Last year, Coastal Connection followed along as Belfast, Maine, builder Chris Corson built his first custom home designed to the strict Passive House standard for new construction. That house was completed in January of 2012, and was featured in a two-part article in May and June in The Journal of Light Construction (see “An Affordable Passive House" by Christian Corson, Part 1 and Part 2).
Corson is currently working on another near-zero-energy new home in Montville, Maine, and has two more custom homes on the drawing board. But during the spring lull, he had time for a different sort of job: a deep energy retrofit of his own house near the Penobscot Bay in Belfast.
Extending zero-energy methods to existing homes is important, Corson argues. “Maine not only has one of the oldest populations in the country, we also have some of the oldest housing stock," he says. “Passive House is great, and what we're doing in new construction is great, but to not address this existing housing stock is a major problem."
Energy issues aside, Corson's own house was overdue for an exterior makeover. “It was a perfect candidate for exactly what we're trying to do," he says. “There were things we have been putting off. We needed to re-side it anyway — the cedar shakes are forty years old, and the asphalt roof was fifteen years old. And then we had a window of opportunity this spring, so I said 'Let's do a deep energy retrofit on this house.' Then I started running the design through PHPP [the Passive House Planning Package], and I said, 'Hey, let's see how close we can get this house to Passive House.'"
The Passive House program has different, slighty more lenient criteria for a deep energy retrofit than for a new construction project. For example, the retrofit standard, EnerPHit, allows an airtightness blower door test value of 1 air change per hour at 50 pascals of pressure (ACH50), rather than the 0.6 ACH50 required for a new house to pass. With the project still in progress, Corson has already gotten his older home down to 0.9 ACH50. And he hopes that by chasing down and plugging stray leaks, he may actually be able to meet the Passive House standard for new construction.
The above-ground part of the building has already attained a near-perfect air seal, Corson says. “I struggled with the concept of using Larsen trusses or TJI's on the outside of the house, wrapping the house in a cellulose jacket and really doing an R-50 wall," he says — the same approach he succeeded with on the new home in Knox last year. “But I just couldn't make it work for a retrofit. So instead I bought a whole truckload of recycled polyiso rigid foam insulation, and we just wrapped the entire house with polyiso."
Step one was to strip off all the existing siding down to the board sheathing (see photos, above). Corson also trimmed all the existing roof overhangs back to the wall plane. Then he wrapped the entire structure — walls and roof — with Grace Ice and Water Shield bituminous membrane. The 5.5 inches of R-7 rigid foam were applied over that air-tight, vapor-tight inner skin. “It's an old-school, 1980's-style chain saw retrofit," says Corson.
The crew also replaced all the home's old windows with triple-glazed Intus windows — the same brand Corson used for last year's new home project.
The existing basement, however, proved a trickier problem. Corson decided to exclude the basement from the conditioned envelope by insulating the underfloor framing cavities with six inches of low-density, vapor-open spray foam. “I've got R twenty-something in my floor, which is great from an insulation standpoint," he says. “And it works from an airtightness standpoint, but it's not as good as I would like it to be. And I think this house could have gotten to Passive House airtightness levels — 0.6 ACH50 — if I had a better way of air-sealing the basement."
