West Coast Passive House Meets Target
Olympia, Washington, is in a marine climate, but it’s a northern marine climate, not a beach-lounging one. Because Olympia has 5,130 heating degree-days (HDDs) on average—more than Pittsburgh, Pennsylvania—clients of the Artisans Group, a design/build firm in Olympia, want a strong dose of energy efficiency mixed into their high-quality custom homes. These days, what many of them are seeking is a Passive House (PH), including the owners of the North Project, a PH that was completed in 2011.
North Project Details
The North Project’s owners have been feeling mighty fine in their home, and they’ve got the data to prove it. Before it was completed, the North Project attracted the attention of Luke Howard, Michael Lubliner, and David Hales from Washington State University’s Energy Program. Lubliner secured funding from DOE’s Building America program for design analysis and simulation, which Hales conducted. Howard led the effort to set up a modest monitoring system so that they could see how the home’s performance would compare with the energy use predicted by the Passive House Planning Package (PHPP) and by REM/Rate, another energy-modeling program (see Table 1). They installed temperature data loggers in every room of the three-bedroom, two-and-a-half-bath home except for the powder room. In addition, thermistors were installed on the heat recovery ventilator (HRV), the water heater, and the heating coil.
The 2,300 ft2 home has a master bedroom and bath on the main floor, along with a kitchen, pantry, great room, mechanical room, and powder room, and an “away room” for watching movies, practicing drum solos, or any other activities that the whole family might not want to do together. Upstairs are two more bedrooms and a bathroom, plus a loft sitting area.
To meet the PH standard, Randy Foster and Tessa Smith of the Artisans Group built the North Project using a triple-stud wall assembly. They applied site-built Larsen trusses to a standard 2 x 6 wall 24 inches on center. For insulation they used blown-in dense-pack fiberglass in both the trusses and the conventional walls. The sheathing on the outside of the 2 x 6 wall serves as the moisture and air barrier. The sheathing on the outside of the Larsen truss is an open-diffusion fiberboard. The exterior cladding is fiber-cement paneling. A ¾-inch rain screen made of 1 x 2s that are applied as battens between the sheathing and the exterior cladding helps to keep the Olympia rain at bay and give any moisture that does seep in a space to dry out. The windows are all triple pane. Altogether, the wall assembly provides a total R-value of 49.
Testing and Results
Foster ran some insulation tests at this project, and at others, taking core samples of the blown-in fiberglass around the home to make sure the correct densities were being installed, and he was satisfied with the results. He has also conducted drill testing on other projects to verify that there has been no settling, and has found none. When he is working in more forgiving climates, such as that of the San Juan Islands, he prefers using cellulose as an insulation material, because of its lower embodied energy.
Air sealing at the North Project relied primarily on taping the sheathing on the 2 x 6 interior walls, using various Siga tape products. This strategy paid off, resulting in an airtightness rating of 0.46 ACH50. Retesting by Howard almost two years later showed the home’s airtightness holding fairly steady, having shifted just to 0.57 ACH50.
Heating the structure relies on a combination of passively captured solar heat through the windows, internal heat gains from occupant activities and appliances, an HRV, and a Navien condensing on-demand gas-fired water heater, which has an energy factor rating of 0.96. The Navien has two loops, one of which heats potable water, and a separate circuit that has hydronic fluid. The hydronic fluid circulates through a water-to-air heat exchanger coil that is integrated into the HRV. The HRV delivers a continuous supply of fresh air at 103 CFM, which can be boosted up to 130 CFM in the bathrooms when needed. A booster switch allows cooks to increase the exhaust ventilation in the kitchen, and a charcoal-based filter in the recirculating range hood helps with grease collection and reduces cooking smells.
When there is a need for heat, the warm air is delivered through the ventilation ducts, because the energy modeling showed that ductwork specifically sized for ventilation would be adequate to supply the heat needed for this high-performance home. It’s an elegant solution, and as Foster says, to achieve that level of simplicity there has to be a compatibility between the ventilation and the heating requirements that is difficult to design for from an architectural perspective. “Many times the air volume required for ventilation is different from the heating needs,” says Foster.
In spite of the design challenges, a comparison of the PHPP model results to those predicted by REM/Rate and the actual space- and water-heating loads shows that the PHPP was a very accurate tool and that the heating loads were indeed quite low. “I was pleasantly astounded by the tiny variance in the heating loads of the home—within 1–2%,” says Foster. And the homeowners didn’t achieve this low heating energy usage by skimping on comfort; average interior temperature during the heating season was 70°F. PHs are designed to have consistent temperatures throughout the house, with no particularly cold or hot rooms. The North Project lived up to this expectation as well, with the highest monthly average difference in temperatures among the various rooms of the house being only 1.7°F. The average indoor temperature during August—usually Olympia’s hottest month—was 74°F; the North Project has no cooling system.
The difference in actual compared to predicted electrical loads, however, is another matter—a 70% increase over the PHPP modeled results. “Everyone in the American PH community knows that those tend to be higher than the PHPP model predicts,” says Foster. And unfortunately, that trend is unlikely to be reversing soon—at least not without some serious educational efforts—as plug loads have been a growing share of U.S. residential energy use. However, as Foster points out, those loads are largely in the control of the homeowner. “As long as the builder puts in the capability for LED [light-emitting diode] fixtures,” says Foster, “homeowners are empowered to make those loads as low as they want.”
Pros of the PHPP
Despite this variance, I ask Foster and Smith if they find the PHPP to be helpful, and there is an almost stunned silence. That is a fantastic understatement, they both reply. “It’s a remarkable gift to mankind,” says Foster. “In my experience in our climate, it’s a fantastic tool,” he adds. “I’ve been doing green home design for quite a few years,” Smith chimes in, “and absolutely the PHPP has been much more helpful than any of the other programs.” Previous certification programs for them were mostly about jumping through a series of hoops, and if the actual energy use of a home came in at 50% more than modeled, that was not at all surprising.
By following rules of thumb, says Smith, an architect can create a generally sustainable home, but the PHPP takes optimal design to a different level by enabling her to tailor a project’s design so that it works optimally for a given site. “I learn things on every project from modeling with the PHPP,” she adds. For example, on a recent project that was located on a heavily shaded parcel with good southern exposure, she came up with a solid design that performed fairly well, but not well enough that it would meet the PH standard. Smith started adjusting the location of the house on the site, rotating the house 3 degrees at a time, and also trying out different roof overhangs. After Foster, who is the one in the firm who most enjoys modeling, tweaked the PHPP little by little perhaps 50 times, they came up with a final design that would meet the PH standard. Not incidentally, all those changes led to a reduction in the home’s predicted energy use of 20–25%. “There is nothing available to the design world except for the PHPP that can do that,” says Smith.
Although cooling is a minor load in their climate, there are homes in Olympia that need it—but not those designed by the Artisans Group. In the 20–25 houses that they have modeled, says Foster, they have managed to bring the percentage of the year that the house will overheat to less than 1%. However, their clients have to actively manage the home and still might be uncomfortable on some evenings. When they hear about an uncomfortable occasion in one of their homes, says Foster, it’s inevitably because the kids arrived at the house two hours before the parents, for example, and didn’t open the windows. “In our climate you have to use the windows for night flushing,” says Smith. “It always cools down by midnight.” Some homes may also require operable shades on the exterior to keep the house cool in summer. But even if a house does get overheated, it takes a few hours—not days—to cool it down.
In response to the North Project’s monitoring results and subsequent experience with the PHPP, Smith and Foster have modified their envelopes a bit, but mostly they are just honing the details and emphasizing value engineering—finding better open-diffusion sheathing or outstanding windows at a better price point. They feel fortunate to have found a mechanical engineer who is not afraid of designing systems for houses with low heating loads. They continue to use the Navien combi system, only with the second loop going to in-floor hydronic loops or to radiators.
Get more information about the Artisans Group.
In addition to building custom PHs, the Artisans Group has lately been designing cost-effective PH pocket houses and townhouses for its local markets. With predesigned floor plans, these homes allow buyers to enjoy the comfort and energy efficiency benefits of a PH without paying custom-home prices. Smith and Foster are excited to extend their PH expertise to a wider range of buyers, allowing more Washington residents to slash their energy bills without shivering in the dark.
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