Passive House has come a long way since its origins in the US in the 1970s and the first “official” Passivhaus in Germany in 1991; through its US domestication as Phius (2003), the thousands of US projects, and the dozens we’ve done here at TBDA (2012-present), it is now a term many people have heard.
Yet passive house or passive design and building is not a simple concept to describe. This is because it is both a design methodology and a building certification program, and the certification program requires the training of designers, builders, and third-party inspectors (called raters). Furthermore, the certification program has a complex set of criteria that need to be met. In this article, we will lay these out clearly.
What is Passive House and What Are Its Design Methodologies?
Passive design means prioritizing energy conservation. The shape, insulation levels, assembly methods, window sizes, orientations, shading, and quality are the most important factors determining energy consumption for most buildings. That’s right; it’s not the solar panels, type of heat pumps, or other gadgetry–it’s the built-in, long-lasting, passive elements. Most of these are not readily visible: you can’t see how airtight a building is or how much insulation it has, but it makes all the difference in its performance.
So, how do you know you’re doing the right thing? When you’re trained as a Certified Passive House Consultant, or CPHC (by Phius, as our TBDA staff is, or by PHI), you learn the building science and energy modeling tools to analyze the energy implications of your design decisions thoroughly. To ecological architects like us, this is empowering! Phius has developed a set of performance metrics that provide the designer with clear-cut targets for certification.
Key Passive House Design Targets
1. Space Conditioning Limits
Space conditioning limits are the primary and challenging metric–how much energy the building uses for heating and cooling annually and in peak demand (on the coldest and hottest days). To achieve these, you have to use the right amount of insulation for the shape of your building and its windows/doors and achieve enough efficiency with the mechanical system. This metric also requires architects to avoid thermal bridges–common elements still for many architects, but designed out of Passive projects.
2. Airtightness
Airtightness is another primary metric. This is of critical importance to the durability of buildings, as well as an energy-saving technique. It is also part of healthy building design (see next section) and can even decrease fire risk, as recently demonstrated in Los Angeles. The airtightness limit is several times more stringent than the building code. This criterion requires designers to figure out a continuous airtight boundary–not part of most architects’ and builders’ lexicons, unfortunately, but fundamental to the Passive House architect.
3. Heat- or Energy-recovery ventilation
Heat or energy-recovery ventilation is a requirement for passive buildings. We call these systems “the lungs of the building” because they are a controlled way to exhaust contaminated air from cooking and laundry areas, bathrooms, and any other areas with indoor pollutants–painting studios, cat litter box areas, you name it. As air is being cleared from these areas, an equal amount of air is supplied to habitable rooms (bedrooms, living spaces, classrooms, galleries), creating a pressure-equalized building with circulating air. Best of all, the air is filtered at nearly HEPA levels (or can be fully HEPA level if desired), meaning that pollen, dust, wildfire smoke, and even some viruses can be removed from the indoor air. The target for this metric is the overall air flow (based on occupants and use) and the efficiency of heat- or energy-recovery of the system.
4. Overall energy use (“Source Energy”)
This is the final primary target: the energy the building needs for all functions–heating/cooling, water heating, de- and humidification, ventilation, lighting, appliances, and other auxiliary equipment. This metric ensures that the remainder of the building’s energy-consuming systems are also treated mindfully.
Meeting these targets results in buildings that, in our cold climate, use 7-10 times less energy to heat and often less than half the overall energy of a merely code-compliant building. These are staggering results–and while they are mostly not readily visible, occupants feel the comfort of the stable thermal environment and benefit from the excellent indoor air quality. It also explains why Phius says that Passive is the path to Net Zero–if you’re not consuming so much, you can fit the solar PV on the roof!
To summarize the design methodology, when I got my Phius training in 2010, it was like finishing school in architecture. It opened my eyes to how a building should be designed to perform well–it’s a combination of factors and elements within the architect’s control, with an energy model to show how well you’re doing. Since it is not prescriptive, you have design freedom–you can design anything you want as long as it performs as a Passive building should. Once you know, you can’t go back: all our projects, whether pursuing certification or not, use the Passive House methodology.
Certification
Once all that design is done, owners should be sure they’re getting the results they expect. That requires third-party review as a quality control system: this is the essence of, and reason for, certification. The first step is for the CPHC to send the energy model and design to Phius for review. There, the expert reviewers comb through the model and check for errors, unaccounted-for thermal bridges, mistaken insulation values, etc. Once that’s complete and the design team has successfully responded, Design Certification is met. Then, the construction fun begins.
The Energy Rater is the key player during construction (and in the mid-to-later stages of design, ideally). Their role is to visit the construction site to see that the design is being implemented as specified. One of the key tools they use is the blower door kit–a fan that tests the airtightness of the building. This key diagnostic is something we recommend on all projects, whether certified or not–test it when you can get the thermal envelope airtight (not after the project is done), and seal it up! As construction wraps up, the Rater and Architect/CPHC work together to get the paperwork to Phius for final certification.
Builders
In addition to training CPHCs and Raters, Phius also trains builders. This is key for advancing what I see as a sorry state of the industry standard, especially in homebuilding. The typical build is so far removed from the Passive way that huge amounts of waste are being built into our homes and other buildings. It doesn’t need to be this way, but the waste goes on while builders are untrained and inspectors don’t require better. In Phius training, builders learn Passive House design principles, building science, and specific techniques and materials for air sealing, insulation, and HVAC system installations.
Summary
Passive House fundamentally is an evolution of green design and building. It is a way to bring buildings to a higher performance standard, using timeless design combined with data (energy modeling) and modern materials and equipment.
One thing I realized as I got into Passive design was that, before we had powerful HVAC systems and readily available window systems, buildings had to be more passive: more compact, with moderate window sizes, and as good as a thermal envelope as possible (which wasn’t that good a century ago!).
There is still a great range of design expression possible while meeting the highest levels of energy efficiency, as these examples show. While the TBDA approach is rooted in the vernacular, more contemporary styling is definitely possible!
In the post-war decades, cheap fuel and the International Style led to glassy, wasteful, uncomfortable buildings as a new standard. By the 1970s, we realized, thanks to the energy crisis, that we could do much better, but by the 1980s, we (in the US) stopped caring as gas prices fell and consumption rose. McMansions and glass boxes were thrown together, and the baseline consumption of buildings rose to its peak: buildings from the early 1900s typically outperformed buildings from the 1950s through the 1990s.
Now, with the ecological reality of climate change, which we’ve known about for over 50 years, we must do better–and Passive House shows us how. Stop the waste–it’s pretty simple once you understand the methodology and commit to making buildings perform as well as they look.