Writing from northeast Ohio, a reader with the screen name “User-6877304” — let’s call him Steve — is seeking comments on his plans to build an affordable net-zero energy home. The house, to be built on a 30-foot by 50-foot slab-on-grade foundation, seems to have many characteristics of a “Pretty Good House” — that is, it’s well insulated and ventilated but not attempting to hit the Passive House metric.
Steve plans to install R-10 rigid insulation beneath the slab. The house will have double-stud walls insulated with cellulose to R-40 and a raised-heel truss roof insulated to R-60.
“A simple gable metal roof,” he writes in a Q&A post at Green Building Advisor. “Solar panels. Basic interior finishes: nothing custom.”
His questions boil down to a consideration of specific features: the best heating and cooling option, windows, drain-water heat recovery, and whether a heat-recovery or energy-recovery ventilator is a good investment.
That’s today’s Q&A Spotlight.
Start with this document
Dana Dorsett suggests that Steve start by reviewing the recommendations in a Building America report authored by John Straube in 2010 and updated the following year. (For more information on this report, see R-Value Advice from Building Science Corporation.) Because the report is a few years old, Dorsett points out, the efficiencies of photovoltaic systems and heat pumps have changed somewhat, meaning that Steve can probably hit net-zero performance with the recommendations for Climate Zone 4 rather than Climate Zone 5 where Steve is actually going to build.
But, he adds, it’s important to remember the effect of thermal bridging.
“Note, those are ‘whole-assembly R-values,’ not center-of-cavity R-values, factoring in all the thermal bridging,” Dorsett say. “An R-25 wall could be a 2×6 / R-20 wall with 2 inches of exterior polyiso foam.…