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My basement already has framing and insulation around the outside walls, along with a few outlets in them. If I were to do 3/4" OSB over 1" XPS for a subfloor the sole plates of those outer walls would be completely covered. Would there be any way to make this configuration work short of tearing out and rebuilding the outside walls?

subfloor cross-section

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There's no structural issue. How are you handling drainage to floor drains? – HerrBag Mar 28 '13 at 3:14
@HerrBag The only floor drain is in the utility room, which will stay as a bare cement floor. – Brad Mace Mar 28 '13 at 17:20
You should seal the edge of the XPS (around the perimeter of the utility room) to dam in any leaks and keep any leaks from getting under your new floor. – HerrBag Mar 28 '13 at 17:34
up vote 4 down vote accepted

If you have open studs on the side walls and then want to add inside finish panneling or drywall to those walls after the flooring is in you would need to go along and glue/nail in a spacer block in between each pair of studs so that there is something to nail the bottom edge of the wall material to.

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Sounds like a good plan to me. Just fit some nailing blocks (2X4) snugly between the studs, couple of nails or screws, done. – shirlock homes Mar 28 '13 at 8:44
Do you really need to add nailing blocks? Presumably the drywall is supported every 16", do you really need to fully support the the bottom edge? Should you add blocking at every seam where two pieces of drywall come together? – Tester101 Mar 28 '13 at 11:09
@Tester101 It seems like you're right about the drywall. I was also wondering whether it would cause any problems for installing trim, or anything else I'm forgetting? – Brad Mace Mar 28 '13 at 15:15
@BradMace The trim should be able to attach to the studs as well, though you may run in to a bit of trouble hitting them unless you locate them first. – Tester101 Mar 28 '13 at 17:17
@Tester101 -- I wouldn't dream of installing drywall along a floor without edge nailing support for the drywall runs between the studs. Unless of course the basebards are nice wide and sturdy boards like used 100 years ago. These days when baseboards are often nothing more than cheap plastic junk that is less than two inches wide it is just too easy to strike the drywall near the floor with a vacuum cleaner, stroller or someone's foot and cause a breakout of the material. The suggested backer blocks before the drywall installation are a cheap and easy safeguard against future problems. – Michael Karas Mar 28 '13 at 19:47

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