Page 42 - Zero Net Energy Case Study Homes-Volume 2

CASE STUDY NO. 7
PERLITA PASSIVE HOUSE
Low Energy Design Strategies
The new house utilizes the ground floor slab and wood framing of the single story original house, keeping the existing footprint of this structure and simply adding a new floor directly above it and a new stair connecting the two levels. The completed structure received its Passive House Certi- fication in 2017, based on the design characteristics as recorded in the Passive House Planning Package (PHPP). The high standards achieved for this certification incorporate the best compre- hensive low-energy design strategies and are described in the following sections.
Building Envelope — Insulation and Windows
Passive House standards call for heavily insulated walls and roof, usually requiring a layer of rigid insulation on the outside of the wall studs to eliminate thermal bridging. Because the house is on a relatively narrow lot with the narrow driveway typical of houses built in the early part of the twentieth century, the owners did not want to expand the wall thickness toward the outside. Instead, they framed a second row of studs on the inside to maintain the driveway clearance. The new studs in the interior layer of framing were offset from the row of exterior studs in order to eliminate the thermal bridging effect. This type of double-framing is repeated on the second level for a consistent structural solution even though it is all new framing. (See the wall details on the opposite page that illustrate this framing method.)
The interior floor area is reduced very slightly, but the overall floor area on the two levels meets the program satisfactorily and the result is a building envelope that is needed to meet the rigor- ous Passive House standards for heating and cooling. Mineral wool insulation is used throughout because of its inorganic properties (no mold possible) and its slightly better thermal performance per inch. The final overall R-values are R = 22 for the exterior walls and R = 38 for the roof, which includes the effect of thermal bridging even with the offset wooden studs.
A small “attic space” or mechanical room was created above the eastern half of the house, es- sentially to house the air-handling unit. This small room is within the heavily insulated and air- sealed roof structure.
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PHOTO: LAWRENCE ANDERSON
(Above) The renovated house, completed in 2017, as viewed from the street.
Zero Net Energy Case Study Homes: Volume 2