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Insulation Between Rim Joist and Top of Foam Insulation [basement]

Insulation Between Rim Joist and Top of Foam Insulation [basement]


  #1  
Old 04-14-14, 11:44 AM
J
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Insulation Between Rim Joist and Top of Foam Insulation [basement]

I am in the beginning phases of finishing my basement, newer house with poured cement foundation and engineered I-beam floor joists. No moisture issues in the basement.

Rim joist currently has a batt for insulation and that's it. My plan is to replace that with 2" XPS and foam spray and then another batt (probably just reuse what's already there).

On the foundation walls, I am going to use a thinner XPS all the way up to the joists, frame out with 2x4 and insulate with batts, then drywall.

I think I have a good understanding of these 2 parts, but am not sure what to do where they intersect. It seems to me that if I don't somehow seal the top to the foam wall insulation it will allow air to flow in behind the rim joist insulation. I've not seen this particular issue addressed anywhere, so maybe it's not really a problem? I'm trying to err on the side of extreme caution since I am doing this myself, and dont want to headaches down the road.
 
  #2  
Old 04-14-14, 12:25 PM
B
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Figure 10 in the link below may help.
Only comments I can add would be air sealing as you mentioned and termite inspections.

You want to be sure that warm humid air does not reach a cool condensing surface.

In areas where termites are abundant, some states have requirements that the wood rim be accessible for inspection. I've never had to do that, but the description I read seemed to create a removable rigid foam panel over some extra batt insulation.

Bud
BSD-103: Understanding Basements — Building Science Information
 
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Old 04-14-14, 01:38 PM
J
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Thanks Bud, also great web site!

Quick follow-up if you don't mind, looking at this picture:
http://www.buildingscience.com/docum..._figure_02.jpg

The red lines indicate 'sealant, adhesive or gasket' what would you recommend using here?

(image found here)
 
  #4  
Old 04-14-14, 02:32 PM
B
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Most any caulking that is compatible with foam board would be fine. I use a construction adhesive (liquid nails) primarily because it is inexpensive in the large size tubes, and I can use it almost anywhere. When caulking the important part is to do 100% of the seams as you can't really tell what is beyond them.

Note their sequence of placing the foam board against the wood. That way, if any air gets through it will find a warmer surface and do no harm.

Depending upon local codes, some will want all foam covered with a fire barrier. Dow, thermax I think, has a foil faced foam board that will meet those requirements, maybe. The maybe is beacuse each jurisdiction can follow or make different rules. Where you are installing studs and then probably drywall that area is all set. Generally what is above that is less important.

Bud
 
 

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