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Rim Joist Insulation/Basement finishing steps

12 years ago

Hello all,

Quick question...

I am working on finishing parts of my basement (after a gut).

I tore out 1" styrofoam insulation and some old Fiberglass Batts in the studs.

My plan is this:

1. Caulk all rim joists with spray foam where the sill meets the foundation (poured foundation)

2. Buy Roxul insulation and cut to size for rim joists cavity.

3. Cover Roxul insulation by installing a 1x2 to each side of joist and screwing either (a) cement board or (b) green board. What do you recommend?

4. Glue 2" XPS to walls (do I need to attach any other way?)

5. Frame and electrical

6. Install FG Batts (maybe, not sure if necessary)

7. Drywall (tape, mud, etc)

8. Party

What do you think of this? I know worthy is the best person on the insulation piece and I've read alot. I don't have any moisture problems in the area I'm working in.

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Comments (3)

worthy
12 years ago
Scrap the Roxul. Fibrous insulation in the rim joists allows the buildup of moisture and mould. Instead, use foam or XPS caulked and sealed.

Gluing XPS to poured walls generally doesn't work as the walls are too irregular. In any case, it may not meet some Codes, which require mechanical attachment. Instead, you can attach each foam board with concrete screws and fender washers or run strapping across multiple foam boards. Fibrous insulation to the interior can add up to R13.65 for a 2x4 wall.

Use shiplap XPS or tape the seams for optimal airtightness. In some markets, you can obtain XPS in 4' wide sizes.

Here is a link that might be useful: Building Science Corp. on Rim Joists
Graeme
Original Author
12 years ago
worthy,

I know that in my city, we are currently required to follow the IRC (or at least that's what the building inspectors follow). It's nice to know I don't need to firebreak the foam/xps in the rim joists.

Do you recommend having the rim joist spray foamed? This is right below my kitchen and the kitchen is the furthest heat run from the furnace. Would this help with heating/cooling this area?

Can you provide the link to the mechanical attachment document that would help? I am able to get 4'x8'x2" XPS foam for $26 a sheet.

You should probably "re-do" a basement and have a link to exactly how you would do everything. It would probably save on answers..

Thanks for your help. It's really useful for a new DIY project.
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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!
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I used 2" and cut the pieces out of a 4x8 sheet and cut them 1/4" or so shy of the average bay sizes to give room for the foam to go in. I had a few that didn't want to stay against the rim joist as they were foamed and I didn't have any washer nails to hold them in tight. The few that gave me trouble got marked with a marker and I drilled a few holes in the foam board and shot foam in the back side and let it sneak back out the holes and shaved the excess off. it is a night and day difference between the foam board and just having batting stuffed up in the spots. I don't have a heated basement and can tell a world of difference. I think I got around 30 plugs out of one 2" 4x8 sheet of foam board with a few pieces left over for odd sized spots. One can of great stuff foam did between 8-10 plugs with a price around 6 bucks a can and the sheet price of $22.50. Once you get set up and going it doesn't take too awful long to do. I got the ones with obstructions fitted with a hand saw and in place first and then stuck 10-12 in a row so I just had to concentrate on foaming. I noticed that if you spend too much time putting the foam blocks in after the can has been partially used it wouldn't go near as far as if you started foaming and went from one to the next.
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Comments (9)
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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According to the Polyisocynanurate Insulation Manufacturer's Association, "polyiso...is generally required to have a thermal barrier. But, it has the capability to be used without a thermal barrier when proven by test. ...Consult a specific polyiso manufacturer for code approval data allowing a productÃ¢ÂÂs use without a thermal barrier." And the question is whether 1/2" of the polyiso you choose meets the testing requirements of ASTM E84.
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worthy
12 years ago
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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