by Nathaniel Dick
Senior Director Energy & Capital Projects
Designing housing to Passive House Institute United States (PHIUS) standards is the most responsible approach to limit operational carbon and lower our communities’ risk against future unknown energy costs and intensifying climate events. Passive house design successes are helping manage cost premiums to achieve cost parity with code-built buildings.
POAH certified our first passive house in late 2022 with Brewster Woods, a 30-unit family affordable housing site in Brewster, MA on Cape Cod and recently certified the Kenzi, a 50-unit senior affordable site in Boston and The Loop at Mattapan Station. POAH has 232 units of housing in construction across four locations in DC, Massachusetts and Illinois that are targeted for PHIUS certification and another 510 units in active design to achieve PHIUS certification in DC, Massachusetts, and Maine.
Passive house defined
A building performance standard designed to produce ultra-low energy consuming buildings with long term durability, comfort, and enhanced indoor air quality. Two primary certifications provide standards to achieve a passive building goal, these include Passive House Institute United States (PHIUS) certification or Passive House Institute (PHI) certification. POAH works with PHIUS standards to achieve passive house in our projects. PHIUS outlines specific energy limits by climate zone, size, and unit count. The limits specify annual heating and cooling demand (energy use per square foot per year), peak heating and cooling loads (energy use per square foot per hour), and source energy per person per year limits (includes energy losses in generation and transmission of energy from the source). Five basic design principles are necessary to achieve the passive house standard including continuous insulation without thermal bridging, airtight building assemblies, high performance windows, balanced heat and moisture recovery ventilation, and minimal space conditioning systems.
Continuous insulation without thermal bridging
Continuous insulation is achieved when there are adequate levels of insulation around the conditioned space with no gaps or “thermal bridges” where heat transfer occurs more readily from the building’s interior conditioned space. For example, typical construction uses wood frame walls. Drywall is installed on the interior and plywood sheathing on the exterior before siding is applied (along with air/water/vapor control layers). Heat transfer through wood occurs much faster than insulation; this measurement is called R-value, or resistance (to heat flow) value. Higher R-value materials are better at limiting heat transfer, so insulation has a higher R-value compared to wood. If insulation is only installed between the studs in a wall, heat can transfer more readily to the exterior through low R-value materials such as wood from the interior (i.e., drywall to wood stud to wood sheathing). This is called a thermal bridge. To eliminate thermal bridging in a wall, insulation is often run continuously on the exterior of the wall assembly which creates a “break” in the thermal transmittance of heat from the interior, thus eliminating large losses in thermal performance.
Airtight building assembly
An airtight building assembly has a continuous air impermeable layer around the conditioned space of the building. The air barrier can consist of a collection of materials and sealants that resist air flow from the building to the outside. Most buildings do not effectively manage air leakage that can negatively impact the building’s durability, energy use, and the indoor air quality. When air escapes a building, new air needs to replace the air that was lost. The air lost was heated or cooled, possibly filtered, and may have had ideal humidity levels. The air replacing it needs to be conditioned, it may carry airborne contaminants from outside or neighboring spaces, and it may bring unwanted humidity with it. The PHIUS air leakage target is .06 cubic feet per minute per square foot of exterior enclosure at 50 pascals of pressure difference. This is a hard metric to understand without some context. POAH’s 60,000 + square foot, 43-unit building in Chicago, Fifth City Commons in Chicago provides this. During its mid-construction blower door test it produced a 4065 CFM50 leakage rate, which is just slightly below the PHIUS requirement.
I have seen homes that are 3,000 square feet get similar leakage rates, so it is no small feat to achieve the PHIUS air leakage rate especially with hundreds if not thousands of different hands that construct a passive house building.
PHIUS is POAH’s targeted certification
POAH’s pursuit of passive house certification complies with our 5-year strategic plan that challenges us to prioritize sustainability and environmental justice by targeting Passive House standards in new construction projects. PHIUS certification is based on specific energy use limits, air leakage limits, and rigorous 3rd party design review, which are often smaller or non-existent facets of other “green building” certifications. In conjunction with our goal to reduce our carbon footprint through electrification and onsite generation, POAH will continue to fulfill its mission to provide sustainable, healthy, affordable housing for the nearly 21,000 residents who call our communities home.