To reduce energy use, green builders often install above-code levels of insulation. Thick insulation is expensive, however, so it’s sometimes hard to know how much insulation is optimal.
To help guide builders wrestling with R-value questions, I wrote an article in May 2016 (“How Much Insulation Is Too Much?”) reporting on R-value recommendations from three energy experts: David White, Marc Rosenbaum, and Rachel Wagner.
A few years earlier, building scientists John Straube and Joseph Lstiburek (along with several other co-authors) addressed the same questions in a paper called Produced by the Building Science Corporation (BSC) and published by the Building America program, the 2010 paper included an oft-reproduced table with R-value recommendations for all climate zones in North America. As might be expected, the colder the climate zone, the higher the recommended R-values (see image below).
Although these BSC recommendations were interpreted by some readers as appropriate for net-zero-energy homes, the authors of the paper did not make that claim. Straube and Lstiburek didn’t connect the recommendations in the table to net-zero-energy home design (although they did write that their recommendations were appropriate for those with a “desire to provide new homes that will be ready to be powered by renewable energy sources immediately or in the future”).
The dropping price of PV
Some net-zero builders who read the 2010 paper nevertheless used the R-value recommendations in the BSC table as a starting point for envelope design. A few experts have recently questioned these R-value recommendations, however, in light of the steeply falling price of photovoltaic (PV) modules. In the seven years since the paper was written, the installed price of a residential PV system in the U.S. has dropped from about…
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