Passive House enthusiasts enjoyed a bonanza last week: the NY14 Passive House Conference and Expo shared the craft’s latest insights and techniques, and Urban Green’s tour of a Passive House retrofit of a Park Slope brownstone offered an inside look at these ideas in action. Since the air barrier and mechanical systems were installed but not yet closed up, the insulation, HVAC and solar hot water systems were fully visible to tour attendees.
The completed row house will have miniscule heating and cooling needs as well as important creature comforts like near-soundproof windows and walls and multiple filter systems for superior indoor air quality—features that are especially important for a project located on a bus route.
But getting to the finish line is complex; construction entails no fewer than six different types of insulation (XPS, EPS, dense pack cellulose, spray foam, polyisocyanurate, and rock wool) and three different types of air barrier.
There’s a reason for this range of products, and each material has its place. Take spray foam; it’s often the easiest way for semiskilled personnel to add insulation, especially in rough or irregularly shaped cavities. But project contractor Jeremy Shannon (Build with Prospect, Inc.) advised not to rely on spray foam as an air barrier as it can pull away from masonry and wood studs, even on prepared surfaces. Instead, he recommends non-foam air barriers such as membranes and tape. (Nevertheless, we know many projects do use spray foam as an air barrier. What’s your experience? Let us know on Facebook.)
Jane Sanders (Jane Sanders Architect) is both the homeowner and the project architect, so she has both a personal and professional interest in making the house the best it can be for her family and as her workspace. “One of my principal motivations was to go through the process myself so that I could better help clients who want Passive House-level performance,” said Sanders.
But Ken Levenson of NY Passive House points out that a PH retrofit is no small undertaking. “You generally need to take down plaster against exterior walls, or else it’s nearly impossible to hit Passive House metrics” In his opinion, this means that gut renovations are the best candidates.
Once the work of defining high-performing insulation and air sealing characteristics is complete (this project aims for continuous insulation with an R-50 roof, R-22 walls, R-10 slab, and one air change per hour at 50 Pascals test pressure), the building mechanicals can be radically downsized. Project engineer Cramer Silkworth (Baukraft) of Baukraft told us in this case a “four-story home has only two small air handling units for heating and cooling—totaling just two tons.” This would normally only be enough for a 500-square-foot one-bedroom apartment, but with a Passive House retrofit the system’s capacity is ample for a 3,500-square-foot home!
Another key benefit of high insulation and air sealing levels is increased resiliency against extreme weather. Silkworth said the building will be able to coast through brief cold periods; in fact, during a power outage, occupants might not even notice a lack of heating for several days.
While Passive House brings a range of advantages, it also poses novel retrofit issues. For example, Shannon noted that it’s possible for old exterior brick walls to become damaged after an ultra-insulated retrofit, since they are no longer being “baked dry” from leaked indoor heat and therefore can repeatedly freeze and thaw until they crack. This generally isn’t a major problem in NYC because when it’s very cold, it’s usually not very wet, and because the type of bricks used in most NYC construction absorb little water and are less vulnerable in the first place. Still, he recommended having an expert weigh in on the façade condition before starting work.