Insulation plays a crucial role in Passive House Design. Passive houses require significantly more insulation than what is required by current building regulations.
This is achieved through airtight construction techniques, effective sunshades for windows and a continuous insulation wrap around the whole building envelope. Eliminating interruptions in the insulation – called thermal bridges – is also essential.
Energy Efficiency
The airtight building envelope and super-insulated walls, roof, and floor of a passive house reduce energy use by keeping desired warmth in the winter and unwanted heat out. This minimizes internal gains from people, cooking, plug loads and sunlight. In the summer, shade, insulation and low SHGC windows keep outdoor temperatures outside, while a ventilation system with a high efficiency heat recovery or energy recovery unit provides for fresh air without introducing excessive amounts of heat to the interior.
The effectiveness of insulation depends on its R value, which measures how well it resists thermal transfer. The higher the R value, the better it is. However, a high R value alone is not enough to make a building energy efficient; it must also be properly installed. Insulation that is installed incorrectly can allow cold air to leak in and hot air to escape, reducing its performance. Air leaks can be a major source of heat loss in buildings, so it is important to ensure that insulation is tightly sealed.
A key to achieving high R values and the energy savings that come with it is the use of continuous insulation. This means that the insulation wraps around the entire building, uninterrupted by any materials used for structural reasons, like drywall or wood frame. Minimizing these thermal bridges, as well as avoiding the use of metals, helps to maximize insulation’s performance.
Indoor Air Quality
Many people are aware that outdoor air pollution can negatively affect their health. Less recognized is that indoor pollutants may have similar impacts — and potentially more severe ones. EPA studies suggest that indoor levels of some pollutants can be up to 100 times higher than outdoors.
A key element of the passive house concept is to achieve a strict level of airtightness that may significantly influence indoor air quality (IAQ). To achieve this, Passive Houses typically have triple-pane windows, insulated frames and glass spacers, and high-performance doors.
In addition to a high degree of superinsulation, passive houses minimize thermal bridges. Insulation is most effective when it wraps the building uninterrupted, but some interface details present challenges, particularly with very conductive materials like metal. These include gaps around components, such as electrical fixtures, or the way a window meets a balcony. Passive House designers strive to eliminate these bridges by using minimal repeating insulation and limiting the number of thermal interfaces.
This approach also helps to minimize moisture accumulation and thus reduce the risk of mold and dampness. Occupants of Passive Houses report that they experience much less condensation on windows and doors than conventional homes, and that the indoor environment is generally odourless. Further research should investigate odours and other chemical, biological and physical parameters in a larger sample of Passive Houses during both heating and non-heating seasons.
Indoor Comfort
The passive house concept combines high quality insulation with a very efficient ventilation system, making it an excellent choice for any climate. Its energy savings, comfort levels, and air quality set a new standard for sustainability.
A key element in any passive design is ensuring the continuity of insulation and eliminating air leaks that can reduce the building’s thermal efficiency. The ideal is to have a continuous layer of insulation encasing the whole building, but this can be difficult when the building needs to incorporate components that are used for structural reasons or for aesthetics (e.g., studs, dormers, or bump-outs). These areas of bypass are known as thermal bridges and can significantly reduce the effectiveness of insulation. Minimizing the number of these bridges by using a single material for framing, keeping corners and bump-outs well insulated, and avoiding repeated insulation at edges and joints helps improve overall thermal performance.
Superinsulation is a crucial aspect of the passive design, with assemblies having up to double or triple the R-values of current building codes. Careful consideration is also given to the use of natural light and the positioning of skylights to avoid excessive glare and heat gain. Another key feature is careful construction that focuses on sealing the building envelope with effective air barriers and detailed detailing for penetrations and terminations. This involves a high level of coordination amongst the design team, general contractor, and subcontractors.
Materials
Passive houses are built with high-performance building materials that help reduce energy consumption. These include orient strand board (OSB), metal, and spray foam, and they are often assembled from structural insulated panels (SIPs)—a composite that functions as framing, insulation, and sheathing all in one.
The type of insulation used in the house plays a major role, too. Bulk insulators are ideal for passive homes because they resist conductive heat flow and provide a large surface area for thermal mass, but the choice of insulation also depends on how the building is built. A key consideration is avoiding thermal bridges—areas where other materials bypass the insulation. This can occur around window and door frames, sill plates, recessed lighting fixtures, and electrical outlets. The best way to minimize thermal bridging is to limit repeating insulation layers and to ensure that the insulation wraps completely around components without any gaps.
Other material considerations include minimizing the use of drywall and plaster, which are known to cause air leakage, as well as selecting non-toxic insulation. Ideally, non-toxic insulation should also be low-VOC, since VOCs can contribute to poor indoor air quality.
In addition to reducing air infiltration, passive house design includes a very tight building envelope that limits the amount of fresh air brought into the home—a requirement that requires careful attention and rigorous testing with a blower door. The airtightness is aided by the use of membranes and tapes that are carefully installed, as well as by sealing any openings for plumbing, electrical, or ventilation systems.