Tolerances and techniques for installing sheet foam below slab
dsmcn
| Posted in GBA Pro Help on
Zone 4A. I will be adding 2″ of 25 PSI foam below my slab, across a width of 40′, enclosed on all sides by Superior Walls. Two questions:
First, how close must the flatness tolerance be between compacted crushed rock and foam?
Second, please offer suggestions & tips for achieving this tolerance. If screeding is suggested, please be specific about how to set up screeds across a distance of 40′.
I have seen several articles here and in Fine Homebuilding about installing foam on walls, but nothing very detailed about installing below a slab.
Thanks to all.
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Replies
David,
When I've done this work, I've specified 3/4-inch crushed stone -- not larger -- because 3/4-inch stone is easy to rake. I rake the stone by hand, using a garden rake, and verify that the crushed stone is level with a long, straight 2x4.
I simply put a 4-foot level on the 2x4 to see how I'm doing. You can use the 2x4 as a straightedge to locate hills and dips.
If you pay attention, these are all the tools you need.
Note, for a 3-4" residential slab you don't need anything like 25psi foam- 15psi still has huge margins over what it would actually experience, even if you regularly host crowded polka-parties where the participant jump up and down with with kegs on their heads, or parked your Caterpillar D10 in the living room.
Dana,
Roxul ComfortBoard IS (8 lbs./cu.ft.) is now recommended for sub slab insulation (ASTM C 165
745 psf (35.5 kPa) at 10 %, 1270 psf (61 kPa) at 25 %). Given your comment, do you think this is a good choice versus XPS?