Passive House as Energy Retrofit
Paul Kando
Bringing an existing conventional building up to passive house (PH) standards follows the principles of building a new PH. The existing structure is essentially wrapped in a seamless blanket of an air/moisture barrier, followed by a thick layer of “super-insulation”. Our example started life as a modest 1,458 ft2 home and barn at 40 River Rd. Newcastle. It now has 3,065 ft2 of living space yet uses far less energy. The original typically burned 857 gallons of oil per winter, the current version requires less than 10% of that. Not bad for a much expanded early 20th century Maine house.
photo credit: Naomi Beale
Key elements of passive house (PH) construction include (1) Super-insulation (R50 – R60 in Maine), (2) No thermal bridges, (3) Air-tightness (virtually no air leaks), (4) Heat-recovery ventilation, and (5) Warm (PH-rated) windows and doors. Incorporating renewable energy features can turn a PH into a zero energy building.
As a first step, this house was first stripped down to the outside sheathing. The original leaky windows were removed and disposed of and eventually replaced by new PH-rated airtight units with 7 to 9 times the R value. Next a moisture and air-impermeable building fabric was stapled to the outside of the existing board sheathing, extending over the shingle-free roof surface. All joints and staples were carefully taped, forming a seamless air barrier around the whole structure, with the bottom edge taped to the existing concrete foundation.
Window and exterior door rough openings were extended 12” outward. All joints were carefully taped to the seamless air-barrier and the new rough openings were temporarily covered with a heavy duty plastic sheet. Through-the-wall ventilator casings were installed and carefully sealed to the continuous air barrier.
12” I-joists were next attached to the exterior, to function as “studs” of a new exterior “wall” to house added insulation, The tops of the I-joist studs were tapered to fit the existing roof line, forming an extension to it. The 12” bottom plate was taped to the foundation. A semi-permeable building fabric was stapled to the outside of the I-joists, carefully taped, creating a moisture-proof enclosure for the blown-in insulation.
A network of 1x4 nailers was attached to the outside to add lateral stability to the I-joists and support new siding. Strips of a plastic honeycomb were added to the top and bottom of the network of nailers, creating an insect-barrier and breathing and weep-holes to facilitate post-construction dry-out behind the siding. Through blow holes cut into the outside building fabric, the 12” deep wall cavity was blown full of densely packed cellulose insulation, after which the holes were covered and taped. The installation of PH rated windows and doors conventional siding and standing seam metal roofing completed the exterior. Solar panels were installed on the southern roof slope.
A 16” layer of blown-in cellulose was added to the attic. The basement window openings were filled with insulating foam, sealing them permanently shut. The basement ceiling was sealed and insulated. The interior, including the former barn, was finished the customary way.