Passive Building On The Rise
Passive building is a design methodology defined by a set of principles that prioritize energy conservation and best practices. Together, these principles produce a product where the whole is greater than the sum of the parts. Passive building is associated with lower energy use and, specifically, lower space-conditioning loads. However, the methodology produces other benefits: comfort, improved indoor air quality, durability, and resilience. Passive building principles can be applied to all building typologies—from single-family homes to multifamily apartment buildings, offices, and skyscrapers.
Three concepts shape the design principles: thermal control, radiation control, and air control.
Thermal Control is achieved by continuous insulation, or thermal resistance, in the building envelope. Typically, this is increased thermal resistance relative to code construction, as well as attention to detail at connection points to avoid thermal bridging.
Radiation Control is achieved through optimal glazing selection, considering glazing size, solar heat gain, and appropriate shading strategies.
Air Control is achieved by creating an airtight boundary in the building envelope and then employing balanced, fresh air ventilation with filtration. Heat or energy recovery is often also employed as an energy-saving or load-reduction strategy.
These form the main passive building principles: (1) high-performance insulation, (2) thermal bridge elimination, (3) optimal glazing, (4) shading/daylighting, (5) airtightness, and (6) energy recovery ventilation (Figure 1).
The fundamental principles that encompass this concept date back almost 50 years to research done in Canada, in the wake of the oil embargo in the 1970s. The history is well explained in an article in the Journal of Building Physics.1
In the U.S., the PHIUS+ Passive Building Standard was developed as a guideline on “how far to go” with the passive building principles.
The standard is pass/fail, with the following three performance pillars:
- Space-conditioning targets for annual and peak heating and cooling loads.
- Overall energy use, in terms of source energy.
The space-conditioning targets are met using the design principles described earlier. They represent a cost-effective sweet spot in investment for conservation measures. They were developed in a comprehensive study, optimizing for high source energy savings and low total cost, which includes the up-front cost of the conservation measures, as well as ongoing operating costs over the projected life of the building. The targets recognize diminishing returns on investment in energy-conserving measures—the energy savings resulting from the last inch of insulation in the building envelope does not pay off in the same way as the first inch.
The targets are set for that optimum point in investment, for minimum life-cycle cost. They vary based on climate, building size (envelope to floor area ratio), and occupant density, and guide the designer as to “when to stop” with passive building conservation measures. A full report on this target setting process can be found in a 2019 ASHRAE Conference proceeding.2
The source energy target can be met with passive and active conservation strategies, including renewable energy generation. It is based on the “fair share” principle for carbon emissions in the building sector to limit global warming to <2°C (3.6°F) by 2050. Source energy is used instead of site energy because it is a better proxy for emissions associated with the energy use. This target is on the glide-path to zero, recognizing that emissions must go to zero overall. Three tiers of performance are available, with special recognition for those designed to be source-zero right away.
The airtightness target requires an airtight building envelope and passing a field test at final construction. The metric is based on research3 that determined an acceptable level of airtightness in a building assembly to avoid moisture or durability concerns. Of course, as a secondary benefit, airtightness leads to energy savings in many climates. The assemblies and construction details are also scrutinized for other kinds of moisture risk and are subject to rules to limit such risks.
A critical element of the certification framework is on-site quality assurance. The PHIUS+ Standard utilizes well-known prerequisite programs that ensure quality elements that do not show up in kWh or kBtu. PHIUS+ certification requires Energy Star, DOE Zero Energy Ready Home, and EPA Indoor airPLUS for eligible* projects.
Energy-modeling software guides design and verifies compliance with the standard. The building is certified based on a combination of modeled performance and field inspection for quality assurance and performance testing.
First Passive, Then Zero
Passive building provides a proven methodology for designing a net zero energy building. The first step is conservation—first through passive measures, then through active measures. Once conservation targets are met, on-site or off-site renewable energy is used to offset remaining energy use. With reduced loads, less renewable energy is needed, and less grid support is needed when the building is not powered by renewable energy production. Up-front conservation efforts will be critical for the widespread facilitation of net zero buildings into the existing electric grid.
The benefits of passive building include the following:
- Reduced energy use, specifically for heating and cooling needs. This is quantifiable, and varying levels of savings can be achieved across all building types in all climates. These lower annual loads result in a monetary benefit—lower utility costs for owners or tenants.
- Similarly, lower peak loads result in lower up-front costs for heating and cooling systems because smaller equipment sizes can be used. Low-load buildings also inherently result in more resilience against outages, or “passive survivability”—the building’s ability to maintain livable interior conditions during outages or lack of fuel.
- The extremely airtight envelope, combined with balanced ventilation, provides an extremely durable envelope plus great indoor air quality. The source of the air brought into the building is controlled and continuous rather than being provided unintentionally through leaks, cracks, etc. The airtight boundary minimizes the amount of air that comes in unintentionally, and fresh-air ventilation provides filtered air while extracting stale air from exhaust points, such as kitchens and bathrooms.
- The envelope creates comfortable spaces because the temperature of the interior surfaces of the envelope (walls, windows, floors, etc.) remain near the interior setpoint temperature, rather than colder or warmer, which causes drafts and discomfort.
- The passive building shell also creates sound attenuation, providing some peace and quiet. This is especially beneficial in urban areas with high levels of noise pollution, or developments along transit systems or highways.
Building Certification Trends
The square footage of certified or pre-certified† PHIUS+ projects has doubled each year over the past three years. In 2019 alone, 2 million square feet were certified or pre-certified, which brings the running total to about 4.5 million square feet, ~4,200 units, creating comfortable, healthy homes for around 10,000 people. This trend is expected to continue, as over 7 million square feet have been submitted for PHIUS+ certification—about half of that in 2019 alone (Figure 2).
Professionals Certification Trends
Alongside projects, the market of professionals proficient in passive building design strategies, energy modeling, construction techniques, and on-site testing has also significantly increased. Education for design professionals (CPHC®) was introduced first, which was soon followed by the need for the education of contractors (CPHB®) and on-site testing professionals (PHIUS+ Raters/Verifiers). Together, these practitioners are geared up to deliver high-quality passive buildings throughout design and construction.
This growth can partially be attributed to the increase in incentive programs, and the increase in incentive programs can be traced to the growth in passive buildings—they are fueling one another.
Jurisdictions around North America have written passive building into their incentive programs in varying ways. A few examples are as follows:
Low-Income-Housing Tax Credit (LIHTC) Applications. Multiple housing agencies are awarding additional points in the low-income-housing tax credit (LIHTC) application for affordable housing projects built to passive building standards. The Pennsylvania Housing Finance Authority (PHFA) was the first to do this, starting in 2014. That agency has found no direct correlation between the cost of development and achieving passive house levels of performance.‡ A handful of additional jurisdictions have followed the PHFA framework, utilizing the LIHTC program to incentivize passive building, as follows: Connecticut Housing Finance Authority (CHFA), New Hampshire Housing Finance Authority (NHHFA), California Tax Credit Allocation Committee (CTCAC), Ohio Housing Finance Agency (OHFA), Illinois Housing Development Authority (IHDA), Virginia Housing Development Authority (VHDA), Idaho Housing and Finance Association (IHFA).
Cash Incentives. Other organizations such as the New York State Energy Research & Development Authority (NYSERDA), Mass Save, and National Grid offer cash rebates for both the design and construction costs of dwelling units achieving passive house certification, as well as reduced cost for professional training.
Expedited Services. The City of Seattle is offering expedited permitting for projects aiming to achieve PHIUS+ certification.
Grant Funding. The Illinois Clean Energy Community Foundation is offering grant funding for projects that achieve both PHIUS+ certification and net zero (design and operation), and the Washington State Housing Trust Fund Program includes certified passive buildings as an acceptance path to qualify for their funding.
Stretch Codes. Some states such as Massachusetts and New York have gone as far as putting passive building into their stretch codes, which can be adopted and made mandatory for local jurisdictions within the state.
Alternative Energy Code Compliance Path. The State of Washington has included PHIUS+ certification as an alternate compliance path to meet the state energy code.
Preferential Pricing from Lenders. Organizations such as Fannie Mae offer preferential pricing on loans secured for multifamily properties obtaining passive building certification, making the up-front conservation measures more affordable.
|Pioneering Passive House in the U.S.|
One of architect and builder Adam Cohen’s projects, the Center for Energy Efficient Design in Franklin County, Va., was the first K–12 public school in the United States designed to the passive building performance standard. The school showcases renewable, sustainable and energy-efficient technologies.
Photo: Jim Stroup
Delivering a passive building at market price is a pioneer’s specialty.
Architect and builder Adam Cohen was an early adopter of passive house design strategy and is credited with building the world’s first passive house dental clinic and North America’s first passive house-certified public school. He delivered the projects at market rate.
One of his projects, the Center for Energy Efficient Design in Franklin County, Va., was the first K-12 public school in the United States designed to the passive building performance standard. Cohen said the school district’s superintendent said that the building could not cost more to achieve passive house standards.
After redesigning the school, the building, as designed, actually cost less and still met passive house standards, said Cohen.
Occupancy was one of the building’s challenges. One teacher and about 24 students mainly use the building, but the school can also house up to 100 people for tours and events. The school had to be designed to be thermally comfortable at capacity, so the team modeled the building under a “worst-case” scenario of 100 people in mid-summer, with high humidity, to determine mechanical system loads. The school uses a variable-speed rotary ERV with a two-stage, ground-coupled heating and cooling system.
He honed his skills at market rate, Cohen said. He designed a 40,000 ft2 (3,716 m2) dormitory at Emory and Henry College in Emory, Va., for less than market rate. The dorm is pre-certified by Passive House Institute US, Inc.
The first passive house dental office he designed was built at the low-end of normal market rate and used 68% less energy than standard construction, according to Cohen.
Another strategy Cohen used to increase passive house adoption was creating a modular construction system to make the design and construction processes of these highly efficient buildings intuitive and customizable.
He created a prefabrication service that could create passive house-level buildings. He said the “Lego system for building passive house[s]” is easy for energy engineers to customize in terms of R-value for insulation. For architects, the details are standardized pre-certified. And the system is intuitive for builders.
Cohen said this strategy reduced the time and energy it takes to deliver a passive house.
Thermal isolation, especially in colder climates in the winter, is another strategy. Disconnecting a building from the ground is beneficial during the winter. During the summer, the building can use passive thermal cooling loops in the concrete slab to achieve thermal comfort, he said.
Interview of Adam Cohen was conducted by Mary Kate McGowan, Associate Editor, News.
Design Challenges in Climate Zone 5A
Designing passive buildings in mixed climates has its challenges, but also presents a lot of opportunities. Climates with long or harsh heating or cooling seasons benefit the most from passive building, and can achieve the highest percentage of savings relative to a “code-baseline” building. In these climates, balance is key—it is easy to design a building that performs very well at freezing temperatures, or one that performs well on a scorching, hot, summer day. People in Chicago know they need a heavy winter coat, but that they also need shorts and tank tops. A building cannot take off its heavily insulated coat; therefore, solar gain and insulation levels need to be balanced for both heating and cooling, not overly favored to one or the other, and seasonal shading techniques are beneficial. The climate-specific PHIUS+ space-conditioning targets guide the designer toward this balance point.
In addition, many passive buildings choose to go “all electric” so that they can run solely on renewables and be net zero. This can pose a challenge when selecting heating equipment—the capacity of heat pumps drops as outdoor temperatures drop, so ensuring enough capacity on peak days without oversizing equipment can be a challenge.
In January 2019, ASHRAE released news that work is starting on a Passive Building Design Standard (ASHRAE Standard 227P). The standard’s purpose is to provide requirements for the design of buildings that have exceptionally low energy use and that are durable, resilient, comfortable, and healthy. An open standard like this, that jurisdictions can adopt or make mandatory, is critical for wider adoption.
Passive building represents the future of building design. This simple, yet effective, design strategy creates healthy, long-lasting, efficient buildings.
- Klingenberg, K., M. Kernagis, M. Knezovich 2016. “Zero energy and carbon buildings based on climate-specific passive building standards for North America.” Journal of Building Physics, 39(6):503-521.
- Wright G., and L. White. 2019. Setting the heating/cooling performance criteria for the PHIUS+ 2018 passive building standard. Proceedings of the Thermal Buildings XIV Conference. Atlanta: ASHRAE.
- Salonvaara, M, A, Karagiozis. 2015. “Acceptable Air Tightness of Walls in Passive Houses.” Passive House Institute US.
- PHIUS. 2019. “PHIUS+ 2018 Passive Building Standard Certification Guidebook.” Passive House Institute US.
- Passive House Accelerator. 2019. “Can You Build Affordable Passive Houses? The Importance of the New Gravity Conference.”
Beach Green Dunes
Aerial view of Beach Green Dunes, highlighting the proximity to the rail line and Atlantic Ocean.
Photo: The Bluestone Organization
Client/Owner The Bluestone Organization
Location Far Rockaways, N.Y. (climate zone 4A)
Certification PHIUS+ 2015
Size 109,000 ft2, 101 dwelling units
Architect Curtis + Ginsberg Architects LLP
Contractor The Bluestone Organization
Window Glazing U-0.25, SHGC 0.36
Window Frame U-0.17
Window-to-Wall Area 22%
Roof R-value Concrete + polyiso (R-40)
Wall Insulated concrete forms–2.5 in. EPS, concrete, 2.5 in. EPS (R-24)
Floor Floor over crawl—mineral insulation board + concrete (R-28)
Airtightness 0.08 cfm50/ft2
Heating/Cooling Variable refrigerant flow (VRF) air-source heat pumps
Ventilation One ERV per dwelling unit, additional in common spaces
Hot Water Cogeneration with high-efficiency boiler backup
Renewable Energy 130 kW rooftop PV
Other Black-start microturbine
Modeled Site EUI 23.6 kBtu/ft2·yr
The mechanical code states that energy recovery ventilator (ERV) exhaust and intake ports are required to be separated a minimum of 10 ft in order to avoid cross-contamination. Each of the 101 dwelling units has an individual ERV, running around ~45 cfm. The supply and exhaust ducts ran vertically up and down from the unit to the exterior, which challenged the 10 ft separation from the unit above or below. The manufacturer of the small ERV suggested at least 6 ft of separation, which was achievable and accepted by New York City code because the ERVs that drove the higher required separation distance ran at much higher airflows (>1,000 cfm).
This project was part of a neighborhood rebuild after the destruction of Hurricane Sandy, with resilience as a priority. It incorporated resilient design features outside of passive building, including on-site storm water retention, concrete construction elevated above a crawlspace with flood vents, mechanical equipment on the top story/roof, and a black-start microturbine that could be used to power critical loads during an outage.
Design also focused on low-flow water fixtures and efficient water distribution, which resulted in using about 50% less water relative to other similar buildings owned by the same developer, and saved significant water/sewer costs.
At the time of completion, this project was the largest PHIUS+ certified project in the country. Modeling this larger building shed light on the high internal gains and the favorable envelope-to-floor area ratio relative to a smaller building. Because of this, less up-front investment was needed in the envelope to meet the performance metrics. It set an example for affordable multifamily buildings to come. It showed developers that it could be done for little additional cost. The savings from reduced heating/cooling system sizes paid for all upgrades needed to bring it up to a certified passive building.
Street view of the 6-flat Tierra Linda, the first PHIUS+ certified multifamily project in the city of Chicago.
Photo: Mark Ballogg
Tierra Linda is a six-flat, affordable multifamily development in the Humboldt Park neighborhood of Chicago, and the first PHIUS+ certified project within the city limits of Chicago.
Client/Owner Latin United Community Housing Association (LUCHA)
Location Chicago (climate zone 5A)
Size 9,300 ft2, 6 dwelling units
Certification PHIUS+ 2015
Architect Landon Bone Baker
Window Glazing U-0.10, SHGC 0.529
Window Frame U-0.19
Window-to-Wall Area 22%
Roof Tapered polyiso (R-60)
Wall 6 in. steel stud w/blown cellulose, 4 in. EPS (R-32)
Floor 4 in. sub-slab graphite EPS (R-20)
Airtightness 0.046 cfm50/ft2
Heating/Cooling Individual unit ducted heat pumps
Ventilation One ERV per dwelling unit
Hot Water High-efficiency gas heater
Modeled Site EUI 23 kBtu/ft2·yr
While a mixed/cold climate such as Chicago presented challenges, the Chicago Building Code presented the biggest hurdles, as follows:
- The Chicago Building Code requires noncombustible framing for exterior walls in six-flat structures. Therefore, the wall assembly used steel studs for framing. This required many iterations of the wall assembly, reducing reliance on insulation within the structural framing (because the effective R-value is very low due to the thermal bridging), and instead insulating more to the outside of the structure to provide most of the insulative value. More attention had to be paid to how the exterior insulation was connected to the structure because of the conductivity of the steel framing relative to wood framing. Instead of using unfamiliar, thermally broken clips to attach exterior rigid insulation and cladding to the structure, the team chose to adhere a 4 in. (R-20), 3-coat finish EIFS system, which was a single-source cladding system. This simplified the detail, reduced cost, and streamlined constructability.
- The Chicago Building Code also requires that any air delivered to the conditioned space must be within 10°F of the interior temperature. This must be calculated at a design temperature of –10°F, although the ASHRAE 99.6% (2017) temperatures for Chicago O’Hare and Chicago Midway are –1°F and 0.5°F. Using an ERV with 82% sensible recovery efficiency meant that if a –10°F outside temperature and an exhaust air temperature of 70°F, the interior air would be brought in at ~55.6°F (about 14°F lower than the interior temperature). The team was able to obtain administrative relief for the ERV fresh air supply temperature during the design condition because the fresh air was being introduced at a low airflow rate, and there was a heating system sized to overcome this load and maintain an interior setpoint temperature of 68°F.
- The Chicago Building code allowed for only metal, fixed-plate, heat-recovery ventilators. This would limit the project to heat recovery ventilators (HRVs) only, which is not ideal for a climate like that of Chicago, with humid summers. It would not allow for using wheel-type units that were equivalent in performance and available at competitive costs. The team was able to get administrative relief to use a fixed-plate ERV that utilized a non-metal plate in order to exchange both heat and moisture between the incoming and exhaust ventilation air. This was approved because the unit selected had third-party test data, verifying 1.6% cross-leakage, which fell below 5%, stated as a maximum in the code for the metal fixed plate.
This project is part of a scattered-site development and has a “twin” building just across the neighborhood, designed to comply with code-minimum energy requirements. Both buildings have comprehensive monitoring systems, sponsored by the local utility, which will allow for comparison of energy use and indoor air quality between the buildings.
As an affordable project, one of the main project goals was to decrease the utility costs for the tenants. This objective was achieved, but one element could have been improved. In order to save on up-front costs, a high-efficiency gas water heater was used. This required a gas hookup and, therefore, a monthly gas customer charge from the local gas utility. This charge ended up being ~$23/month, or $276/year, per unit. In the future, it will be important to consider both the operating cost and the fixed customer charge when weighing the benefits of purchasing more costly equipment up front to go all electric.
Prairie Activity and Recreation Center
Prairie Activity Recreation Center, the first PHIUS+ certified and PHIUS+ Source Zero-certified recreation center in the United States.
Photo: ©Kmiecik Imagery
Client/Owner City of Plainfield
Location Plainfield, IL (climate zone 5A)
Size 35,200 ft2
Certification PHIUS+ 2015, PHIUS+ Source Zero
Architect & contractor Wight & Company
Window Glazing U-0.12, SHGC 0.239
Window Frame U-0.20
Window-to-Wall Ratio 8.2%
Roof Polyiso over concrete deck (R-54) min
Wall Precast concrete panel (R-34)
Floor Under slab perimeter insulation for 4 ft (R-8)
Airtightness 0.045 cfm50/ft2
Heating/Cooling Rooftop units with ASHP and backup gas heat (gym), VRF (school/office)
Ventilation Rooftop units with enthalpy wheel (gym), ERV (school/office)
Hot Water High-efficiency gas heater
Renewables 213 kW rooftop PV
Modeled Site EUI 24.6 kBtu/ft2·yr
This is the first park district project in the U.S. to achieve PHIUS+ certification, so the team was stepping into uncharted territory right at the start. The construction team chose to use precast panels because they are ideal for durability inside and out. The panels are insulated, but included seams that did not maintain insulation thickness and, in some cases, did not provide continuity in insulation. The thermal bridging at these junctions was accounted for, lowered the effective performance of the panel, and required additional work on site. Even so, monitored data show that the constructed envelope is effective at maintaining a consistent indoor temperature, even during unoccupied periods with HVAC setbacks.
The project was awarded a $1 million grant from the Illinois Clean Energy Community Foundation for achieving PHIUS+ certification as the baseline standard for their net zero energy building design. The building is equipped with interactive, educational displays that present information on carbon, net zero, and the concepts behind the building’s operation. A portion of the grant is provided up front, but in order to receive the remaining amount, the building must operate at net zero for a straight calendar year.
Given the grant contingency described above, there was a focus on monitoring and operation. One key lesson learned is to not “wait for a lot of data to analyze it,” but to continuously analyze data so problems can be addressed early on in the process. For example, lower output of the PV array was expected to be due to “cloudier than usual days,” but, in reality, it was a problem with an inverter. Acting on the issue sooner would have led to a quicker fix. •
About the Author
Lisa White is associate director at Passive House Institute US (PHIUS), Chicago.