In a bustling neighborhood south of downtown Seattle, a 35-unit multifamily Passive House building is under construction. It’s the brainchild of Sloan Ritchie, president of Cascade Built, and developer, builder, and owner of Pax Futura. Although Cascade Built has had plenty of Passive House experience, this project is Ritchie’s first Passive House apartment building, and is also a first for Seattle.
“It’s new territory,” Ritchie says succinctly. He is referring to the design of the project’s cooling system, but the same can be said about other aspects of this ambitiously forward-looking development.
Filling a pressing need for new rental housing, the four-story building will offer 6 one-bedroom units, 26 studios, and 3 live/work spaces on the ground floor. Located close to a light-rail station and numerous retail establishments, the 50-foot by 100-foot site has a walk score of 94. In keeping with Seattle code allowing developers to eschew parking garages in specified dense neighborhoods, Ritchie opted for bike parking instead of a garage that only would have been able to accommodate six cars—an economically unsound choice and a waste of valuable living space, he points out. By limiting the building height to four stories he also was able to forego building an elevator, eliminating a potential primary energy hit.
With Passive House the goal from the outset, designing the building envelope was pretty straightforward, says Brittany Porter, architect and certified Passive House consultant with NK Architects, the project’s designer. “This project validated what we already knew about the compatibility between multifamily and Passive House,” says Porter. Careful detailing and modeling are critical to any project’s success, no matter the scale, she emphasizes, but multifamily buildings come with inherent advantages.
The foundation assembly is a fairly standard slab-ongrade with 4 inches of rigid insulation and a vapor barrier below the slab. The R-68 roof assembly includes more insulation than is typical but not by a huge amount. The exterior roof insulation consists of tapered polyiso—up to 1 foot thick in some locations. Interior to that is the air barrier and sheathing, supported by 9½-inch I-joists insulated with fiberglass batts.
A primary question was how to get enough R-value in the walls yet not take up too much space on a small lot. The answer relied heavily on a structural insulated panel wall that uses a graphite-enhanced EPS to deliver a roughly 15%–20% higher R-value. The taped panels will serve as the air barrier. “It’s a typical 6-inch wall, but it’s significantly more thermally efficient,” says Porter.
Thanks partly to this thermal efficiency and partly to the high occupancy per square foot, the building easily meets the Passive House heating target using double-pane windows. Seven household-sized HRVs, each one serving three to five dwelling units, will supply continuous fresh air. Thanks to this semidecentralized system, duct runs were minimized, sharply reducing typical efficiency losses. When needed, electric-resistance cove heaters, mounted on the ceiling, will provide supplemental heating.