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rigid foam board insulation over uneven stone?


kyselad's Avatar
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11-30-13, 08:26 PM   #1  
rigid foam board insulation over uneven stone?

Our old house has a dirt floor crawlspace with uneven stone walls. We'd like to install a vapor barrier over the floor and insulate the walls. Since uneven walls and foam board don't mix, it seems the "correct" solution would involve spray foam on the walls. But we got a steep quote from a spray foam company; the space is too low to fit their equipment, so there would be a lot of labor involved in digging out access.

As a DIY solution, I wonder whether we can't still use foam board. Could it be installed straight up and down, with the inevitable gap at the top sealed with canned spray foam? Or could we just frame a simple, but flat, narrow wall in front of the foundation and then insulate that with foam board?

 
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12-01-13, 09:09 AM   #2  
If you search
"Dow Styrospan and Roxul Batts"
The GBA dot com has an article that may provide some guidance. Unfortunately you need to sign up for a free 10 day membership and I have exhausted my free time. But GBA is a great source and they are discussing a similar issue.

Do some reading on Roxul as well as I have read where it can be used against basement walls. No first hand experience to help you.

Bud

 
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12-01-13, 11:33 AM   #3  
Interesting, I got to the GBA thread by looking at Google's cached copy without having to login to the site Seems people are warning against the fiberglass, but still don't think the foam board will work on the uneven surface, again suggesting closed cell spray is the way to go.

I'm now wondering whether it wouldn't be simple enough just to build a sort of wall + ledge out of foam board. The idea would be to mount foam board along the uneven wall, which would leave a couple of inch gap up at the top. I could then mount a narrow shelf of foam board over the top to span the gap between the vertical foam board and the rim joist. A bead of canned spray foam would then be sufficient to seal up the joint between the pieces of foam board.

This sounds very simple, but is it somehow unworkable? Any suggestions on how best to fasten the vertical board to the wall if I were to go this route? The rim joist is actually nice and flat, to fastening at the top is relatively straightforward.

 
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12-01-13, 11:51 AM   #4  
You will have to tell me how to access those cashed files .
One problem with the gap behind the rigid is the moisture and heat from below grade will create a circulation that pushes the moist air to the top where the foundation is colder. Frost can built up and later melt.

The mineral wool I mentioned (not fiberglass) might be able to be placed against the stone wall to fill that gap and minimize the circulation. Roxul is very dense.

There is a second source of moisture, being air inside the crawlspace. Your final solution needs to ensure no air flow to the cold stone wall.

Bud

 
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12-01-13, 12:53 PM   #5  
I agree with Bud. Again. Roxul is an air permeable fibrous insulation, be sure to use a membrane first (and interior drain-if needed- or it will pool), to stop air/moisture as said; BSI-041: Rubble Foundations — Building Science Information
The condensing plane is still on the stone (rather on inside face of fb), with the fb stopping any room heat to keep the cavity warm enough to raise the dew-point. It may appear dry now with the air currents wicking away moisture as it forms.

Gary
PS. regardless, cover the dirt ASAP with 12mil.

 
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12-01-13, 09:38 PM   #6  
undo

undo, posted in wrong thread

 
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