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Actively Seeking Passive House

Scott Alan

Buildings account for a one-third of carbon emissions in the United States. The 2030 Challenge was introduced in 2006 to address this, charting a voluntary path for architects to lead the way for carbon neutrality in buildings. However, reported progress by 2030 signatory firms is falling far short of the goals. While 2030 firm reporting is increasing and results are trending in the right direction, much more is needed to drive adoption of low-carbon building design. 

While a long time in the making, promising passive building standards are gaining traction, and could lead to breakthroughs in cutting energy use and carbon emissions in buildings. According to a study by the Pembina Institute, “Accelerating Market Transformation for High-Performance Building Enclosures,” the adoption of zero-energy building laws is pushing interest in passive buildings.

Passive What?

Passive building concepts, and the building science research that support them, started decades ago in North America, spurred in part by the energy crisis in the 1970s. The concept is simple, design and construct buildings with really good envelopes so that very little energy is required to maintain comfort conditions inside. The first quantified performance standard was introduced by the PassivHaus Institute (PHI) in Germany in the 1990s. Passive building was first introduced in the US in 2003 by Katrin Klingenberg with a residential project in Urbana, IL. With Mike Kernagis, she co-founded the non-profit Passive House Institute US (PHIUS) in 2007.

In English, the name can be misleading, as “house” implies a residential dwelling. The German word “haus” means building, so PassiveHaus is inclusive of all types of buildings. Both PHI and PHIUS standards can be applied to almost any building, from a single-family house, to an office tower. The attributes of a passive building include continuous insulation with no thermal bridging, an airtight building envelope preventing infiltration and exfiltration, high-performance windows (triple-pane) and doors, minimal space conditioning, continuous mechanical heat-recovery ventilation and managed solar gain (orientation, shading, etc.). This aggressive approach to energy demand reduction results in buildings that use 60 to 90 percent less energy than a code complaint baseline.

PassivHaus

The performance-based PassivHaus (PHI) standard was developed specifically for buildings in the Northern European climate, but is applicable (if not ideal) in many locations. It includes four key requirements:

  • A Low infiltration rate (≤0.60 air changes per hour at 50 Pascals)
  • Maximum annual heating energy use of 15 kWh per square meter (4,755 BTU per square foot)
  • Maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot)
  • Total primary energy use no greater than 120 kWh per square meter (11.1 kWh per square foot)

Certified buildings must also satisfy requirements for thermal comfort, humidity and acoustics. Compliance of the design is determined using the PassivHaus Planning Package (PHPP) modeling tool.

Passive House

The Passive House (PHIUS+) standard is similar to PassivHaus but has adapted in a few key ways. Rather than the one-size-fits all energy use limits, (well suited to Germany), the PHIUS+ standard (released in 2015) has variable certification criteria for annual heating and cooling demand and peak heating and cooling loads based on climate zone, and a different airtightness criterion (≤0.05 cfm/square foot of enclosure at 50Pa). Certification is determined using the WUFI Passive 3.0 modeling tool, which allows both static and dynamic energy modeling, and hygrothermal analysis to prevent moisture issues inside the tight envelope.

Adoption

Data compiled in the Pembina Institute study shows steady growth in passive building in North America and around the world. According to the PHIUS project database, there are now over 1,200 PHIUS+ Certified and Pre-certified buildings totaling over 1.1 million square feet in the US. The list includes nine commercial buildings, and 19 multi-family residential projects, with the vast majority being single-family homes. The most notable commercial building is the new headquarters for the Rocky Mountain Institute (RMI), a 15,610-square-foot, zero-net-energy office building in Basalt, Colo. In 2015, Orchards at Orenco Woods, in Hillsboro, OR, a 42,584-square-foot, 150-unit affordable housing project, broke new ground as the largest PHIUS+ certified project in the U.S. Multi-family housing is the fasting growing sector for PHIUS+ projects, and this this record has since been eclipsed by several other apartment towers. 

Last year, New York City-based FX|FOWLE Architects completed an important study (funded by NYSERDA) demonstrating the feasibility of passive building for larger projects. The study compared options for a 26-story, 593,000-square-foot multi-family residential building in New York City, one designed to achieve LEED v2009 Silver Certification and the other meeting PHI standards. The proposed PHI upgrade did not require any aesthetic changes, and relied on triple-pane windows, a thicker wall assembly with continuous rigid insulation, and enhanced insulation effectiveness using thermally improved rain screen attachment clips (typical rain screen clips cause significant thermal bridging). Energy-recovery ventilation systems were added, Air Barrier Association of America (ABAA) quality procedures were proposed for construction, and blower door testing would be required to validate airtightness. The authors estimated these measures would have a 2.4 percent capital cost premium, cut energy use by 47 percent, and have a 24-year simple payback.

Clearly, Passive House certification is feasible and beneficial, but it also presents some challenges. The envelope-focused energy demand reduction approach provides a quantum leap forward in energy use and climate impact reduction, with the added benefit of reducing heating and cooling system sizing and costs. This also increases the passive resilience of the building (maintains comfort in prolonged power outage) and positions it well to be zero net energy. The careful attention to infiltration reduction and moisture control, coupled with continuous energy-recovery ventilation result in improved comfort and health for occupants. I also see the reduced energy costs and improved indoor environmental quality being particularly beneficial in affordable housing communities.

Conversely, it is worth considering if the rigor required in envelop design and construction pushes Passive House certified projects past the point of diminishing returns. The thicker wall assembly will increase the gross floor area, or decrease net usable (leasable) square-footage, the airtightness requirements demand significantly more coordination between trades for enclosure construction, and with multiple studies showing an average incremental cost increase of 6 percent (can be as low as 2 percent), certification may require initial investments with returns outside what most proformas will support. In all but the most extreme of climates, would a less rigorous effort provide most of the same benefit at a fraction of the cost?

Regardless of whether a passive building certification is pursued, the envelope is the first place an owner and their design team should focus to create long-term value, in energy savings, durability and human comfort. The enclosure will be there through multiple HVAC retrofits. Once in place it is difficult to upgrade, and failures can be disruptive and costly to repair. Because of its longevity, a high-performance envelope will not only reduce the size and initial cost of HVAC systems, along with energy use, but that savings will be replicated each time the systems are replaced over the life of the building. This long-term, enclosure-first approach lends itself to new funding models like Property Assessed Clean Energy (PACE) financing, which stays with the building through property sales, and is repaid over 20 years. Some municipalities are introducing incentives that support a passive building approach. We all need to take a more active look at passive building.


Alan Scott, FAIA, LEED Fellow, LEED AP BD+C, O+M, WELL AP, CEM, is an architect with 30 years of experience in sustainable building design. He is a Senior Associate with WSP in Portland, Ore. To learn more, visit www.wsp.com/usbuiltecology and follow Scott on Twitter @alanscott_faia.