Second: Thermal Performance of the Envelope
All those careful preparations will only go so far, however, if the building’s shell doesn’t perform well enough to keep indoor temperatures stable. Achieving that thermos-like performance requires two things: more insulation and attention to thermal bridging.
And while avoiding thermal bridging is critical to a home’s thermal performance, failure to carefully design wall systems to avoid bridges can also create a health risk. “If the bridging causes the internal surface to drop below dew point, it can cause condensation and the mold and rot problems that brings,” Hindle says.
In order to maximize insulation, some builders have started replacing traditional framing materials with engineered wood I-joists typically used for floor and roof structures, says Mike Knezovich, director of communications at PHIUS. “You get thicker walls that allow for great amounts of insulation, using a commonly available material that’s cheaper than standard framing.”
For another alternative, Hindle suggests substituting staggered-stud or double-stud walls, which leave a cavity for blown-in insulation between studs, for 2×4 walls. If 2×4 or 2×6 construction is a must, he suggests external insulation, which caps thermal bridging of the framing and delivers better thermal performance at intersections where the stud wall meets the floor plate, foundation, or roof—junctures with a particularly high risk of bridging.
To keep the envelope tight, foundations and roofs also require additional insulation. For the roof, Hindle recommends using trusses or attic spaces fitted with high levels of insulation. Foundations, however, can be harder as they require rigid foam insulation, which raises environmental concerns.
While it’s no secret that not all insulations were created equal, Hindle warns in particular against extruded polystyrene and blown-in closed-cell foam insulations, due to their high global warming potential and the carcinogens they hold.
“Studies suggest that if you did an R-40 wall, which is what a Passive House probably needs to be in moderate heating–dominated climates, using blown-in closed-cell insulation or XPS, by the time you’ve built the house you would have already done 80 years’ worth of damage in terms of the carbon output equivalent compared to a typical house running on fossil fuel,” he says.
To avoid problematic materials, he suggests well-insulated framed floors over unheated crawl spaces or basements.
In addition to well-designed wall systems and appropriate insulation, Bill Arthur, an architect and Passive House consultant at Coalesce Architecture in Salt Lake City, suggests gathering plumbing stacks to allow them to share a single hole in the roof.
Subs should also know what the goals of the project are and receive the training to achieve them, says Jarrod Denton, a partner at St. Helena, Calif.–based Signum Architecture and a Passive House consultant. He recommends at least a half-day of education on passive building for everyone involved on the house. “If an electrician knows that if he punches a hole in the wall that will create a thermal bridge, then everyone is on the same page.”
The PHPP can also be a helpful resource for ensuring a tight shell, even when a home isn’t fully committed to reaching Passive House standards, says Alan Abrams of Abrams Design Build in Takoma Park, Md. When he’s working on a remodeling project, Abrams will use the portion of the PHPP spreadsheet dedicated to the building envelope to calculate how a change in the plan will affect BTU output. “For example, if I change my walls from 6 inches to 8 inches thick or if I use triple-glazed windows, then it’s a really quick analysis of what the BTUs saved will be.”
