Maine Passive House: Windows and Blower Door ~
Work continued in October on the Passive House project in Knox, Maine, under construction by Chris Corson and EcoCor Builders. This month saw the installation of the home’s high-tech Intus windows, manufactured in Lithuania. Given the Passive House standard’s strict energy efficiency requirements and detailed modeling, window energy performance is a big deal for this project. “The windows are important not only from an air-tightness standpoint, but also from a thermal resistance or conductivity standpoint, and also for their solar heat gain, generating actual BTUs,” said Corson. “And the biggest thing is the comfort standard. These windows with these high R-values allow for there to never be a fluctuation of more than 3 degrees Celsius, or like 7 degrees Fahrenheit, between the ambient temperature and the surface temperature of the window itself.” Window unit thermal resistance (R-value) or emissivity (U-value) has its own Passive House standard, Corson says: “In order to meet the Passive House standard for windows, the window has to be less than or equal to .5 watts per meter squared Kelvin, so that’s equivalent to a whole window U-value of about 0.14. Now if you have windows that don’t quite meet those criteria, you can still have a Passive House. But then you would have to address the windows in a traditional manner — like supplementing point-source heating underneath windows to combat condensation. And it would definitely increase the potential for convective current to occur because of temperature differentials, not because of air leakage. But you could actually build a Passive House with windows that performed slightly worse than that.” Corson’s Intus windows could pass the Passive House window standard, says Corson, but they haven’t been formally certified by the Passive House program. That’s by Corson’s deliberate choice, he says: official Passive House testing costs money and adds to a window’s price tag. “I chose a window where I could get the values for the metrics of the spacers, the frame, and the glass separately and just do the math. And by those calculations, they’re right on the cusp — exactly .14 U-value.” Corson is now a U.S. product rep for Intus windows, after meeting the company at last year’s Northeast Sustainable Energy Association ( NESEA) conference in Boston. “I was specifically shopping for windows for this project,” he says. “I was looking for a European-made U-PVC window that had the values that I needed and was relatively green, without paying an arm and a leg. There were some companies that met those criteria, but not to the level that Intus did — and their price point jibed with my business model, which is making Passive House affordable, by building these homes at such a price point that it’s almost stupid to not build one. That seems to dovetail with their goal of bringing Passive House windows to a market that isn’t million-dollar or half-million-dollar build outs, like some of these other Passive Houses that have been built already. It kind of fit with what I was doing.” Window characteristics aside, installation is also a critical part of the process. Before setting the windows (a major feat of strength in itself, given that the 7-foot by 6-foot triple-glazed units with one large operating pane weigh upwards of 350 lbs. apiece), the crew installed Vycor modified-bitumen flashing tape on the interior windowsills, and 3M tape on the exterior sills, for water management purposes, lapping the outboard edges of the tape over the home’s Tyvek drainage plane. And after screwing the windows to their rough openings (Intus windows install with metal brackets, not with a window flange), the crew took extreme measures to make sure that the window perimeter was tightly air-sealed, using expanding injected foam insulation as well as 3M construction tape to seal the joints. By now, the house is taped up like a Christmas present from a compulsive mother: tape on every wall sheathing joint, tape between the frame and the foundation, tape where the walls meet the second-story ceilings, and tape on the underside of the ceilings themselves. But the results paid off this Monday, when a third-party verifier arrived to test the shell’s air-tightness using a blower door. The Passive House airtightness spec is the tightest in the industry: 0.6 air changes per hour at 50 Pascals of pressure. “With just the shell, no insulation, and conservative volume calculations — although I did measure everything pretty precisely — we are at .545 without any cellulose or anything,” Corson reported jubilantly. “Just the OSB.” Clearing the bar at this stage means that the additional value of dense-blown cellulose in the cavities on both sides of the OSB air barrier will be just icing on the cake. “We were running around to chase leaks, but there was no place to find the leaks,” Corson says. “The windows aren’t leaking, nothing’s leaking. I really think half of the measured leakage was the blower door. There were more gaps around the blower door than there were in the rest of the house. And this is just the rough shell. After we blow cellulose in all the bays, and drywall and tape and caulk and mud everything, we should be about as tight as it gets.”
