In Part 3 of a five-part series on building a Passivhaus, architect Steve Baczek explains how he and his crew designed and built a concrete slab insulated to R-50. He chose to use 10 inches of EPS under the slab: a 4-inch layer atop a 6-inch layer.

Using foam this thick meant developing some new techniques. The crew used a reciprocating saw rather than a tablesaw or circular saw to cut the foam; they drilled a pilot hole for each penetration, then drilled from both sides of the foam with a spade bit; and they tacked the inside pieces of foam to the first layer of foam to keep them from shifting.

After the slab was poured, workers bumped up the R-value of the floor by fastening pressure-treated 2×4 sleepers to the slab, and then fitted pieces of rigid foam between the sleepers and filled remaining gaps with spray foam. Only then did they install the subfloor.

Watch the video above; read the companion Fine Homebuilding article at right; and then join the conversation with the designer of this house, Architect Steve Baczek.

Over the course of the next several months — as each new issue of Fine Homebuilding magazine is released — this collection of articles and videos will cover:

Passive House design

Airtight mudsills

Superinsulated slab

Double-stud walls and insulation

Windows and doors

To watch Episode 1 of this video series, click here.

To watch Episode 2 of this video series, click here.