Slab Insulation Retrofit
Hello,
I have been hired to remodel a basement addition that was built on an existing slab. When the plumbers cut out some of the slab, we realized that there is only 1 inch of rigid insulation beneath the 4 inch concrete slab. The clients don’t want a cold floor, so we plan on adding rigid on top of the slab. The plan is for one inch of rigid on top of the concrete topped with 3/4 plywood as a sub floor. The exterior sliding door was installed to accommodate this plan. The clients haven’t decided finish flooring material yet but it will probably be some kind of engineered wood thinner than 3/4 inch.
My question is simple. There is no plan for sleepers since the height at the threshold won’t allow for it. How would I adhere the rigid to the concrete and how shall I adhere the plywood to the rigid without having sleepers to nail into? Its a small space, 250 square feet. Also, if i taped the rigid (unfortunately XPS) would this be a proper moisture barrier?
Thanks,
Stephen
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Replies
Stephen, I might glue the foam to the floor with appropriate construction adhesive, a type that won't melt the foam, then cover the surface with a vapor retarder, then use long Tapcons to fasten the plywood to the concrete. It will be slow but I would not trust a single layer of plywood to be only glued to the foam, and one inch of XPS is about 1.1 perms; 3/4" would be more like 1.5 perms, so not a good vapor retarder.
If you don't want to use that many Tapcons, you could use 3/4" foam and two layers of 1/2" plywood screwed together. That's what Andy Engel did here, in the article that inspired Steve Demetrick and Steve Baczek to eliminate the concrete slab entirely, which kicked off the current trend of concrete-free slabs: https://www.finehomebuilding.com/2005/03/01/the-no-mold-finished-basement
If you use 3/4" tongue and groove plywood or Advantech and stagger the joints the whole thing locks together and won't move. You still have to use fasteners but not as many. Three inch Tapcons will work. What I like to do is use three drills to make it go quickly: a cordless drill with a 3/16" wood bit and a 1" depth stop for drilling pilot holes in the plywood, a SDS drill with a 3-1/4" depth stop for drilling into the concrete, and a driver for putting the Tapcons in. Then it's just bang, bang, bang. Put enough fasteners in the first row of plywood that you can really bang on it without it moving so you can hammer the joints closed.
If you cut your first row of plywood to 3' you can put down a row of foam and then a row of plywood, rather than covering the whole floor with foam first. That way there's only about 1' of foam exposed at any given time and it doesn't get beat up from being walked on.
Usually when you raise the floor the rise of the stairs to the floor above becomes an issue.