XPS over ratslab
Hello all,
I was reading a discussion about polyuso on a basement wall and am looking at using the same technique for my crawl space in NJ. However during the discussion Dana mentioned insulating the footing before putting on a rat slab something I can’t do in m1950s built house.
So my plan was to run a 12 mil vapor barrier over the slab and 1 foot up the wall then cover the wall in R20 polyiso Rmax foam from Home Depot. But I am wondering if it would make more sense to add a 1″ layer of xps on the perimeter of the ratslab floor to help resist thermal bridging from the footing? I was thinking about 1 foot in all the way around the floor.
For reference my crawl space is 5 block high and I am in southern NJ along the east coast and below is the article where I was thought of this idea.
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Replies
Chris,
If you want to install some horizontal rigid foam insulation under your rat slab, there is no reason why not. You're right that you should start at the perimeter (although there is nothing wrong with installing horizontal rigid foam under your entire rat slab if you want to).
The payback period for rigid foam under a rat slab in your climate (New Jersey) is likely to be quite long, but that doesn't mean that you shouldn't do it.
One more comment: most green builders try to avoid using XPS, because XPS is manufactured with a blowing agent with a high global warming potential. EPS is preferred.
Thanks Martin for that comment about XPS I'll see what Home Depot has for EPS. Just want to clarify my rat slab is already installed so there is no way to get under it anymore. I am trying to determine if just putting some insulation on the top of the ratslab adjacent to the interior foundation walls would be worth it. Or if I should just do the walls.
Chris,
If I were you, I would probably just insulate the walls. Here is a link to an article with more information: Building an Unvented Crawl Space.
Home Depot carries 3/4" thick EPS pre-cut into 14.5" wide strips. The cost is pretty minimal, the labor is easy with it pre-cut, and it would offer some benefit to use that as you suggest.