An architectural cliché from the 1970s — the passive solar home with large expanses of south-facing glass — is making a comeback. In recent years, we’ve seen North American designers of Passivhaus buildings increase the area of south-facing glass to levels rarely seen since the Carter administration.
What’s the explanation for all this south-facing glass? We’re told that there’s no other way for designers to meet the energy limit for space heating required by the Passivhaus standard: namely, a maximum of 15 kWh per square meter per year.
Struggling to meet this goal, many Passivhaus designers have found that the typical triple-glazed windows sold in North America have U-factors that aren’t quite low enough (or SHGCs that aren’t quite high enough) for their designs to meet the standard. Because of this, these designers often end up specifying very expensive triple-glazed windows from Germany or Austria.
What about cost-effectiveness?
As I have often noted, these Herculean efforts to meet the Passivhaus standard pay no attention to cost-effectiveness. Even when designers find it necessary to invest in measures that are much more expensive than a photovoltaic array, they plow ahead because they have to meet the numbers dictated by the PHPP software.
These investments in very expensive building materials are probably a waste of money. An excellent paper by Gary Proskiw, “Identifying Affordable Net Zero Energy Housing Solutions,” looks into the cost-effectiveness of large expanses of south-facing glazing as well as the cost-effectiveness of low-U-factor windows. Proskiw, a mechanical engineer from Winnipeg, Manitoba, who specializes in residential energy issues, concludes that heroic window measures don’t pay worthwhile dividends.
Proskiw’s analysis and conclusions are fascinating and thought-provoking, and I believe that most designers of low-energy homes will want to read Proskiw’s paper. (I’d like to credit GBA reader Sasha…