How to insulate without rim joists?

10 years ago

Hello,

So, we have a 1935 Cape Cod and (as some of you have seen) we're moving ahead on finishing our basement (much thanks to worthy for insulation guidance and pointers).

One of the regularly recommended steps is to insulate the rim joists. However, it seems that we don't have any.

Hopefully, you can see from the picture I'm including that, on one end of the house, there's a gap behind the joist closest to the cinder block wall. That cinder block continues up an inch or two behind the joist and is immediately topped by a brick-like material (sure looks like bricks but there seem to be some places where it's something that's brick-like, but hollow on the inside, like cinderblock but brick). There is nothing like a horizontal beam -- sill plate -- across the top of the foundation wall and there is no rim joist on top of that (as I've seen in pictures of modern houses on the web). By the way, this situation replicates itself at the 180-degree opposite end of the house.

Also, I hope you can see that at the end of that joist, where it goes into the wall, there is no rim joist there, either. This, of course, is the case for every parallel joist, from one end of the house to the other.

So, I'd been thinking that I'd use an expanding foam to seal the gap between the outermost joist and the parallel wall (though in some places that gap is two inches wide). Should I do that? What else could/should I do?

Similarly, what to do between the joists where they enter the wall at the far end?

Thanks!

Tiger

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Comments (9)

worthy
10 years ago
Yep, same solution as where there are rim joists: spray foam or foam board and caulking.
tigerninety
Original Author
10 years ago
Thanks for the above. What kind of spray foam should I use? Is there a particular kind that is consistent with the plan to use XPS against the cinderblocks below the joists? And, I presume I should be trying to spray as much foam up between the wall and the joist as possible, to fill the entire area up to the floor boards above, true?
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Early blower doors tests in the 1980s revealed that leaky rim joist areas accounted for unexpectedly large amounts of air infiltration. You can seal rim joist areas with caulking and polyurethane foams (Great Stuff, for instance) and then either insulate with boards or several inches of closed or open celll foams. Spray or seal with foam board?" is essentially an economic question. What makes sense for a do-it-yourselfer homeowner may be impractical for a homebuilder. As a general rule, foaming is faster and tighter with fewer air leaks and thermal bridges than achieved by applying boards. But it's more expensive. As a sort of middle ground, diy foam kits sold in the big-box stores can be useful for doing the rims. (I've not personally tried them.) The mechanical attachment of the boards is straightforward. You can use concrete screws (Tapcon is a well-known brand) and fender washers on individual boards. Or you can run strapping secured by concrete screws across several boards. Here is a link that might be useful: Attaching and Taping XPS to Basement Walls
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worthy
10 years ago
Either closed cell polyurethane spray foam (ccspf) or open cell polyurethane spray foam (ocspf) is approved. However, open cell may require a further vapour barrier in cold climates.

There are lots of diy kits. But you have to follow all the safety precautions to the letter and there is clearly a learning curve. Or you can get a contractor.

You can get the same energy savings by cutting XPS boards to size and sealing with caulk and/or handheld single component sprays such as Great Stuff.

Installing Two-Component Polyurethane Spray Foam
Photo: Applegate Foam

Here is a link that might be useful: Building Science Corp.: Spray Foam at Rim Joist
worthy
10 years ago
Either closed cell polyurethane spray foam (ccspf) or open cell polyurethane spray foam (ocspf) is approved. However, open cell may require a further vapour barrier in cold climates.

There are lots of diy kits. But you have to follow all the safety precautions to the letter and there is clearly a learning curve. Or you can get a contractor.

You can get the same energy savings by cutting XPS boards to size and sealing with caulk and/or handheld single component sprays such as Great Stuff.

Installing Two-Component Polyurethane Spray Foam
Photo: Applegate Foam

{{!gwi}}

Here is a link that might be useful: Building Science Corp.: Spray Foam at Rim Joist
tigerninety
Original Author
10 years ago
Hello,

Many thanks, as always, for the speedy replies.

I'm afraid, though, that I probably wasn't as precise as I should have been. My fault.

I fully expect to use XPS between the joists perpendicular to the wall, cut to fit, and sealed with caulk or Great Stuff. No need to use the spray foam as they are doing in the pictures.

What I was wondering about is what I should use to fill the space between the joist that runs _parallel_ to the cinderblock wall and the cinderblock wall itself. In my original picture, it's that dark gap that starts in the center at the top of the photo and then "runs" away from you toward the bottom of the photo. That gap is the entire height and length of the joist (but the gap is not even and there are various random mortar protrusions, some of which you can see in the picture).

Should I use Great Stuff to fill that void, before I start adhering my XPS sheets below that? If not, what should I put up in that void (because I don't have a rim joist)?

Thank you very much!

Tiger
worthy
10 years ago
Too small a job for an outside contractor.

As you've probably found, the single component handhelds don't work well on overhead jobs. Wear goggles at all times!

But there are some professional guns for handheld foams that will work, such as those from Fomo, that retail for about $50.

Or you could push in sheets of XPS and make sure they're jammed tight against the exterior concrete using scraps of 2x2s or 2x4s. Not a perfect air seal, but it should preclude condensation problems.

(Sorry for the double posts and misunderstanding the question. Wouldn't be the first time!)
hendricus
10 years ago
I've used Great Stuff in the same kind of space, tape some straws together to get a longer reach. If you use a good brand of tape it should work for 3 to 6 cans and you can get right into all those hard to reach spaces.
vanamala_dutt
last year
I have the exact same kind of construction …. my question!is 1) I am not doing my walls , shiuld I still seal those places with the material mentioned here? There is lot of air leaking into the basment2) can I fill the gaps in the concrete around the block with concrete first?
worthy
last year
Concrete is not an air seal. And how you would place it overhead beats me. Just use the air sealing discussed above.