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, a professor of building envelope science at the University of Waterloo, and a very smart guy.

I asked Straube whether the differing recommendations resulted from uncertainties related to soil temperature measurements. No, Straube answered, the reason for the disparity lies elsewhere.

Is There a Cheaper Way To Do It?
As it turns out, the PHPP software never considers whether the incremental cost of thicker and thicker insulation is greater than the cost of an alternative method of meeting the home’s energy needs — namely, a photovoltaic (PV) array.

Straube explained to me that Passivhaus designers have only a few dials to turn when adjusting a home’s specifications. (Straube gave credit to energy consultant Marc Rosenbaum for the control-panel metaphor.) The Passivhaus standard does not allow the use of site-generated PV to help meet the 15 kWh per square meter and 120 kWh per square meter goals. Once the designers have specified the best available windows, for example, the window dial can’t be turned down any further. These houses are just about as airtight as buildings can be built, so the airtightness dial has basically been bottomed out. Once the easy dials have been turned, the only remaining variable under the designer’s control is insulation thickness — so, to make the house work, that’s what gets adjusted, even when the resulting insulation thickness is illogical or uneconomic.

Sometimes PV is Cheaper Than Thicker Foam
It’s easy to understand why many Passivhaus advocates disdain PV: the electricity produced by a PV array is very expensive. “In most of climate zones 5 and 6, PV (at $8 per peak watt for the total system) will generate electricity at around 50 to 60 cents per kWh, if financed by a 25-year 6% mortgage,” Straube said recently. That’s why many builders have concluded that PV arrays are toys for the wealthy.

Figures provided by Dr. Wolfgang Feist, the director of the Passivhaus Institut in Darmstadt, Germany, are very close to those provided by Straube. “At the moment, the cost of electricity produced by photovoltaics is in the range of 40 to 50 cents per kWh, which is still ten times the cost of electricity produced by oil or gas,” Feist told me in 2007.

What Feist failed to mention — but Straube went on to prove — is that the logic of the Passivhaus standard drives cold-climate designers to use sub-slab polystyrene that is even more expensive than an unsubsidized PV array.

The Cost of PV-Powered Heat
Before making his back-of-the-envelope calculation, Straube studied soil-temperature data. “A slab on grade insulated to R-32 in Finland had an average heating seasons soil temperature of 12.5°C (55°F),” Straube wrote in an e-mail. “Hence, during the heating seasons the average temperature difference between soil and indoor air is about 15°F.” In other words, the delta-T across an insulated slab is much less than the delta-T across an insulated wall — at least in cold northern climates.

If a home uses electricity to supply heat, a heat pump is obviously more efficient than a resistance heater. “If you account for a coefficient of performance of 2.5 for the heat pump over the season (3.3 in 40°F weather but 2 in -10°F weather), the cost of PV-powered heat is no more than 60/2.5 = 24 cents per kWh. Note that many central forms of renewable electricity production work at 25 cents per kWh, such as wind, microhydro, tidal, biomass, concentrating solar thermal, etc. So this seems like the high end of electric production costs.”

Once this number is known, it becomes a simple matter to calculate whether the incremental cost of very thick foam insulation is cheaper or more expensive than PV. Most builders would agree that it makes little sense to invest in foam when a PV array is cheaper.

It Makes Sense To Stop At R-15 to R-25
Straube wrote, “The cost of insulation becomes more than the cost of generating energy for the walls in a typical house in a 7,200-HDD climate at about R-60 (using the Building Science Corporation approach), and slabs [on grade] at about R-20 to R-25, depending the cost of placing EPS (which costs around 10 cents per R per square foot). Basements have less heat loss [than slabs on grade], so the cut-off point is more like R-15 to R-20 for a basement slab. Heating a slab with radiant tubes increases the temperature of the slab from around 68°F or so to 80°F or so on average, so the insulation levels need to be increased by about 50% over this for radiantly heated slabs.”

Building Energy Optimization (BEopt), an energy modeling program developed in 2004 at the National Renewable Energy Laboratory in Golden, Colorado, shows designers the “least-cost path” to building optimization by performing calculations similar to those made by Straube. The idea behind BEopt is that no building should have a PV array until the designer has implemented every building envelope measure cheaper than PV. Once this point is reached, an investment in PV may make sense. Certainly such an investment would make more sense than spending additional dollars on insulation — since the insulation would provide even less benefit per dollar invested than PV.

What About Maintenance Costs?
Those who have been following the discussion this far may have noticed a flaw in the “PV is better” argument: PV equipment and heat-pumps have a shorter life, and require more maintenance, than sub-slab insulation. In fact, this point may be enough to convince some builders to choose 14 inches of foam over a PV array. It’s a defensible position, but it’s one that should only be made after considering the fact that the homeowners would get more bang for their buck from a PV array than from the last 10 inches of foam.