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 “High R-Value Enclosures for High Performance Residential Buildings in All Climate Zones.” 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 $4.50 or $5.00 per watt…
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