spray foam but less airtight because you'd have to foam around it and make sure to fill all gaps.
I don't know what the solution is to keeping your joists safe but you could look into Bora-Care. You spray a dilution onto the wood and it keeps rot, ants, and termites away. Soaks in, spreads with moisture, supposedly lasts 30 years. Not a bad idea to treat the foundation, sill, and anything in that neighborhood if you have an old house.
Thanks for the Boracare suggestion. I had forgotten about that product. But used it successfully in the past.
Here's a picture of our situation. I'm going to be framing walls 2" offset from the concrete, so there will be a neat little hollow rectangle between each joist. Maybe open cell foam insulation there?
Open cell foam sounds like it'd have the same problem as the mineral wool but if you have an airtight wall there maybe it'd be different. Sorry, don't know.
Closed cell foam can be applied to bora-care-treated wood but give the bora-care a week to absorb and dry out a bit to be safe.
Thank you. Just been talking to a scientist at nisuscorp that makes bora-care. I think you are spot on with this recommendation. Thank you.
I believe the closed cell foam will best prevent condensation (as a previous reply mentioned), but because the joists can still wick air and vapor both within the concrete pockets and into the air space interior to the wall you are framing, you will not actually be fully encapsulating the joist. Therefore the joists will continue to be able to dry (and wet) as before?
Thanks @exinreno. That's a really good point that I haven't considered. Lost a lot of sleep over this matter, but your comment makes a great amount of sense.