Adding Flooring Over Uninsulated Slab
I’m renovating a room in my 1957 ranch built on a slab foundation (at grade), located in Massachusetts, climate zone 5A. Given the age of the house, I’m making the assumption there’s no insulation or vapor barrier below the slab.
The current floor is carpet, subfloor, 2×4 joists, slab foundation. I plan to replace this with an engineered hardwood floor.
My plan for the new floor: 6mil poly vapor barrier, 1.5″ xps, a second layer of 1.5″ xps rotated 90 degrees and glued to the first and seams taped, 3/4″ plywood floated over xps, a second layer of 3/4″ plywood rotated 90 degrees and screwed to the first, underlayment, 3/8″ engineered hardwood nailed to subfloor. Spray foam insulation applied along the edge. See attached detail.
1. Is the poly vapor barrier needed and what’s the proper way to terminate it at an existing wall? Is there reason to be concerned that I might drive moisture that enters the slab toward the wall/sill plate?
2. I’m planning on two layers of xps because 3″ xps is harder for me to acquire. Other than additional labor, are there reasons I shouldn’t take this approach?
3. Due to the presence of water piping in the slab, I’m not comfortable securing the subfloor through the foam into the slab with tap-cons. The plan is to float the subfloor on the xps. Will this work? Do I risk squeaky rigid insulation noise?
4. Any other advice or observations are greatly appreciated.
Thank you all for you expertise and advice!
GBA Detail Library
A collection of one thousand construction details organized by climate and house part