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insulating deep rim joist cavity - fill entirely with foam board or just 4"?

insulating deep rim joist cavity - fill entirely with foam board or just 4"?

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  #1  
Old 09-13-16, 04:33 AM
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insulating deep rim joist cavity - fill entirely with foam board or just 4"?

Is it okay to use just 4" thickness of foam board sealed up with caulk in a deep rim joist cavity? I worry about mold forming behind the insulation, but tedious to insulate a deep cavity one 2" piece at a time.

I have 22" rim joist cavities in my basement extending into the unheated garage area (upper image). I had fiberglass pink in the cavities (no vapour barrier had yet been installed) and mold formed on the insulation (no big surprise based on what I have been reading about insulating these spaces).

I only have 5 of these longer cavities the rest are about 5 inches, so I have been filling them with 2 x 2" thick pieces of foam board - 4" total thickness (lower image) as many recommend for colder climates (southwestern Ontario, CA)
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  #2  
Old 09-13-16, 05:53 AM
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Number one would be to be sure those cavities are air sealed. The rigid foam board with edges sealed may accomplish that.

Number 2 is to make sure no inside air can reach an exterior surface that is cold enough to create condensation. Using enough rigid insulation does this, as long as you don't add a lot of air permeable fiber insulation to the inside. Example, with 1" of rigid and 6" of fiber insulation the inside surface of the 1" rigid is insulated from the basement heat and may get sold enough for the air that reaches it to cause a problem. There is an article I can dig out about this ratio of rigid to inboard fiber insulation.

Your current mold formed because the fiber insulation allowed the air to reach the cold exterior surface, as you know.

4" of rigid is a good number and maybe 3.5" of Roxul over that. Check local code requirements about any thermal barrier needed over the rigid.

Bud
 
  #3  
Old 09-13-16, 03:27 PM
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Thanks Bud
I was thinking 4" or maybe 6" foam board at the front (interior) of the cavity only (2 or 3 pieces of 2" thick foam stacked) sealed around the edges with either tuck tape or silicone or both with nothing in the rest of the cavity - just leave about 16" of the cavity on the garage side empty so if by chance it fails, I don't have a bunch of moldy fiberglass pink on my hands. You think it is better to fill that remaining space with fibre?

thanks.
 
  #4  
Old 09-13-16, 03:51 PM
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What's on the bottom of those cavities? Your description sounds like you are proposing just the end of the cavity to get 6" of rigid. Bring me up to speed?

Bud
 
  #5  
Old 09-15-16, 11:34 AM
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There are at least two layers of drywall on the bottom of the cavities. They extend under a bump out (originally a closet, now a built-in area with shelves) into the garage. The thing is, I don't know what, if anything is on the outside of the drywall - I cannot tell in the garage, there is a gap between the bottom of the bumped out bit and the floor - about 10 inches - I could drill a large hole to find out what is there I suppose. There is stucco on the outside in the garage. I doubt it matters, but we insulated built-in from the inside upstairs when we converted the closet.

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So maybe fill these cavities entirely with foam board to be safe? Maybe even vacuum up all the mouse droppings first...
thanks!
 
  #6  
Old 09-15-16, 11:48 AM
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I'm not really picturing what you have, but the cavity we see just needs to be sealed at all seams and then install your insulation against the cold surfaces, which I assume are the bottom and the end.

Those mouse droppings are one reason I don't like to see any fiber insulation used unless it is enclosed on all 6 sides and well sealed. Some of the discoveries I have made have been totally disgusting.

Bud
 
  #7  
Old 09-22-16, 08:25 AM
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In the end I stuffed the entire cavity with foam board, using canned spray foam to fill gaps around each individual rectangle of foam. It was tedious, but I think this should all the problems: mice, mold, vapours, heat/cold. The first photo shows the 11 pieces needed to fill the cavity. The second shows sealing things up to start, and the third, sealing each piece. The forth shows where this cavity comes out in the garage. The trick of course is to pre-cut the pieces and work reasonably fast once the foam is sprayed, it firms up fairly quickly. Now to move on the concrete walls. Thanks Bud.
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