A Passive House That Fits in Your Pocket (Almost)
Show time nears for the Mini-B prototype, a backyard cottage designed with Passive House performance features
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 HouseA residential building construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. Developed in the early 1990s by Bo Adamson and Wolfgang Feist, the standard is now promoted by the Passivhaus Institut in Darmstadt, Germany. To meet the standard, a home must have an infiltration rate no greater than 0.60 AC/H @ 50 pascals, a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot), a maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot), and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot). The standard recommends, but does not require, a maximum design heating load of 10 W per square meter and windows with a maximum U-factor of 0.14. The Passivhaus standard was developed for buildings in central and northern Europe; efforts are underway to clarify the best techniques to achieve the standard for buildings in hot climates., 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 GBAGreenBuildingAdvisor.com 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 testTest used to determine a home’s airtightness: a powerful fan is mounted in an exterior door opening and used to pressurize or depressurize the house. By measuring the force needed to maintain a certain pressure difference, a measure of the home’s airtightness can be determined. Operating the blower door also exaggerates air leakage and permits a weatherization contractor to find and seal those leakage areas., 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 2x4 framing with half-inch plywood sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. 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 1x4 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 2x10 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.
- Mini-B Passive House / Johnson Braund Design Group
- Martin Holladay
Dec 22, 2010 12:58 PM ET
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