Difficult Rim Joist Insulation

Joe1980February 23, 2014

Hello all, I'm in the process of remodeling my basement, and have been doing significant research on insulating. My hang up is the rim joists. Mine overhang the house by about 6 inches all the way around, leaving a pretty deep cavity to deal with. I understand the concept of cutting XPS board and sealing them in. Currently, they are all stuffed with fiberglass, so I understand I have to remove it, install the XPS, and reinstall the fiberglass.

On the one wall (garage on opposite side) that the joists run parallel to the block wall, I have about 1.5" of space between the block wall, and the last joist. From the last joist inside, perpendicular framing members extend out to the actual rim joist, which leaves me with a bunch of rim joist cavities that are inaccessible, and have fiberglass in them that I can't get out. I assume they were stuffed with the fiberglass before the first floor was covered.

So, how do I go about sealing up these cavities? Also, am I right in assuming I have to cut XPS for the bottom of the overhanging part of the rim joist cavities, as they contact outside air?

For the record, I'm not SUPER concerned about saving every dime in heating costs, but more concerned about moisture from air movement and condensation. Thanks in advance.


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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.


    Bookmark   February 24, 2014 at 7:23PM
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do I still need the ones on the rim joist face, even though the outside is insulated?

Not likely, assuming you're in a moderate climate.

effectively blocking the entire wall worth of rim joist space from the interior air.

I fail to see the benefit of air circulating on the interior of rim joists.

See the linked article below for various sections that may apply to your situation.

Here is a link that might be useful: Building Science Corp.: Basement Insulation

This post was edited by worthy on Mon, Feb 24, 14 at 23:52

    Bookmark   February 24, 2014 at 11:48PM
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I live in Wisconsin, so not quite moderate.

As for the rim joist space, I guess I was worried about creating an isolated space between the rim joist and the next joist over, filled with fiberglass, should I plug the 1.5" space between the 2nd joist and the block wall with XPS. I would imagine there will still be some outside air infiltration into that space, and I cannot access the space to seal it up tight. All I can do it seal at the sill, and will have to leave the fiberglass in the cavities, without completely air sealing them. I have searched the web up and down, and haven't found anything similar to this scenario. Thanks for the reply so far.


    Bookmark   February 25, 2014 at 10:51AM
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Most of Wisconsin is Zone 6; the upper one third is in the same climate zone as Alaska. So, not moderate.

Why fibreglass can be a problem in the rim is that it doesn't block interior warm moist air from reaching the wood and stopping frost from forming; and fibreglass doesn't do well in damp conditions.

If you can't get the fg out, I'd totally block off that last rim with foam.

The exterior foam may well be enough to keep the rims warm enough that frost won't form in the winter anyway.

Here is a link that might be useful: Insulating Rim Joists

    Bookmark   February 26, 2014 at 12:28AM
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Thanks for the suggestions. In the rim joist cavities I can get to, there are no signs of moisture at all, and the house is about 20 years old. Some of the fiberglass has dust, a sure sign of air infiltration, but only on the front side that I described as having that 2" or so gap between the sill and the boards nailed under the rim.

I also intend to keep the basement dehumidified, as I do now, to 50% or less, so I suppose that helps keep any condensation/frost down. Being that I'm not in a "moderate" climate area, with the exterior insulation on the rims, would it still be beneficial to apply XPS to the inside as well, or can I skip that? Thank you much,


    Bookmark   February 26, 2014 at 7:37PM
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Code minimum for basement walls in Zones 6-8 is R15 continuous (as in foamboard sheathing) or R19 otherwise.

BTW, if space is not a big consideration, you can usually achieve the same R factor with cheaper EPS rigid foam.

This post was edited by worthy on Wed, Feb 26, 14 at 23:55

    Bookmark   February 26, 2014 at 11:54PM
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