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Another Rim Joist Insulation Question

13 years ago

Newbie here: Please correct me if I am wrong. I want to use XPS for the rim joist area in my basement. Do I just cut the desired shape and then fill the sides with Great Stuff or caulk? Do I need to place the XPS to the complete back of the area or can I place it in the front because some areas are extremely deep and not accessible? Do I still need bat? I live in Chicago land area.

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Comments (10)

worthy
13 years ago
Caulk or foam, whichever works best. Best tight to the rim. But in no case should a batt be used other than on the inside of the XPS, that is, towards the interior of the basement. Use a minimum 1" thick XPS.
brickeyee
13 years ago
Cut the pieces about 1 inch undersized in both dimensions, then fill with foam.

If you wedge them in you cannot get foam in to seal things up.

I usually put a bead of foam on the bottom of the piece, then set it in place and foam the other three sides.
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I have to post a bit of an update: After thorough examination, I've discovered that the back side of the house does not extend out beyond the block wall, so a shallow rim joist cavity. Also, the outside walls all have exterior rigid insulation, including over the rim joists, so according to building science, I don't need to put XPS on the inside of it. However, the front of the house extends out past the sill plate, and another board is attached to the bottom of the joists outside, but with a 2" gap from the sill plate. There is aluminum soffit on it, which I can touch through the gap from the inside, by reaching under a tar paper of sorts, which is on top of the sill. Lots of air leakage in these cavities. Also, the sills have a woolly type gasket under them, but still have air leakage, so I will seal those up. With that added, I would imagine that I need to cut XPS to fit the bottom of these cavities for sure, and seal them up. But, do I still need the ones on the rim joist face, even though the outside is insulated? Also, I still have that wall with the inaccessible cavities, filled with fiberglass. This wall does extend out from the block wall, and also is insulated on the exterior. I can't feel near the draft I could from the front wall, except a bit from under the sill in spots, which I can squeeze a great stuff straw into. Building science shows that I should insulate the top of the block wall that is exposed, but if I were to do this on this particular wall, it would fit tight up against the last inside joist I mentioned above, effectively blocking the entire wall worth of rim joist space from the interior air. Would this not create a big problem? I am a bit puzzled on how I should deal with this area. Joe
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I build in what may be a similar climate. (Please note that gardening zones are not climate or hygrothermal zones. See International Energy Conservation Code (IECC) energy zone map below.) I only use high-density spray foam on the rims. But the way you describe your retrofit, the combination of Roxul with sealing may be more appropriate if you see a need for accessing any of the pipes that would be buried in the foam. (I can't imagine why you would need to access the wiring.) Unlike fiberglass, on which mould grows when wet, Roxul will not wick water and does not have to be replaced after heavy wetting. However, unless great effort is put into cutting the insulation to fit, there will be gaps that reduce the R value. Whatever method you use, you might want to meet IECC minimums. Beside referring to the table above, you can use the map on this link to find IECC county-by-county insulation requirements. If this is an unfinished basement, termite shelter tubes will be visible on the foundation walls inside; and, for the same reason, be sure the foundation is exposed on the exterior. There's no particular moisture problem. Sources of ignition will have to be shut off during application. Don't forget the water heater. This post was edited by worthy on Wed, Oct 22, 14 at 0:02
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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
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I'm unclear on the flooring myself - I guess there are beams going the opposite direction of the joists. Ok - I can picture it. What about your r-19 grouped with some foam? I am imaging 1 inch of foam on the plaster/drywall sprayed in to seal it. It also adds some R-value and thickness. Then if you wanted some more r-value, you could do 1 inch rigid foam (or 2 inches) on top of the joists. This would help with thermal coupling of the joists. Getting to a magical R-value is not the only concern. Sealing properly is worth a lot. That being said, you would be at r-27. Spray foam is about a dollar per sq foot at an inch thickness. So you are looking at $1500 - but you might use less because of the joists. The rigid foam is about 1/3 of that so you would need $500 for that. Not cheap but 30% tax credit would count. I think you would be fine from a vapor barrier standpoint. 1 inch of rigid foam does not count as a barrier but 3 inches would. I would want some independent confirmation on the vapor barrier question.
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the-tech-guy
Original Author
13 years ago
Is bat even needed or will the XPS take care of everything?
worthy
13 years ago
One inch of XPS has an R Value of 4.5-5, which should be sufficient to prevent condensation on the rim. However, it is way below the recommended insulation values for your climate. The US Department of Energy recommends R30 for walls. The rim area, which is abovegrade is, in effect, a wall. So any suitable insulation up to that R value will be effective. At the least, I would add batts. You could also use EPS (expanded polystyrene) in addition to or instead of the XPS. It has a lower R Value per inch than XPS, so you'll need more. But it's cheaper. Polyisocyanurate boards with a foil covering will give you up to R8 per inch and are usually not required to be covered by any other fire resistant materials.

******
I find it virtually impossible to use the handheld foam can upside down in the cramped spaces between the joists. So I cut the foam tight and use caulking. But maybe that's just me!
brickeyee
13 years ago
"I find it virtually impossible to use the handheld foam can upside down in the cramped spaces between the joists. "

Throw the little straw that comes with the cans away and purchase some nylon tubing.

Slip the tubing over the nozzle and you have a nice flexible applicator to put the foam were you want it.

The tubing is cheap enough I do not even waste acetone trying to clear it out.
worthy
13 years ago
Neat trick!
andrelaplume2
13 years ago
I think you need to face the xps with drywall too for fire safety....
worthy
13 years ago
need to face the xps with drywall too for fire safety

Yes.

But if you spray foam the rim joist area with medium to high density foam, no thermal barrier is needed under the International Residential Code, adopted in most of the US, as long as the maximum thickness is 3.25".
andrelaplume2
13 years ago
that interesting...does it not burn?
worthy
13 years ago
I don't get it either!

A plumber accidentally lit a tiny section of ccspf in a house I was building. No flames, but the acrid odour from the one square inch section drove everybody out of the house tout suite. I would never leave it uncovered in a house I built. In that case, I sprayed the rims with a 1/2 inch of plaster.

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