A concrete first floor over a basement: how architect Steve Baczek kept the slab edge warm and airtight

Baczek said the owners liked how a concrete floor looks but would not give up the basement. So he borrowed from commercial work: a cast-in-place slab on fluted metal deck, carried by steel beams. He called the house Passive House inspired: the owners skipped certification and spent that money on the building. Baczek placed it in New Hampshire, while the episode's YouTube notes describe a 2019 home on the Maine coast.

The episode centers on the slab edge. Each steel beam bears in a pocket in the poured foundation wall, and the deck stops short of the outside face so rigid foam can wrap the slab perimeter. That puts the beams and the deck, which Baczek called the "two worst culprits" in the assembly, on the warm side. The structural engineer asked for 60 psi foam instead of the usual 25 psi. Baczek thought that was overly cautious, since the foam carries almost no load. The owners ruled out spray foam, so the wall has board foam and a rock wool batt below a double-stud wall filled with 16 inches of cellulose.

The crew laid sill sealer on the foundation, ran a sealant bead on each side and set the mudsill on top. Another bead joins the plywood sheathing to the sill and the concrete. SIGA Majvest, a self-adhered membrane, bonds to the plywood, which makes it part of the air barrier. Baczek said the house tested at about 0.38 ACH50. Risinger noted that a builder in Boston has to test at 3 ACH50 or lower. Baczek rates a result under 1.0 as good and under 0.6 as great, but said a single 2-inch gap next to cold steel or concrete can lead to condensation there.

Outside, untreated 1x3 furring leaves a 3/4-inch rainscreen gap. Baczek said about 1% of rain gets behind the cladding and that a gap wider than half an inch both drains and dries. The gap is vented at the bottom and the top. Risinger said he once vented only the bottom of a rainscreen and had to add top vents later to fix problems.