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Crawlspace Insulation and Vapor Barrier


kramttocs's Avatar
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01-12-15, 06:32 AM   #1  
Crawlspace Insulation and Vapor Barrier

I am insulating and working to reduce the humidity in my crawlspace (built in 1970) and had a couple of questions.
I am using the blue dow foam board in 1in thickness. I went with the 1in vs the 2in due to ease of cutting it via the table saw and another reason I will explain below. I already have 2 inches of it in the rim joist cavities (minus the vents) sealed with spray foam and I have spray foamed the gap between the sill plate and foundation. In the crawlspace I can see 6-7 inches of the footer all around and then 2 feet of the foundation. I currently have 2 inches of the foam pressure fitted in the 2 foot portion (so two 2'x8' strips). I have a section in one corner of the crawlspace where the water line comes in/up that gets damp in the winter. Not standing water wet but leave knee indentions wet. There isn't a vapor barrier installed although I just bought a 15mil one that's in the crawlspace still rolled up. I am waiting to install it after a scheduled french drain is put in along that damp side of the house this spring. Once I do that and verify the humidity is down low enough I will put 3 inches of foam in the vents.
The second reason I went with 1inch was that I was thinking of adhering (pl300?) the first inch of blue foam to the foundation, bring the vapor barrier over the footer and lip and then about 6 inches up the foam, use mastic to attach it, and then adhere the second inch of blue foam to the remaining 18 of the first inch. I was thinking this would be about as good of a vp seal as I could get.

1. Is there any reason why I shouldn't do this (the vp between the foam)?
2. Do I need to water seal the foundation first? In all of my research I only found one mention of this and it may have been dealing with a basement.
3. Is pl300 the best product to adhere the blue foam to the concrete?
4. Any reason why I can't go ahead and adhere the first inch right now even though I don't have the vb installed?
5. There are two inch of concrete on top of the foundation before the sill plate that are uncovered and a little slick in a couple of corners. The foundation is 8 inches but the sill plate is a 2x6. Should I put a strip of foam over this as well? Or just go crazy with the Big Gap spray foam?
6. Should I put an inch or two thickness of foam on the 6 inches of exposed footer prior to the vb or is it far enough down that it won't make a thermal difference?


Last edited by kramttocs; 01-12-15 at 08:06 AM.
 
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01-12-15, 01:10 PM   #2  
Doesn't look like I can edit it anymore. I mistakenly put 'vp' in places where I meant 'vb' for the vapor barrier.

 
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01-12-15, 01:54 PM   #3  
Give me some time, long posts fall at the bottom of my list, sorry.

Bud

 
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01-12-15, 02:01 PM   #4  
Hey Bud!
No problem. I was hoping you'd chime in as I knew you had experience with this.

 
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01-12-15, 06:12 PM   #5  
1. Not a problem with a layer of vb in there, it just makes that 3" more resistant to vapor movement, which with your short walls is not a problem.
2. Foundations need the ability to dry in at least one direction, usually to the inside below grade. But, with 2' foundations, your walls will dry to the exposed upper area of those walls, assuming about 6" of exposed foundation. Attempting to seal the foundation on the inside should not be necessary. Any drainage issues on the outside should be addressed as always.
3. The tube I happen to have at arm's reach is Liquid Nails and includes foam board in the list of approved surfaces. Always read the label. I usually like to use a mechanical fastener or two so the foam stays in place while it hardens.
4. You can go ahead with the added one inch.
5. They usually show gluing a piece of foam to the exposed top.
6. Sounds like your frost line is not that deep so no need to go crazy detailing foam over that footing.

Bud

 
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01-13-15, 06:19 AM   #6  
Thanks Bud - I appreciate it.

 
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