PASSIVE HOUSE: BUILDING FOR TODAY, TOMORROW, AND THE FUTUREBy Sara Shelton and Serena Kacharos Warren
WHAT IS PASSIVE HOUSE?
Serena: Passive House is a performance standard that focuses on reducing the energy usage of buildings and through passive building elements rather than active systems. The standard focuses some core principles including airtight construction, super insulation, minimizing thermal bridges, highly effective triple pane windows, and efficient ventilation systems
Sara: Since Passive House is a performance standard, it means that field testing needs to be done to ensure compliance. There are thresholds that must be met for annual heat demand, peak heating demand, cooling demand, and airtightness.
COULD YOU DIVE INTO SOME OF THE HISTORY AND THE BACKGROUND OF PASSIVE HOUSE DESIGN?
Sara: The concept started in the 1960s. There was a lot of experimentation being done with architectural design, specifically related to its influence on thermal comfort. A lot of the strategies that were experimented with can be seen in vernacular architecture. So Passive House is really not a new concept – it’s just the codification of design strategies that have been tested throughout history.
A major proponent for Passive House was Dr. Wolfgang Feist who founded the Passive House Institute (PHI) in 1996 as an independent research institute. An abundance of research was conducted in extreme climates, where they were looking to build a more thermally insulated envelope in order to keep indoor spaces at a steady temperature during the winter while also reducing energy use.
Serena: The first certified Passive House was the Saskatchewan Conservation House. The construction of this house is the first time all the key principles were incorporated, not just passive solar or some other green building strategies that were seen prior. The client wanted the house to be powered off solar energy, but technology and existing systems weren’t advanced enough to store enough solar power to keep up with the energy demand of the house. So, the design team set out to reduce the overall energy load of the house, which today is one of the key definitions of Passive House. With that in mind, they started incorporating super insulation around the entire envelope and focused on airtightness so there wouldn’t be as much energy loss through the envelope due to leakage. Additionally, triple-pane windows were introduced which limited that transfer of energy from interior to exterior and vice versa. While passive house has evolved since then, these early methods became the driving force behind achieving a Passive House performance standard.
WHAT ARE THE CORE PRINCIPLES OF PASSIVE HOME DESIGN?
Serena: A key principle is having high R-value assemblies and continuous insulation. R-values measure an insulating material’s resistance to heat flow – the higher the R-value, the more effective the insulating material is. That is the number one way to reduce heat loss and heat gains, and has the biggest effects in terms of energy usage. This applies for new homes, as well as passive house renovations. For renovations, continuous insulation is one of the first steps that they take towards achieving passive house.
Sara: The next core principle is a continuous air-seal layer, which I would argue is probably the biggest difference between passive house and conventional construction. In passive house, you have to have a continuous air barrier, such as a film, similar to a waterproof barrier, that keeps the air out. Similar to insulation, it requires careful detailing in order to form a closed-loop. Passive House certifiers advocate for you to print a plan or a section and then draw a red line all the way around the envelope to make sure that every single point where there is a transition has an air seal. For instance, windows, pipes, and conduit that penetrate exterior walls all need to be air sealed. These details are often missed, so by sealing them, a lot of lost energy can be saved right away.
Serena: This also improves the air quality inside; proper sealing helps prevent dust, smoke and pollutants from coming into the house. This leads to the third principle which is to eliminate – or greatly reduce – thermal bridges.
Sara: To simplify the explanation of a thermal bridge, it is anytime you have something interrupting the insulation value of the envelope. A stud, for instance, acts as a thermal bridge because it reduces the amount of insulation between the interior and exterior where it occurs. Linear thermal bridges, such as where an exterior wall meets the slab, often cause a lot of avoidable energy loss.
Serena: Basically, when you’re incorporating these high R-value elements such as windows and insulation, there are going to be points the thermal envelope that may be more difficult to thermally protect. For example, when the exterior wall meets the roof rafters it creates a transition that can be difficult to insulate properly, and this can lead to a weak point in the thermal envelope. Passive house really focuses on those transitions with careful detailing and construction to minimize as much energy loss as possible.
Sara: The fourth core principle is high-performance glazing. For the most part, this means installing triple-pane windows. However, since passive house is a performance standard, the necessary efficiency of windows varies depending on building orientation and climate zone. In general, the idea is that the glazing should be acting better than the walls. In the summer, when there is an excess of heat, you don’t want heat coming in through the windows, so you want them to have a high R-value. In the winter windows with a high R value can act in your benefit by allowing heat in, which solid walls cannot do.
Serena: The last principle is mechanical ventilation and heat recovery. Once your detailing is complete, your insulation is in, and you have an air seal, your energy usage is now lower. However, you still must account for some energy usage. Typically, they install HRVs or ERVs which stand for Heat Recovery Ventilation systems and Energy Recovery Ventilation systems respectively. These systems are always running, consistently taking air in, filtering it, and then pumping it throughout the house. So the occupants have fresh air, clean air. It is the only mechanical system that you need in the house, and it’s way smaller than what is used in conventional builds.
HOW ARE PASSIVE HOMES CONSTRUCTED?
Serena: It really depends on where your project is located. A lot of the long-term success of passive houses is based on the information and data collected at the very beginning. You’ll need accurate weather data for your site and you’ll have to figure out solar orientation to take avantage of heat gain and proper shading throughout the year. One of the great things about Passive House is that it there is no set way to achieve it. It’s not required that passive house projects use wood stud wall assemblies, a passive house could incorporate CMU or Metal stud wall assemblies. Sure, some approaches are more standardized, but there is no set way to do it. The main factor is that there is high attention to detail during both design and construction.
Sara: Exactly. I would say the beauty of Passive House is that it’s completely dependent on climate, location, and construction type, and it varies widely to really suit whatever the project needs are. That being said, since there is not a single standard, you are often testing new details to make sure that they work. It requires a very high level of careful detailing and extensive coordination with an experienced contractor who is on board with the idea of Passive House. Passing the certification relies heavily on proper execution of the details in the field.
Serena: Another thing to note about detailing, is that there are Passive House websites that have generic or suggested instructions on details, such as wall assemblies, that are Passive House certified, so people don’t have to reinvent the wheel. These standard details won’t ensure passive house certification, but they are available to help guide passive house design.
HOW DO PASSIVE HOMES CONTRIBUTE TO A MORE SUSTAINABLE FUTURE? AND WHAT ARE THE SUSTAINABLE BENEFITS OF PASSIVE HOMES?
Sara: There are a lot of sustainable elements of Passive homes. To name a few, passive homes contribute to a more sustainable future by increasing the air quality for the residents. They are cost-effective. They have increased durability due to air sealing. They have low energy use by providing a long-lasting, comfortable interior temperature. They have next to no condensation risk (which also removes the risk for mold).
Serena: I think the biggest one that would contribute to a sustainable future would be the reduced energy usage through the minimization of mechanical systems. Durability is also a big one that some people might not think of. They design passive houses so they are able to last around 100 years. Is that right, Sara?
Sara: Yes – they have a longer life span than typical construction. Since you are sealing everything in with an air barrier, in turn a lot of the mold and condensation risk is reduced. Everything lasts longer because you don’t have contaminants going through your wall and messing up the finishes or messing up the insulation. All of your materials on the inside of the building are preserved a lot better than they would be in a typical building, so in terms of sustainability, that’s huge because you are increasing the lifespan of materials and you don’t have to demo or replace things as often.
Serena: Instead of building two houses in 100 years, you can build one and maybe make some tweaks here and there.
Finally, cost-effectiveness is also important from a client perspective and even from a designer perspective. There may be a little bit more upfront cost, but over time the money that you save on the mechanical systems makes up for that.
Sara: That’s right. It’s a passive house so you’re saving in the long-term on mechanical systems and other things that you’d traditionally have to add in. Take solar for instance: solar is great, but that is an active system, so you’re making up for energy that you are using. Solar panels are great, but if you can reduce the amount of energy you need to begin with, you don’t need as many solar panels. The idea here is to completely eliminate or greatly reduce the need for mechanical systems.
Serena: One of the big ideas from passive house was that moving parts break, and therefore must be repaired and maintained. So if we can make a stationary house, that’s better simply because of the construction, methods and materials used, then you’re going to have fewer problems and you’ll have a more efficient house.
HOW DOES PASSIVE HOUSE COMPARE OR RELATE TO OTHER SUSTAINABLE DESIGN CERTIFICATIONS OR EFFORTS?
Sara: Passive House, as we’ve mentioned, is primarily focused on thermal comfort and interior air quality. The continuous air seal largely eliminates the need for air conditioning and heating, whereas other certifications or Green Building Standards, such as LEED, are focused on a broader range of components such as getting EnergyStar rated appliances or sustainable materials. Passive House does not address the embodied carbon or the lifecycle of products therefore, it doesn’t require a specific type of insulation. If you use foam insulation, for instance, that might work well in a passive house because it has a really high R-value, but it’s not the best material in terms of embodied carbon. Passive house is definitely a great standard for thermal comfort, air quality, and developing a long-lasting building, but it’s not meant to be looked at in a silo. It’s helpful to have other certifications in mind and be looking at building components from multiple perspectives.
WHAT SHOULD BE CONSIDERED BEFORE CONSTRUCTING A PASSIVE HOUSE?
Serena: There a handful of things, but the three that stand out to me are the location of the project, your team, and experience. Passive House is very much site based so right off the bat, when you know you want to build a passive house, you must consider your site and how that is going to impact the design. For example, the orientation of your building matters to reduce heating and cooling loads for your mechanical systems. Climate also plays a factor in the building assemblies, if you’re in a temperate climate you may only need 2” of exterior insulation however if you’re in an extreme climate you may need more.
Another thing to consider is your team. Every person involved in a passive house from the client to the architect, to the contractor and even the subs, need to keep passive house strategies in mind while progressing on the project. It can require careful coordination to make sure that all the passive house elements get completed properly. If the Plumber doesn’t properly seal and tape all the penetrations created during their time on the job, that can significantly impact the overall function of the passive house because there isn’t a continuous air barrier. In turn, your mechanical systems aren’t going to be able to compensate for that energy lost through the air barrier and the passive house will be unbalanced. All the Passive House key principles must work in harmony for a passive house to function properly. There is a passive house certification for builders and consultants, as well as courses offered to help professionals understand passive house design.
This bring me to the last consideration which is experience. A team’s experience with passive house design can affect a lot of elements such as time, cost, creative solutions, and proper coordination. Make sure that the team fully understands what the scope of a passive house entails before jumping into design and construction.
WHAT ARE THE COMMON MISCONCEPTIONS OF PASSIVE HOUSE?
Sara: One common misconception is that Passive House is only applicable to single-family houses, which is not true. Passive House can be applied to single-family houses, multifamily projects, large buildings, commercial buildings, small buildings, retrofits, really anything. Since it’s a performance standard, it kind of varies depending on the application, but large buildings can actually greatly benefit from passive house because you have a higher volume to surface area ratio, so the thermal envelope that you’re making very energy efficient is now benefiting a larger volume.
Serena: Another misconception is that you can’t open the windows. You can. There are operable windows. The more important factor is that you want really good windows. You want triple-pane windows that have good frames so that you’re not getting thermal bridges in the, dead of winter when you don’t want cold air leaking into your house. Typically people are only going to open windows when it’s more of a moderate temperature outside that matches the thermal comfort zone we’re trying to achieve inside, so it’s totally acceptable to open the windows.
Sara: Yet another misconception is that since you’re sealing your building, the air quality on the inside must be awful, which is also not true. Instead of breathing in the air that comes through walls with mold, you are now breathing in air that is intentionally filtered through a ventilation system. The air quality drastically increases on the inside which many do not realize.
Serena: Another myth is that passive house standards are too restrictive on aesthetics. A lot of people think that passive house is only for single-family homes that end up as these monolithic boxes with thick windows that you can’t open. But that type of design is typically due to inexperience with Passive House – since new passive house designers are so focused on the detailing and making sure that everything is airtight and thermally insulated, pushing the design boundaries isn’t a top priority. Once designers get more comfortable with the standards it becomes easier to push the boundaries.
Sara: I would say that the last common misconception people encounter is that Passive House is too expensive. You may have a bit more upfront costs, especially if the entire design team is new to it and are creating all the details completely from scratch. However, the client will actually be saving money in the long run. There’s a little more labor and material cost in making building airtight and having higher insulation values and triple-pane windows, however, you’re saving a lot of money on the mechanical system and the eventual labor of that mechanical system. At the end of the day, your building is made to last longer and be more energy-efficient, so you are saving those costs as the project’s life goes on.
AS YOUNG FEMALE ARCHITECTS AND DESIGNERS, WHAT DOES PASSIVE HOUSE MEAN TO YOU AND WHAT GOT YOU INITIALLY INTERESTED IN IT?
Sara: In general, I’m just very interested in how we as architects can make buildings more sustainable. I’ve been interested in the concept for a while, but I didn’t know much about the key principles and wanted to learn more, which is why I took a course on it. I think sustainability is really integral to our design process and the duty of architects to make sure we are designing projects to last as long as possible. A lot of what is interesting about Passive House is that it’s primarily a passive system, so you’re not relying on technology that may change over time, like a mechanical system or even the changing technology of solar panels. You’re relying on designing a quality building from the start, that’s made to last and be energy efficient throughout its lifespan.
Serena: I first heard about Passive House in college in a building science course at University of Oregon. From that initial introduction, it’s just seemed like an obvious step in sustainability. Passive house focuses on the bones of a structure and how to improve those first to minimize energy usage. Then I kind of lost touch with it and until I took a Passive House course this past year. Again, it just solidified that there’s room to improve our traditional design/construction strategies in order to create better buildings.
WHY DO YOU THINK MORE PEOPLE SHOULD CONSIDER PASSIVE HOUSE DESIGN?
Serena: I view it from the lens of our mission: we’re trying to create environments people enjoy, passive house focuses on improving thermal comfort and air quality within our building and achieves it through improved building practices. I like quality. I want something that is going to be reliable for the next century. Also, buildings make up 41% of the US primary energy consumption, so if we can reduce that energy consumption while also improving the quality of buildings for its occupants, then it’s a win-win situation.
Sara: Agreed. I think more people should consider passive house for three reasons. The first is that it increases the quality of life (both from an air quality and comfort standpoint.) The second reason is that it decreases one’s carbon footprint by largely eliminating the need for heating and cooling. And finally, because it’s not a complicated or fancy system. There may be some small additional upfront costs, but it averages out and reduces maintenance costs over time.
Sara Shelton, AIA