Five Principles of Passive House Design and Construction
Passive House is considered the most rigorous voluntary energy-based standard in the design and construction industry today. Consuming up to 90% less heating and cooling energy than conventional buildings, and applicable to almost any building type or design, the Passive House high-performance building standard is the only internationally recognized, proven, science-based energy standard in construction delivering this level of performance. Fundamental to the energy efficiency of these buildings, the following five principles are central to Passive House design and construction: 1) superinsulated envelopes, 2) airtight construction, 3) high-performance glazing, 4) thermal-bridge-free detailing, and 5) heat recovery ventilation.
All these key principles are linked to and impact each other in the design. No one principle can be neglected without having a negative impact on the rest. To effectively create a Passive House building, the design should be looked at holistically to incorporate all five design principles.
The building envelope is what separates the interior of the building from the exterior; it consists of outside walls, roofs, and floors. In cold climates like Canada, where inside air is heated to keep the building comfortable, some of that heat will be lost as it moves through the envelope (via the process of conduction). In order to reduce this heat loss, insulation made of low-conductivity materials is installed within the wall and roof assemblies.
Passive House makes the most of the envelope by superinsulating the building in order to minimize the heat loss. For a Passive House, the aim is to use assemblies with enough insulation to double or triple the heat resistance compared to what is required in current Canadian building codes. The result is a significant increase in the thermal performance expected from the building envelope. Insulating to Passive House levels has the added advantages of greater soundproofing, improved durability, and greater building resiliency—including the ability to maintain interior comfort for extended periods even if there is a power failure.
Achieving Passive House levels of heat resistance is not just about how much insulation you have, but whether that insulation is used effectively. Insulation is most effective when it wraps the building uninterrupted by other materials, but there will always be areas where this is not possible, such as around components used for structural reasons. When a material bypasses the insulation, it is known as a thermal bridge and can significantly reduce the effectiveness of insulation, especially if that material is very conductive, like metal.
Minimizing repeating thermal bridges and aiming for continuous insulation where possible, as in the assemblies shown in Figure 1, helps make the most of the insulation within the building envelope.
Figure 1. Example Building Assemblies
Heat can also be lost through the envelope via air leakage. A building’s air barrier is a layer of material (membrane, tape, seals) around the envelope that restricts the movement of air in and out of the building. Gaps in the air barrier can allow air to move in and out of the building uncontrolled; they occur when there is insufficient detailing during construction, when there are numerous ducts or other penetrations in the air barrier, or when construction is of generally poor quality.
High volumes of uncontrolled air exchange with the exterior can lead to a whole host of problems, including increased energy use from having to repeatedly reheat the air, discomfort from cold air drafts near the walls, and localized moisture and condensation problems. While air exchange is necessary for ventilation and providing fresh air, it is far more effective to control air exchange by tightening the envelope and using mechanical ventilation.
There are strict design and construction requirements for a Passive House project to be certified airtight. Quantitatively, this means that when tested the building needs to have less than 0.6 air changes per hour (ACH50) to achieve Passive House certification. This stringent value can be compared to other high-performance building standards, such as the R2000 program, which allows up to 1.5 ACH50 from air leakage. As additional quality assurance for a Passive House project during construction, at least one on-site air leakage test must be completed to demonstrate that the building meets the airtightness requirements.
Achieving this degree of airtightness requires careful planning in the design stage, including making sure that the air barrier is continuous and evident on drawings, that effective air barrier materials are used, and that clear detailing for penetrations and terminations is provided. Construction quality with thorough quality control, from the contractor down to the trades, in the installation of the air barrier is critical. The entire construction team should be aware of the important role that airtightness plays in a Passive House project.
Airtight construction on a Passive House project will further reduce space-heating costs and localized condensation problems and will provide better comfort inside the building. In a Passive House building these advantages cannot be achieved by tightening the building envelope alone but must be coupled with a suitable ventilation strategy to deal with excess humidity in the building.
While the walls typically make up the largest area of a building’s façade, the glazing systems (windows and glazed doors) can play an even bigger role when it comes to contributing to space-heating energy. Due to their function (providing light and visibility), glazing systems cannot be insulated to the same degree as a wall, resulting in the windows being the weakest areas of the envelope in terms of heat-flow resistance. Therefore, it is very important that high-performance glazing systems, such as Passive House-certified windows, are used to reduce that heat flow as much as possible.
Some key characteristics of a high-performance Passive House glazing system, as shown in Figure 2, include nonconductive framing or large thermal breaks; insulated framing; double- or more likely triple-glazed units; argon or krypton gas fill; multiple low-e coatings; and warm-edge or nonconductive spacers.
Figure 2. Example Features of a Passive House Window
It is important not only to make sure to specify high-performance windows, but also to carefully consider how they are incorporated into the building design. Passive House designs take advantage of free passive heating from the sun. Solar heat gain through appropriately placed windows can help offset the amount of heat a building needs during colder months. During the summer months, this needs to be counteracted with shading to prevent too much heat from the sun from getting into the building, causing overheating. For each Passive House project, there will be an ideal number of windows that can balance the advantage of free heat from the sun with minimizing the heat loss from having too many windows.
The final consideration for glazing systems is surface temperatures. When outside temperatures are low in winter months, the inside surface temperatures on low-performing windows can also be quite cold. Low temperatures around the window can result in a higher risk of condensation (and potential mould growth) and feeling colder when you are close to the window because of radiant heat loss or from temperature-induced drafts. To reduce these risks, certified windows must be assessed according to hygiene and comfort criteria that set minimum allowable surface temperatures around the window.
The last envelope consideration is the minimization of thermal bridging. This was discussed earlier for repeating thermal bridges in the general wall and roof assemblies, but Passive House designs also aim to be thermal-bridge-free when it comes to architectural interface details. These are parts of the building where different architectural features meet that require additional attention in construction. Examples include how a window is attached to the walls, how a wall meets a balcony, and how walls meet at corners, as shown in Figure 3. The way these building features connect and are designed can also introduce thermal bridging that’s not always easy to recognize.
Figure 3. Example building interface details
Thermal bridging from interface details can have numerous effects on building performance. For highly insulated envelopes like those in Passive House projects, thermal bridging can significantly reduce the benefits of superinsulating by allowing heat to flow around the insulation and out of the building, and can also create localized cold spots, increasing the risk of condensation and mould growth around these details.
The easiest way to avoid thermal bridging is by making architectural design changes (where possible), such as using self-supported decks and canopies for low-rise buildings or reducing the number of cantilevered balconies and articulating architecture (lots of corners) on larger buildings. This is not always realistic or achievable, and in these cases, special attention needs to be paid to these interfaces. Reducing direct conductive connections between the interior and exterior is important. Examples include installing intermittent connections for shelf angles, overinsulating in front of certain connections around the foundation, wrapping insulation around protruding details, or using special materials such as thermal breaks.
While it may not seem obvious, the thermal bridges caused by window-to-wall interfaces can have a very large impact. The total perimeter of all the window-to-wall connections can add up to several kilometers on some projects, so how a window is installed into an opening plays an important role in minimizing the heat flow. Reducing thermal bridging at this connection involves positioning the window to line up with the insulation layer, overinsulating in front of the frame, and minimizing how far closure flashings penetrate the rough opening while still maintaining adequate drainage paths. Eliminating or minimizing thermal bridging on Passive House projects helps ensure the effectiveness of the envelope performance in reducing space-heating energy use.
Heat Recovery Ventilation
Since Passive House projects are airtight, a ventilation system is needed to bring in fresh air and exhaust out built-up pollutants, odours, CO2, and moisture. During winter, this means dumping out warm air and bringing in cooler air that needs to be heated up again, which increases the heating energy. A Passive House ventilation system uses a heat recovery ventilator (HRV) to continuously remove stale or moist air and deliver fresh air. During this process, it extracts heat from the exhaust air and puts it into the incoming air without directly mixing the airstreams together. This way, all the heat in the exhaust air is not completely lost to the outside. For a Passive House HRV, at least 75% of that heat needs to be recovered.
For warmer summer months, most Passive House-certified ventilation systems also feature a summer bypass damper that diverts air around the heat recovery core. That way the system can still bring in fresh air but doesn’t recover heat when it’s not needed.
In dry locations, like the prairies, buildings without humidification in winter can leave the interior spaces at low interior humidity (under 30% RH), which leads to discomfort, potential health issues, and damage to interior materials. In these cases, an energy recovery ventilator (ERV) can be used. Unlike HRVs, which only transfer heat, ERVs can also transfer moisture from the outgoing exhaust to help maintain more-comfortable moisture levels in interior spaces. Occupants can also utilize natural ventilation (using cool summer breezes) from opening windows to exchange stale air by nonmechanical means and are encouraged to do so when it makes sense. Passive House designs utilize both methods to keep ventilation energy to a minimum. While Passive House projects can still be fitted with a heating system (such as air source heat pumps, electric baseboards, or boilers) having heat recovery in ventilation can greatly reduce the size, capacity, and maintenance needs of this equipment, shifting project costs from the mechanical systems to a superior building envelope.
The incredible year-round fresh indoor air quality and stable temperature, the substantial reduction in energy use and operating costs, and the quiet atmosphere that the Passive House standard delivers are directly attributable to these five principles and the way they are integrated into a Passive House building. By following a holistic approach with these five principles through the design and construction on any project, owners, designers, and builders can be confident that they can achieve a truly high-performance building.
Neil Norris, senior industry consultant, Passive House Canada