I've been working on superinsulated houses since the mid-1990s. I designed and built my first Passive House in Knox, Maine, in 2010 and 2011 (see "An Affordable Passive House," May/12 and Jun/12), and since then, I've built six new Passive Houses, designed several more to be built by others, and completed a deep energy retrofit on my own home in Maine that meets the Passive House EnerPHit standard for retrofits.
In recent years I've been approached by a handful of potential clients who have asked me to do EnerPHit retrofits on their existing houses. But in almost all cases, I've declined the work. The sad truth is, it's so expensive to turn a leaky, poorly insulated old house into a Passive House that it's rarely worth the investment. I referred all those clients to a weatherizing contractor who I knew could improve the houses to a moderate degree for a reasonable cost.
But in 2013, I accepted a deep energy retrofit job that, if it doesn't reach EnerPHit levels, will certainly come close. The job won't pay for itself by energy savings, but I decided that it still made sense. By the time the owner, Deb Poor, called me, she was already planning a gut-rehab of the house (to be carried out by another contractor, Tim Andrews of Nobleboro, Maine). Deb had her own reasons to have an energy retrofit done. In particular, she wanted the project to serve as an example to others of what was possible in this community.
I think she's making a wise investment, even though a deep energy retrofit is not the most economical way to pursue the goals of Passive House. Her house is in a drop-dead gorgeous waterfront location in Damariscotta, Maine, a popular vacation spot. She's choosing top quality throughout, from frame to finish—it's going to be a showcase project even without the Passive House package. In this case, giving the house the comfort and independence of near Passive House performance is icing on the cake.
Retrofits are a labor of love—it will always be cheaper to start from scratch. There are also practical concerns. The better the bones of the home, the higher the likelihood that it's a good candidate for a deep energy retrofit. If the bones are sound and the basement is sound and dry, then a roof that needs replacing, or siding that's in shambles is something we can work with. In this case, the basics were all in place.
Existing Conditions
The original house was a typical Maine hodgepodge. The oldest part of the house was built in 1906, with good solid framing, horizontal sawn-board sheathing, and a sound rubble-stone foundation. A garage addition, which has plywood sheathing, was probably framed in the 1970s.
When Tim Andrews' crew ripped the existing clapboard siding off the house, we found an odd mix of paper underneath. The oldest section had asphalt paper as the weather resistive barrier, and the work had been carefully done. The workers had even cut out little squares of paper to back-flash all the clapboard joints. It was nice work for its day.
Other parts of the house had Tyvek and Typar housewrap, which had been carelessly applied in the 1980s or 1990s. There wasn't any air space for a drainage plane behind the clapboards, and by this time, the housewrap had badly deteriorated and was rumpled and torn. But whether it was felt paper or housewrap, none of it could make the house anywhere near airtight, so it all went into the Dumpster. We removed the existing windows and started over at the level of the sheathing.
New Air Barrier
The EnerPHit standard calls for an airtight envelope. In new construction, our high-performance wall system consists of a 2x4 or 2x6 frame sheathed with OSB and taped at the joints so that the layer of OSB forms the airtight control layer for the whole house. Then we apply an outer frame of wood I-joists outboard of this sheathing layer to form a cavity for 12 inches of cellulose insulation. That outer frame is enclosed with vapor-open fabric so that when the cavities are filled with dense-blown cellulose, the assembly can dry to the outside of the building. Everything inboard of the OSB layer is able to dry to the inside.
Our plan was to apply the same I-joist assembly to the outside of the Damariscotta house walls and have it function the same way. Essentially, we'd be wrapping the whole building in a thick jacket of insulation. But we needed something besides the sheathing to form the air barrier under the I-joists. We'd never be able to make the existing board sheathing—or even the existing plywood sheathing on the addition—airtight by using tape. So we chose an airtight vapor barrier fabric from Pro Clima called DA. We applied the fabric over the existing sheathing and taped the fabric joints with Pro Clima Tescon Vana tape.
DA is heavier and tougher than typical housewraps. Although it's waterproof, it doesn't form the drainage plane for this job—it's just an air-control layer within the wall. But in retrofits, DA does offer the advantage of temporarily protecting interior framing, insulation, and finishes from rain and wind while work on the building exterior is in progress. (In this case, temporary protection didn't matter too much—the whole house was being gutted anyway.)
We rolled the DA out nice and flat and stapled it along the studs with pneumatic staples. We carefully laid out the stud locations before stapling and then taped over the staples with Tescon Vana tape to maintain a perfect air seal. Later, we would attach the I-joists at the same locations, driving structural screws through the flange, through the self-healing tape, and into the studs, so that the air barrier would remain intact. We were shooting for the Passive House airtightness spec of 1.0 ACH50 for retrofits, so we had to be meticulous about avoiding incidental punctures in the air barrier.