UPDATED 3/22/2011: New photo added.
It looks like a Solar Decathlon entry, only smaller. At about 300 sq. ft., much smaller, actually. But the Mini-B Passive House, as its creators call it, is designed to pack big performance in a small package.
The product of collaboration between Joe Giampietro, a certified Passive House consultant and architect with Johnson Braund Design Group in Seattle, and instructors and students at Seattle Central Community College’s Wood Construction Center, Mini-B is designed and built to Passive House performance standards, and also conforms to Seattle’s code requirements for backyard cottages (known to the city’s Department of Planning and Development as “detached accessory dwelling units”).
Mini-B’s grand opening is scheduled for January 15 at a community center, the Phinney Neighborhood Association, after final work on the exterior and interior is completed. The house will be displayed at the community center for six months, and then will be sold for between $70,000 and $80,000. Giampietro told GBA that the plan is to eventually build more Mini-Bs (and perhaps some to a larger scale) in concert with a modular-home builder, if customer demand warrants it. The house also can be site-built. But the primary goal at this point, he said, is to highlight the benefits of Passive House construction.
Small, but still airtight
Framing started in March, and by mid-June the house was ready for a blower-door test, during which it yielded about 0.40 air changes per hour at a pressure difference of 50 Pascals, well within the Passive House 0.60 ach limit. Giampietro said the wall and roof construction includes conventional 2×4 framing with half-inch plywood sheathing that was treated with a system of R-Guard liquid-membrane sealing products. R-Guard materials also served as the window wrap and window caulking. The R-Guard sealers, Giampietro added, worked extremely well in assuring the fully assembled enclosure’s airtightness.
A 9-in. blanket of expanded polystyrene was applied over the sheathing and secured with adhesive and 1×4 cedar furring, through which 12-in. structural-insulated-panel screws were used to sandwich the assembly together. The exterior is covered with extra-wide James Hardie lap siding. The resulting thermal resistance is calculated to be R-52.4 for the walls, R-52.2 for the roof, and R-75.5 for the floor, where the space between its 2×10 joists is filled solid with insulation. The floor is topped with 1.5 in. of exposed concrete.
The feasibility of actual Passive House certification for the prototype, Giampietro noted, will depend on how and where the house is sited once it finds a buyer. We’ll add photos of the completed building once they become available.