Passivhaus on Spec in Boston
The design-build firm Placetailor has sold a single-family Passivhaus in the city's Roxbury neighborhood
The Boston design-buildCompany that handles house design and construction. Since both services are provided by the same firm, integrated design can often be more easily achieved. group Placetailor is wrapping up work on a single-family house in the city's Roxbury district that was built on spec to the Passivhaus standard. The has was sold even before work was completed.
The two-story house at 55 Marcella Street has three bedrooms, 2 1/2 baths, and a total of 1,450 square feet of conditioned spaceInsulated, air-sealed part of a building that is actively heated and/or cooled for occupant comfort. . It's right around the corner, literally, from a high-performance four-unit townhouse on Highland Street that was completed last year. Placetailor also had a hand in that one.
Placetailor strategic director Declan Keefe said the house on Marcella recently sold for $572,500. The house went on the market during rough framing and was under contract before Placetailor had finished plastering the interior.
The buyers apparently don't care whether their new house is officially certified as a Passivhaus, and Keefe says Placetailor is still discussing whether to pursue official certification on its own. The house, which Placetailor calls the “Rocksberry 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.,” was built on an infill lot right across the street from a small city park and not far from public transportation and bike paths.
A step-by-step progress report
The company has created a construction blog that walks visitors through the process, beginning last September when the building site was an overgrown lot through February when builders were working down their punch list.
There are 21 entries in all, at least so far, and although you'll only get a single photo and a little text at each one, you can see major steps along the way.
Some of the technical highlights:
- A shallow frost-protected slab insulated with 8 inches of expanded polystyrene (EPS) for an R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. of 32. The sides of the footings are insulated with 4 inches of EPS (R-16).
- Above-grade walls are balloon-framed on the outside, 24 inches on center, with a second framed wall inside of that carrying floor joists for the second story. The walls are connected with gusset plates and spanned by window wells. Exterior walls are insulated with 17 inches of blown-in cellulose (R-59).
- The truss-framed roof is insulated with 2 feet or more of cellulose for an R-value of 84.
- Windows are triple-glazed Schuco units with a U-factorMeasure of the heat conducted through a given product or material—the number of British thermal units (Btus) of heat that move through a square foot of the material in one hour for every 1 degree Fahrenheit difference in temperature across the material (Btu/ft2°F hr). U-factor is the inverse of R-value. of 0.176 and a solar-heat gainIncrease in the amount of heat in a space, including heat transferred from outside (in the form of solar radiation) and heat generated within by people, lights, mechanical systems, and other sources. See heat loss. coefficient of 0.50.
- Domestic hot water is provided by a Kingspan FPW30 solar thermal system with a 119-gallon tank.
- Space heating and cooling are provided by a pair of Mitsubishi Mr. Slim ductless minisplits, each with a capacity of 9,000 BTUBritish thermal unit, the amount of heat required to raise one pound of water (about a pint) one degree Fahrenheit in temperature—about the heat content of one wooden kitchen match. One Btu is equivalent to 0.293 watt-hours or 1,055 joules. /h. A Zehnder Comfoair 350 heat-recovery ventilator(HRV). Balanced ventilation system in which most of the heat from outgoing exhaust air is transferred to incoming fresh air via an air-to-air heat exchanger; a similar device, an energy-recovery ventilator, also transfers water vapor. HRVs recover 50% to 80% of the heat in exhausted air. In hot climates, the function is reversed so that the cooler inside air reduces the temperature of the incoming hot air. provides fresh air.
- Airtightness was measured at 0.33 air changes per hour at a pressure difference of 50 pascals. Placetailor used Siga Wigluv tape to seal the 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. seams and Siga Corvum tapes to reduce air leakage around the window frames.
You'll find many more details at the Rocksberry website, including descriptions of interior finishes and exterior claddingMaterials used on the roof and walls to enclose a house, providing protection against weather. .
One neat trick involved the exterior rainscreenConstruction detail appropriate for all but the driest climates to prevent moisture entry and to extend the life of siding and sheathing materials; most commonly produced by installing thin strapping to hold the siding away from the sheathing by a quarter-inch to three-quarters of an inch. . Siding on much of the house runs vertically, so Placetailor looked for rainscreen furring that would allow air to circulate freely (solid material run horizontally would block the flow of air). They settled on a twin-wall PVC product manufactured by Coroplast they could get through a local supplier.
The price is right
The property was snapped up quickly, although Keefe says the eventual buyers weren't shopping specifically for a Passivhaus building. In fact, they hadn't been aware of the Passivhaus standard.
Whether the economy is picking up steam, new single-family homes in the neighborhood are rare, or high-performance homes are more attractive than conventional housing, the Rocksberry House apparently went for an above-average price.
Keefe said the house sold for $325 per square foot in an area where anything above $300 per square foot is uncommon. Even the high-performance "E+" units around the corner sold for $269 per square foot.
Placetailor already has two additional Passivhaus projects in the works, including a two-family condo, also to be built on spec, and a three-family Passivhaus rental that will be built to another firm's design.
Mar 12, 2014 11:12 AM ET