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xps and rim

Posted by andrelaplume2 (My Page) on
Thu, Oct 2, 08 at 13:23

I was reading thru some posts re: XPS and the RIM.

Lets make sure we are talking about the same thing. My XPS will sit against the concrete. The top ledge of the concrete has the framing of the house resting on it...ie the joists sit here and run from one side of the basement to the other resting on an I beam in the middle. Is this waht you are talking about? I have fg insulation is all my joists throughout the basement...ie under the 1st floor. Its also in the nooks and crannies where the joists rest on top the concrete. Are you suggesting I remove the fg here and barracade the area with XPS? What is the down side to NOT doing that...1)its a lot of work and 2) what if I ever need wiring or piping run thru one of those....also a bunch of those have piping or dyer ducts etc running thru them already and that would be tough to XPS around....even a fire hazard no...plus would you not need to drywall over all those 16"w pieces?


Follow-Up Postings:

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RE: xps and rim

The 2x6 sitting atop the concrete foundation is the sill plate. The lumber that closes the space between the joists off from the outside is the rim joist. It's necessary to have this area insulated, but just as important is sealing air leaks, which this area is very prone to. Fiberglass insulation does not prevent air flow, and air flow almost renders fiberglass completely ineffective as an insulator. XPS is an effective air barrier if installed correctly, but as you pointed out, that's difficult and time consuming in this area. I had the entire perimeter sill plate/rim joist area spray foam insulated prior to finishing the basement. I used fiberglass to insulate the rest of the floor/ceiling area, and XPS for walls and floor.


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RE: xps and rim

I see. So by adding the XPS to my walls I am in effect contributing to the warmth of my basement. I can get it warmer by better insulating this rim joist area.

Now, Technically, the air/water vapour goes thru the rim joist into the home. I would think XPSing the rim joist area leaves a pocket where the air/water vapour gets in. It can not get into the basement though anymore since it hits the XPS. Would it not however become a mold / water issue for the rim joist and surrounding beams in the pocket. Are you saying the XPS get shoved all the way to the back against the rim joist...again this sounds like it could lead to water mold issues on the joist?

It would seem the house needs to breath somewhat down there no....


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RE: xps and rim

Any foam at the rim and between the joists is supposed to be covered by a 15 minute thermal barrier. Drywall on the ceilings and walls can serve as that barrier. However, where the basement is only partially finished, spray coatings can be used as well, even one now that claims to go on as easily as paint.

Or you can forget the XPS altogether and instead use a fibrous insulation and a "smart" vapour barrier such as Membrain.

Whatever you do, seal carefully and insulate the above-grade portion of the walls, as they can account for 30% of the heat loss you would experience from the entire basement wall. (See link at p.7)

Here is a link that might be useful: Basement Insulation Systems


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RE: xps and rim

"Any foam at the rim and between the joists is supposed to be covered by a 15 minute thermal barrier. Drywall on the ceilings and walls can serve as that barrier."

Does this mean that a drop ceiling does not provide the 15 minute barrier?


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RE: xps and rim

Fire-rated panels are available that meet Class A (or I) rating.


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RE: xps and rim

Do you allow the foam to fill the entire cavity if you use spray foam in the rim joist? Even if the cavity is deep due to a bay?


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RE: xps and rim

Though I notice other builders limit the spray to enough to block airflow, I fill the cavity to the thickness needed for the R Factor I'm looking to achieve.


pice:I&S Insulation Inc.

(This is not an endorsement of the above insulator.)

Here is a link that might be useful: Spray Insulation Pictures


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RE: xps and rim

The Recommended R-Values chart suggests R-8 in the "slab edge" for most every area. Do they mean the rim joist when they refer to the "slab edge"?


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RE: xps and rim

I'm not clear what table your referring to. The one I most commonly see for the US is this one from the Department of Energy.

The edge of slab is not the same as the rim joist. The rim joists should be protected by the same R-factor as the walls they're part of.


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RE: xps and rim

Is the purpose of XPS in the rim joist to provide insulation against the winter cold coming in, or is it to warm the wall to prevent condensation from forming due to the warmer inside? What impact might I see from the gaps and edges that I cannot seal properly?


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RE: xps and rim

Several of the rim joist bays are deep and run three feet or more out from the house. The floor of the bay runs parallel to the ground. Can I use left over XPS strips to insluate the floor of the bay or should I cut a new piece to fit? The strips will have gaps that I cannot reach to seal.


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