Habitat for Humanity house in Charlotte, Vermont
Photo © Herb Swanson (Vermont Public Radio)

This Habitat for Humanity house in Charlotte, Vermont was designed to meet the strict energy use and airtightness metrics that govern the international Passive House standard. A new certification introduced by Passive House Institute U.S. might change the way some Passive House Buildings are modeled, designed, and commissioned in North America.

A new certification system combining the Passive House standard with the Home Energy Rating System (HERS) Index will issue its first certificates in January 2012, according to Katrin Klingenberg, cofounder and executive director of Passive House Institute U.S. (PHIUS). Called PHIUS+, the certification is separate from international Passive House certification, though projects still must meet the three modeled performance metrics that form the backbone of the international standard.

According to Klingenberg, PHIUS+ adds better quality assurance to projects by requiring that raters qualified by the Residential Energy Services Network (RESNET), creator of the HERS Index, conduct blower-door tests and insulation inspections. She says PHIUS+ is also intended to make Passive House buildings eligible for tax incentive programs as well as more compatible with other green building rating systems that reference the HERS Index.

How readily the new certification will be accepted by Passive House designers and the green building marketplace is so far unclear. Development of the independent PHIUS+ brand was one of three major points of contention leading to a schism in August 2011 between the international Passive House Institute (PHI) and PHIUS—a schism that has left many in the Passive House community uncertain about the future of the standard in the U.S.

“I think adding the HERS Index and alignment with RESNET was a great move on the part of PHIUS,” says Bronwyn Barry, Assoc. AIA, a Certified Passive House Consultant and director of One Sky Homes. However, she adds, “If PHIUS had simply required that the airtightness testing already required for certification be conducted by a qualified HERS rater, that could have been sufficient.” Barry believes that there is more to gain in staying connected to the greater Passive House community than by developing a separate system with no brand recognition.

Klingenberg continues to express disappointment that PHIUS and PHI have officially parted ways, but at the same time she argues that energy models produced by the Passive House Planning Package software program owned and distributed by the international body need to be “translated” for North America. Formerly, any Passive House building modeled in the international software could be rated using the HERS model, but the result was “bogus” due to differing assumptions embedded in the calculations, Klingenberg claims. She characterizes some of these assumptions as climate-related (for example, regarding air-conditioning loads) and others as cultural (relating to plug loads and indoor comfort levels).

In the past, she says, a building that was modeled as net-zero-energy in the Passive House Planning Package might receive an unexpectedly high HERS Index due to these differing assumptions (in HERS, a net-zero-energy home would score a 0). PHIUS+ attempts to “harmonize” the calculations by aligning the underlying assumptions, she explains. Passive House designers will still use the same modeling tools as always; the conversions will be made by project raters. Passive House consultants are unable to evaluate how accurate the new calculations are, as the first PHIUS+ projects have not yet been certified.

But according to Barry, changing the calculations on which Passive House certifications are based may have been one item too many for the Passive House Institute in Germany. “My personal hope is that PHIUS and PHI are able to reconcile their differences,” she says. “Most practitioners of Passive House don't care what the certificate itself is called. They are simply interested in building better buildings.”

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