Conceptually, the Passivhaus is a simple idea for a building standard. in a recent post I wrote "anyone can understand the basic concept of piling on the insulation, sealing up tight, careful siting and good ventilation."
In fact, the reality is not so simple; the Passivhaus standard has target numbers that have to be reached, and it has to be modelled in a software package called PHPP (Passive House Planning Package) to show that it can hit these targets.
The Passive Park House in Seattle that was the subject of the New York Times article is instructive. Writer Sandy Keenan never gave credit for the design to NK Architects; I did in TreeHugger but in fact, it takes more than just an architect, it almost takes a village of consultants to get these things right.
Rob Harrison, another Seattle architect with Passivhaus training, acted as a consultant to NK on the project and explains what a Passivhaus consultant might do:
Here is an example of how a Passivhaus consultant interacts with the designer: NK's first pass at the design showed a 4' wide x 20' high window in the north-facing stair bay. In order to avoid moving to an even thicker external layer of insulation (say 11 7/8" TJIs outboard of the 2x6 structural wall instead of the 9 1/4" TJIs we used) which would have meant losing more floor area on the tiny site, I had to find ways to reduce the heat loss through glazing. Since we were already using the highest performance triple-glazing offered by Intus (Ug=0.105, SHGC=0.62) that meant adding south glazing and reducing north glazing. I proposed and NK executed my idea of chamferring a set of deep-set punched windows instead of the tall fixed window on the bay, using CFC-free polyiso to maintain a decent average R-value in the assembly.
Here you can see the results of Rob's architecture training at the University of Toronto, (my alma mater as well) where Professor Pragnell taught us that the frame of a window and the view it contains is as important as the glass. Rob has essentially put big picture frames around smaller windows. It's a balancing act; architectural detailing takes the place of the large sheet of glass that might have otherwise been used if they didn't have to hit the numbers.
NK Architects did a great job in maximizing the building volume available on the tiny 1,980 SF site, deftly and completely filling the entire zoning envelope in order to accommodate the owners' program. Doing so required complicated massing, which is not conducive to Passive House in a number of ways, including the additional thermal bridging and surface area it created, and necessitating standard 16" on center framing rather than the advanced 24" I had suggested at the initial charrette. At the extremely low levels of energy use of Passivhaus, that meant a performance hit of almost 10% that then had to be made up in other ways.
The siting of passivhaus projects is critical; solar heat gain and heat loss through windows are a significant part of the calculation. Doing it on an urban lot is complicated, so if there is less heat gain through the windows, you have to add more insulation to compensate.
Rob explains, with a great conclusion:
The front of the house (shown the the photos) faces north, and it is shaded by trees east and west, and by a tall house immediately to the south, limiting the solar gain available through the windows. Those design features and site constraints meant that we needed more insulation and higher performance windows than we would have otherwise, but the fact we were able to achieve Passivhaus with this high-design project suggests that there is no excuse for not doing Passivhaus on every project, everywhere.
I asked Bronwyn Barry, a Passivhaus consultant who worked on the project, how hard it is to design a passivhaus project. She said:
The learning curve is steep, but once you have done a few it becomes way easier. The thing is to know what to look for.
Bronwyn Barry has noted that "Passivhaus is a team sport" and the Park Passive is a good example. Outside of NK Architects and their usual engineering sub-consultants, we have Rob Harrison as a Passivhaus consultant, Bronwyn Barry doing detailed THERM analysis, with it all certified by the Passive House Academy out of Ireland. (Why Ireland? that's another story.) Perhaps one reason that there is a cost premium on Passivhaus is that there are so many mouths to feed.
The idea of a Passivhaus is simple; the execution (and the paperwork) is obviously not. I certainly hope that as Bronwyn notes, it gets easier for us all to know what to look for, both as clients and designers.