In the U.S., designers of cutting-edge superinsulated homes generally recommend 2 to 6 inches of rigid foam insulation under residential slabs. For builders who use extruded polystyrene (XPS), the most commonly used sub-slab insulation, that amounts to R-10 to R-30.
As Alex Wilson recently reported, “Building science expert Joe Lstiburek … argues that for any house north of the Mason-Dixon Line we should follow the ‘10-20-40-60 rule’ for R-values: R-10 under foundation floor slabs; R-20 foundation walls; R-40 house walls, and R-60 ceilings or roofs.”
For reasons that are somewhat murky, however, Passivhaus builders install much thicker layers of sub-slab insulation than most superinsulation nerds.
Passivhaus buildings have very thick sub-slab foam
Passivhaus designers use an oft-praised software package developed with the help of German physicists — the Passive House Planning Package (PHPP). The PHPP software helps designers determine how thick insulation needs to be for a house to achieve the Passivhaus standard. Among the key requirements of the standard: the house must have a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot) and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot).
To meet the standard, Katrin Klingenberg, the founder of Passive House Institute U.S., installed 14 inches of expanded polystyrene (EPS) insulation — 7 layers of 2-inch foam (a total of R-56) — under the slab of her home in Urbana, Illinois. The Waldsee Biohaus, a Passivhaus language institute in Bemidji, Minnesota, has 16 inches of EPS under its foundation slab.
What’s the explanation for these differing recommendations?
I recently approached engineer John Straube in hopes of satisfying my curiosity on the surprising disparity between the sub-slab insulation recommendations of North American physicists and Passivhaus advocates. John Straube is a colleague of Joe Lstiburek at the Building Science Corporation,…