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learmoia 10-09-2012 08:36 PM

Rim Joist insulation options?
I'm getting ready to install a ceiling tile system in my basement, and friends have suggested that I install 'more' rim joist insulation before I do so..

The concept makes sense but I'm not sure what route I should go.

Currently I have 3/4" rigid (expanded) foam installed between the rim joists (unsealed, but I can fix that).. this was existing when I purchased the house..
Also I have exterior mounted 1/2" foil faced installed when new vinyl siding was installed. (The 1/2" foil faced was taped at the joints and extends down to the concrete foundation (covers the rim joists on the exterior)

So total insulation is:
1/2" foil faced - Rim Joist - 3/4" expanded foam board.

My friends have said.. "Just get some batten insulation and stuff it in there"..

Well everything I read says don't do that and note potential mold issues and reduced effectiveness... But all the examples assume you currently have no insulation... which I do have..

With the existing insulation in place will that batten work?, Or am I in for trouble down the road? or should I go the extra step and install 2" rigid foam..

Thanks for the input! ~Ian

cleveman 10-09-2012 09:28 PM

I think you have a decent thermal barrier in place and should proceed with the batts.

I regularly have 1" on the exterior and 1" or 2" on the interior (depending on scraps), then I put in some batts.

jklingel 10-10-2012 12:09 AM

Batts, esp fiberglass, are a poor choice here, as they would be exposed to air currents, killing their R value. Depending on where you are, more rigid or spray foam would be my suggestion. Too, you want to be careful sandwiching wood between layers of foam. If too thick, moisture is slow to leave the foam and can condense on the wood if it gets cold. Pick a side and insulate it; air seal, as you suggested you did on the outside.

Windows on Wash 10-10-2012 06:51 AM


Spray foam and seal all the gaps in the EPS board that is in there now.

Install a Roxul batt in there as it will be a much tighter fit and will not suffer convective losses.

Gary in WA 10-11-2012 12:02 AM

Caulk/canned foam the joints at rim to floor sheathing/mudsill to stop air movement. No rigid foam board inside, unfaced insulation only, fig 3:

Here it is again, fig. 10:

This shows foam on both JK said, Fig.2;

Because the wood rim (if solid wood) expands and contracts with the seasons, this cycling movement could break a shallow canned-foam seal around rigid foam board. Using spray foam is different because it would be 3" deep (about 3 times thicker than other), totally stopping any infiltrating/exfiltrating air. I would use 0.50# open-cell foam (SPF) because it "gives" more with movement with solid wood. If the outside foam board was perfectly applied (no air leaks; totally bonded), either foam would work.


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