Brooklyn Passive House: Sealing the Envelope

The Passive House performance-based specifications are simple to describe, but they're not easy to meet. The spec starts with air-tightness—for remodels, a blower-door test value of 1.0 ACH50 or better (meaning one complete air volume change per hour at a depressurization of 50 pascals).

That's much tighter than the 2012 International Energy Conservation Code's requirement for new homes (3 ACH50 in the north, 5 ACH50 in the south). And in Brooklyn, New York, builder Jose Maldonado and Passive House consultant Cramer Silkworth are aiming to pass the tough 1 ACH50 standard in a historic brick row house that's more than 100 years old.

They're doing it with a combination of old and new methods (see slideshow, "Old School Meets High Tech: Air-Sealing a Brick Row House"). First, the crew re-points and patches the old brick walls as needed, and parges the wall with a slurry of lime mortar that matches the original mortar used to construct the home around the year 1900. Then, workers apply Sto Emerald Cote, an elastomeric acrylic coating.

On the stud walls inside the brick—but only at the front and rear of the building—the crew applies an interior air pressure and vapor control layer of ProClima Intello, a so-called "smart" vapor barrier. The Intello layer is also applied to the underside of the rafters.

The Intello fabric is air-tight, but vapor open, and its vapor permeability will be higher in the humid summer than in the dry winter—a fact that Cramer Silkworth is counting on to make the building assembly work as intended.

"The Sto Cote is mainly for the party walls," says Cramer Silkworth, "where it serves as the air and vapor control layer. And because they are warm walls—we have neighbors on the other side of them—we're not concerned about insulating them, except perhaps for acoustic reasons. The Intello is for the front and back walls and the roof, to form an interior air barrier on the inside of any of the insulation that we're installing. So, the Sto and the Intello have the same function, but they are applied under different conditions, depending on the thermal conditions of the particular wall or roof."

The Sto Cote is applied mostly for air-tightness reasons, not for vapor control, says Silkworth—"and to a large extent, the repointing and the parging gets us 90% of the way there. The Sto is just for that last little bit, because the brick and the mortar are somewhat porous. And for passive house levels of air-tightness, we're pulling out all the stops. And Sto is designed to be an air control liquid-applied system, but it's relatively vapor-permeable, so at least we are not creating any vapor trapping issues on the party wall."

The outer brick wall is intended to be "wind-tight," not necessarily "air-tight," Silkworth goes on. "Wind-tightness is important when you're using any sort of loose fill insulation, whether it be cellulose, fiberglass, or maybe even mineral wool. If you have a cold gust getting into the outer part of your wall and filtering through your insulation, that compromises the performance of the insulation. So the outermost layer should be at the very least wind tight, just to prevent the breezes from getting in through the insulation. And then on the inner side of the wall we are putting our Intello air barrier, which is kind of like "wind-tight times ten," —where every last little gap is taken care of. That is really more about containing the indoor air, and preventing moist indoor air in winter from filtering into the wall and getting to the colder surfaces in those walls."