In the main areas of the first floor, where the finished floor will be oak boards, the subfloor is being prepped by pouring gypsum concrete (aka gypcrete) between sleepers and over the pex radiant tubing. Gypcrete is an efficient thermal mass, is fire resistant, and, because it's installed as a liquid, is self-leveling. These were all good reasons to use gypcrete, but the main reason we used this method rather than the pre-routed plywood we used on the upper floors is that gypcrete is a FEMA-approved flood-resistent material. This allowed us to set the subfloor below the base flood elevation (BFE) with the finished floor set exactly at the mandated elevation, preserving an extra 1-1/2" of ceiling height.
While the gypcrete dries enough to be walked on in 24 hours, it "cures" at 1/8" per day. With 1-1/2" of material -- or 12 days for curing -- we can start the flooring installation the first week of March.
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