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Worth insulating joist next to inaccessable rim joist?

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Old 08-21-11, 02:11 PM
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Worth insulating joist next to inaccessable rim joist?

Note the first two background paragraphs on this post is the same as a different post, but it is a different question.

Background
We're re-insulating our rim joist. We decided to cut pink 2" XPS R-10 into blocks for each area, and are actually glueing two pieces back to back to give 4", giving up to R-20. (Bit more work, and we only needed to spend an extra $28 on another sheet of XPS, so why not.) We're leaving room around all sides, using a bit of glue to get it to stick to the rim joist, then filling all 4 sides with Great Stuff. Even though it's a lot more work, we decided to do it this way because it will save a few hundred dollars over the spray foams traditionally used.

The basement is finished with a drop ceiling, and we can easily access the rim joist on the front and back of the house. It appears they did a decent job on the basement walls when finishing. They used a few inches of fiberglass batt's against the concrete walls, made a frame, and drywalled it. Unfortunately, this means we can't get to the rim joist on the sides -- only to the joist next to the rim joist.

Question

The accessable rim joist areas are stuffed with an extremely thin fiberglass batt, that has all sorts of air leaks, so I can only assume the inaccessable rim joist areas are as imeffective as well.

We already bought enough XPS foam for the entire rim joist - I didn't know we couldn't access it on the sides.

There is some sort of "ledge" between the top of the drywall and that joist, so there's no gaping open area to get around the joist, but there's sure leaks.

Would we benefit from putting up the 4" blocks and Great Stuff'ing it, even though it's only going to go on the joist next to the rim joist, or is this a complete waste of time?
 
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Old 08-21-11, 08:24 PM
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Please do not repeat your posts.
 
  #3  
Old 08-21-11, 08:47 PM
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Sorry, I knew this would look like a re-post, because the first two paragraphs are identical, as background to my questions. However, I have different questions on the two posts I made.

This post refers asks whether I should bother insulating the joist after the rim joist, as I can't get to the rim joist along most of this wall. The other post refers to whether to scrunch wall-high batts or cut their tops off, to re-insulate arong the rim joist.
 
  #4  
Old 08-22-11, 07:20 AM
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Guess I could have read it more closely.
The ledge you're referring to is most likely the sill plate that sits on the foundation wall and the isolates the joists.

Your main challenge here is to prevent infiltration of the air trapped between the rim joist and first floor joist into the living space. I assume you're insulating a basement and not a crawlspace. If you can do that, further insulation inside the floor joist won't accomplish much if the rim joist is tight against the plate. The reason is that the insulating products you're employing use trapped air as their primary thermal barrier. As long as there isn't significant exchange with the outside, which you can easily check with a candle flame, sealing the opening between the floor joist and the foundation wall will be as effective as applying insulation to the rim joist.

Good luck with your project.
 
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