At the Passivhaus job site in Falmouth, Massachusetts, architect Steve Baczek specified the installation of 10 inches of EPS under the slab-on-grade foundation. After the concrete had been placed, more rigid foam was installed above the slab, to bring the finished floor assembly to R-50.
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.
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GBA Pro members have access to all of the videos in the Passive House video series.
Here is the link to the “Superinsulated Slab” video.
To see Episode One of the series, click here: “Passive House Design.” (The first episode is available to all GBA readers, including non-members.)