Basement rigid foam extend between floor joists?
Hi,
I recently had a contractor who claims to specialize in energy efficiency describe a basement wall insulation method that I have not seen documented anywhere on the internet. They want to extend rigid foam board straight up from the cmu block wall, notching for the floor joists. Prior to installing the rigid foam, they would air seal the rim joists with caulk.
It would certainly save a lot of work not having to spray foam or “cut and cobble” the rim joists, but it would leave a small strip of insulated floor around the outer edges of the house.
I’m also not sure of the implications for water vapor transport. I can see this potentially allowing the rim joist more opportunity to dry, but I could also see the opportunity for moisture to accumulate in the cavity.
Has anyone heard of this method and can anyone comment on this?
Thanks,
Dave
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Replies
You don't notch around the "rim joistS", since there is only one rim joist, which is what the ends of all the floor joists typically attach to. I think you're confusing the ends of the floor joists with the "rim joist", where the rim joist is usually towards the exterior side of the foundation wall attaching to the ends of each of the floor joists.
If you put rigid foam on the foundation wall and notch it around the floor joists, you leave a big air gap between the back of the rigid foam and the face of the rim joist. I think this is a bad idea, since you're likely to have moisture issues in that space. It's much better to either cut'n'cobble pieces of foam, with each piece installed tightly against the rim joist, or use spray foam to seal the entire assembly (floor joist ends, rim joist, top of the foundation wall, and top of the interior side rigid foam) together. You don't want to create spaces with dead air, since you'll end up with issues in those spaces due to moisture ingress.
I think your contractor came up with a way to save themselves some labor without fully understanding the ramifications of what they're doing. You're not only air sealing the rim joist when insulating this area, you're also taking care to protect the interior (cold) side of the rim joist from moisture to avoid rot. Your contractor is thinking about the air sealing part of things only, not the moisture control aspect. You really need to think about both things to ensure a safe assembly here.
Bill
Unless the sill is flush with the interior of the block wall this is not a fire safe assembly. If you have a horizontal enclosed space it needs to have fire blocking.
Sorry Bill, you are correct, I meant notch around the floor joists. Your comment about stale air is interesting. Once that cavity is closed off, there is no telling what goes on back there without ripping everything out.
The contractor did mention fire blocking, so I think they are aware of the code. I'm just not sure if this is the best assembly from a moisture management perspective.
Any more comments greatly appreciated!
What I don't like about this assembly is that it leaves a strip of floor between the wall and the insulation uninsulated. This is not energy efficient, not to code, and in cold climates uninsulated spots can lead to condensation and mold.
On the BuildingScience.com website they have pictures of the kind of problems this can cause.
Check out photographs 2 and 3 in this article:
https://buildingscience.com/documents/insights/bsi-035-we-need-to-do-it-different-this-time