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algored2deth 04-30-2011 10:56 AM

Concrete in crawlspace
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I am in the beginning of a project where I am working on trying to build stud walls to cover XPS foam in a crawlspace (XPS to be installed later). I have a picture attached of what I am doing wrt concrete work. Note that these walls are NOT load bearing*. Just a means to cover the XPS. Concrete will be about 4" thick and roughly 6.5" wide. The 6.5" width allows me to install 2" XPS and 2x4 studs w/ 1/2" drywall in front of it so that I can add extra insulation in the empty cavities. Crawlspace is about 4 ft high. It is also sealed and I have no water leaks at all. It is a fairly dry crawl beyond the normal seasonal changes for humidity. There will also be a vapor barrier applied to the floor but that is for later. My question is really about concrete.

You will notice in the pic attached that the crawlspace floor appears to have concrete on it. This is true but the quality is pretty horrible and the floor is uneven. It came apart rather easily with a demo hammer. I have done the prep work to excavate a "moat" around the perimeter and made sure that I can lay a somewhat consistent thickness of concrete when I put the wood forms in place. I have done the best I can to compact the soil to minimize any settling.

My question about the concrete is this:

1) Do I need to put in rebar for something like this? I had thought about putting in "pins" vertically every two feet or so. This would give the concrete something else to grab to in order to minimize cracking. It is a sealed crawlspace and I don't expect wild temp swings where soil movement is a problem. I realize all concrete will probably crack to some extent but just trying to prep ahead to minimize this.

2) I also thought about if I do put in rebar pins, I would also run some other wire horizontally, pin to pin, for further strength. While this may be overkill, I just don't know if it is needed? The amount of work to add rebar is low and the cost is not much.


PS: * When I say not load bearing, it is not a part of the house's structure. It will have to hold up the new stud walls, XPS boards, drywall, tape, mud, etc. No significant weight here.

Gary in WA 05-02-2011 02:37 PM

What is the end result of your proposed work?

If you are conditioning the crawl space, just glue rigid foam to the concrete, faced with foil may pass your Building Inspector. Use 2", R-10 which is suggested for your location. Don't need batts or frame wall with your interior drain system exposed to mold them.
If unused, don't worry about the concrete cracking....


algored2deth 05-02-2011 02:48 PM

End result is to insulate the walls. Bldg code here requires that I cover XPS with drywall. Same with the foil faced stuff too. The product says to cover with a thermal barrier.

Ron6519 05-02-2011 03:18 PM

I'd put insulation on the floor as well as the walls. When I built the addition to the kitchen, I laid down 2" foam board with 6 mil plastic under and over it(as per the manufacturers recommendation). I just put down 3" of concrete over the insulation. It more then qualifies as a rat slab and the only weight it's seen is me.

Vincer 05-03-2011 01:28 PM

The concrete on the floor was probably there to prevent rats and other pests from infiltrating the house from below. My house has that in the crawlspace as well. And I found two dead rats ON it. One was fresh, the other might have been there for decades. So much for "rat proofing!"

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