Flooring: On-Grade Concrete Slab
Dear, GBA Community,
I have an on-grade concrete slab (it sits 1-2′ above the ground) that doesn’t seem to be insulated underneath and is damp proofed using cement-based stucco. In late spring and early fall, we have the classic “basement smell” on our first floor due to the humidity present in our home. My goal is to (1) eliminate moisture penetration through our floor (both ways: coming up from the soil underneath the foundation and moving down into the concrete from the floor), (2) provide some modest insulation on top of the slab, and (3) apply an engineered floor on top. Can you recommend a sustainable solution, achieving a maximum material thickness of 1” or less? In addition, I only have about 3’ of space on the sides of our home, so I’m looking for good underground sloping solutions to keep the moisture away from our foundation. We currently use this space for small gardens, and we would love to keep them, if possible.
Best,
Blake
Richmond, VA
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Replies
Blake, I can't recommend anything that will keep you to 1", but on several occasions I have put down 3/4" or 1" sleepers over a vapor retarder, installed rigid foam between sleepers and a 3/4" subfloor on top of that. It's not as well-insulated as some systems but better than others, and it's pretty easy to do in as little as 1 1/2" (plus finished flooring).
Alternatively, this is a decent product: https://dricore.com/products/dricore-subfloor/. They also have an insulated version, but it uses climate-damaging XPS foam and would probably cost you less to go with the sleeper system.
Michael,
This is incredibly helpful. A local green consulstant that I appreciate recommended rigid foam exclusively as my underlayment, but cautioned against the "give" of the material. The sleepers seem like it would solve this issue. (And thanks for the heads up on Dricore. It's good to weigh my options as I negotiate the costs of this project.)
Do you have a preferred vapor retarder?
All the best,
Blake
Blake, in almost all cases, foam rated for 15 psi or higher will be fine under sleepers, with essentially no give. I go into the math a bit here: https://www.finehomebuilding.com/2019/02/27/minimizing-concrete-in-a-slab-on-grade-home. I only put the sleepers directly on the concrete when the assembly height is an issue. It's better for comfort and mold resistance to run the foam under the sleepers (or doubled subfloor, as my friends "The Steves" did here: https://www.jlconline.com/how-to/foundations/a-basement-floor-without-concrete_o)
Pretty much any 6-mil poly sheeting will meet code as a vapor retarder. I usually spec a heavier-duty 10-mil product from Stego or Viper.
My brother used Dricore subfloor (non insulated) to finish his basement and he feels like it made a difference. He find the basement doesn't feel as cold and I have to agree with him when I compare to my basement which just has carpet and underlayment pad over concrete.
If you are doing a large area, it may be easier and very likely cheaper using a roll of something like DMX 1-Step (https://dmx1step.com/) or Delta-FL (https://www.dorken.com/en/our-products/products/residential/delta-fl.php) with either OSB or plywood on top.
Thanks,
Arnold
The spaces under Dricore should provide some insulating value--dead air is R-5.6/in, if there is no airflow or convective looping. It's just harder to ensure than with an insulation product. And the OSB portion is about R-1.2/in.