Superinsulated Slab

Posted on Sep 22 by Rob Wotzak


Superinsulated Slab: If you aren’t insulating the edge, you’re only doing half the job

In part 3 of a five-part series on building a Passive HouseA residential building construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. Developed in the early 1990s by Bo Adamson and Wolfgang Feist, the standard is now promoted by the Passivhaus Institut in Darmstadt, Germany. To meet the standard, a home must have an infiltration rate no greater than 0.60 AC/H @ 50 pascals, a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot), a maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot), and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot). The standard recommends, but does not require, a maximum design heating load of 10 W per square meter and windows with a maximum U-factor of 0.14. The Passivhaus standard was developed for buildings in central and northern Europe; efforts are underway to clarify the best techniques to achieve the standard for buildings in hot climates., architect Steve Baczek explains how he and his crew designed and built a concrete slab insulated to R-50. He chose to use 10 in. of EPSExpanded polystyrene. Type of rigid foam insulation that, unlike extruded polystyrene (XPS), does not contain ozone-depleting HCFCs. EPS frequently has a high recycled content. Its vapor permeability is higher and its R-value lower than XPS insulation. EPS insulation is classified by type: Type I is lowest in density and strength and Type X is highest. under the slab: a 4-in. layer atop a 6-in. layer. Using foam this thick meant that Baczek had to develop some new techniques: using a reciprocating saw rather than a tablesaw or circular saw to cut the foam; drilling a pilot hole for a penetration, then drilling from both sides of the foam with a spade bit; and tacking 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-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. of the floor by fastening pressure-treated 2x4 sleepers to the slab, then fitting pieces of rigid foam between the sleepers and filling remaining gaps with spray foam. Only then did they install the subfloor.

[To read more of Steve Baczek's article from the April/May 2014 issue of Fine Homebuilding, click the link below.]

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