Some lessons have to be learned by doing. And for builders who specialize in advanced super-insulated construction — like Belfast, Maine, Passive House builder Chris Corson of EcoCor, LLC — some of that learning takes place in uncharted territory.
The past year has brought Corson and his crew a few of those lessons. First, Corson discovered that OSB isn’t necessarily an adequate air barrier material for meeting the strict Passive House airtightness criteria. Then, after solving that problem in a new house in Camden, Maine, Corson learned what happens when one of his airtight houses floods with a foot or two of water — from the inside (see Slideshow).
EcoCor’s wall system is an advanced assembly with an inner wall of 2x4 stud framing and an outer wall cavity framed up using wood I-joists screwed to the sheathing of the 2x4 core. The inner 2x4 wall sheathing forms the air barrier for the house. In recent years, Corson has developed a system for panelizing these superinsulated walls in a factory space in Searsmont, Maine. JLC covered the method in 2014 (see: “Panelizing Passive House,” Coastal Connection 6/1014).
Last winter, Corson says, he set one of his panelized packages in Camden, Maine. “They came to us with a Hawaiian and Japanese-inspired design concept,” Corson says, “and we turned it into something that could be built within their budget. We did our Passive House panel thing, and stood the package, and then they took over to install the ventilation system (under our guidance), and they did all the interior finish work. It’s a cute little house — about 1200 square feet — with cedar siding, cedar shakes, and a metal roof.”
Last December, when the panels were set for the Camden house but before drywall was applied, Corson tested the house with a blower door — and failed the Passive House specification. “We were at maybe 0.7 ACH50,” says Corson — outstanding for a typical home, but just a little shy of the strict Passive House 0.6 ACH50 requirement. “I said, ‘What’s going on here?’”
Corson had heard stories of other builders failing the tough spec because of air infiltration through the face of OSB sheathing. So he experimented by taping clear plastic over some of his exposed OSB walls with the blower door running. Sure enough, he says, the plastic “pooched out.” His solution was to coat the inside face of the OSB with a fluid-applied air barrier membrane from Prosoco. “As soon as we did that,” he says, “we blew about 0.28 ACH50.”
That lesson was learned, says Corson: from then on, he switched from regular OSB to ZIP System sheathing for his Passive House panels. “It’s about $600 more per house,” he says, “and it makes a perfect air barrier. It’s a no-brainer.”
Fast forward to June of this year. The new homeowners are almost ready to move in. They’re in the house working on the trim. They finish for the day and go home. When they come back the next morning, they can’t open the door — because there’s two feet of water pushing against it. A pipe fitting on the main water line (supplied by the city) has broken, and the house is flooded. Not a drop has leaked out. “All their tools were floating around in there,” says Corson. “It was like an aquarium.”
Ironically, says Corson, the fluid-applied flashing he put on the walls in mid-construction — which were never part of the plan before the blower door test results were too high — may have saved the house. Corson pulled his crew off other projects for a rush job to repair the water damage. “We had to tear off some sheet rock, pull the wet insulation out, dry everything out, blow in new insulation, and patch the holes back up and re sheet-rock it,” he says. “But without that wet flashing, the water could have wicked up the OSB and ruined the whole first floor. The wet flash literally saved the house.”
The house was built on a slab foundation, with polished concrete for the finish floor. “So there was no wood flooring to ruin,” says Corson. “Basically they lost some tools and we had to fix some walls. It was maybe a $10,000 insurance claim, that’s it.”
So does every Passive House need to come with its own bathtub drain from now on? We’ll let the experts debate that one.