It’s All Greek to Me: Moving Toward Passive House Design Involves a New Alphabet
By Helen Sanders
With building standards from the Passive House Institute (PHI) and U.S. Passive House Institute (PHIUS) becoming more prevalent in multifamily commercial and residential buildings, the terms “psi-value” (from the Greek letter , pronounced like the “psy” in psychology) and “chi-value” (from the Greek letter , pronounced like the “ki” in kite) are being heard more often. But what do they mean and what relevance do they have for the glazing industry?
One of the tenets of passive house design is the elimination of thermal bridges through the building envelope. Thermal bridges are paths of least resistance for heat to flow between the interior and exterior. These bridges can occur when thermally conductive materials penetrate the insulation of the wall or at poorly insulating interfaces between adjacent envelope assemblies.
The psi-value describes the thermal transmittance of an interface between two assemblies—a linear thermal bridge. A psi-value is like a U-factor, but instead of describing heat transfer through an assembly’s area, it describes heat loss through a linear interface. Psi-value thermal transmittances are therefore given in units of “per unit length,” such as BTU/oF.hr.ft or W/m.K, rather than “per unit area” as used for U-factors (e.g. BTU/oF.hr.ft2 or W/m2.K). A chi-value describes the thermal transmittance through a point thermal bridge, as caused by use of conductive fasteners. With no dimensional dependence, the units of chi-value are BTU/oF.hr or W/K.
According to Rick Ziegler at Morrison Hershfield, 13% of the heat loss through a typical steel stud wall with punched opening windows is due to the window-to-wall interface. It can be even higher with poorer edge details.
The International Passive House Standard requires knowledge of the psi-value of the wall-to-window interface (-install), the edge-of-glass (EOG) psi-value ( -spacer), and the frame and the center-of-glass (COG) U-factors. These values are needed for inputs into the Passive House Planning Package tool used by passive house designers to support performance calculations and certification.
This creates several challenges for North American fenestration manufacturers providing information for Passive House projects:
• North America’s thermal performance standard, NFRC 100, delivers an assembly U-factor, not separate frame, COG and EOG thermal transmittances.
• EOG performance is calculated as an area-based EOG U-factor (which is factored into the overall assembly U-factor) in NFRC 100, not as a linear perimeter psi-value.
• The calculation method and assumptions for passive house U-factors differ from NFRC 100.
This problem recently was solved with a calculation method for these Passive House metrics using LBNL’s THERM software, created by RDH Building Science and Peel Passive House Consulting, funded by the Government of British Columbia. This reference procedure and accompanying reporting template are available for free from the Fenestration Association of BC.
Helen Sanders is in strategic business development for Technoform North America Inc. based in Twinsburg, Ohio. Read her blog each month at usglassmag.com/insights.
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