Passive House Building Science Deep Dive
Since completing our Certified Passive House Consultant training, we are pretty excited about all the science behind the best practices for building and home design. For those interested in a Passive House building science deep dive, here it is:
The building science behind Passive House and High-Performance Buildings
The “passive” in Passive House refers to high-performance building strategies that rely on the building envelope to minimize a home or building’s energy needs for heating and cooling as opposed to simply converting to a different energy source without first reducing the energy demand. The first step is a smart site plan that takes into account views, prevailing winds, sun angles during different times of the year, and shading from trees or other structures in the vicinity. Then, we design a compact floor plan that orients the spaces within and around the home or building to take advantage of the sun’s free heat and light and free cooling from breezes. This is a best practice for any building design, but especially important when the goal is achieving a Passive House Certification or the energy savings and comfort benefits associated with building at or near the Passive House standard. From there, Passive House design uses 5 interdependent strategies forming a comprehensive approach to managing a building’s heat, air, and moisture. Read on to understand the building science behind the Passive House standard and why Passive House homes and buildings are so efficient, comfortable, healthy, and quiet.
Optimal Insulation
Heat flows from hot to cold. Like wearing a cozy sweater or putting a coozie around a cold beer, insulation slows the transfer of heat, keeping the interior of a home or building warm in winter and cool in summer. Passive houses and buildings have the optimal amount of insulation for their climate in order to maintain a consistent and comfortable indoor temperature year round using minimal active heating and cooling. Usually this is more insulation than the minimum required by the building code. The thicker walls also absorb more sound. In addition to being more thermally comfortable, Passive Houses and buildings are noticeably quieter inside than conventional construction.
Thermal Bridge Free
To work effectively, the insulation must be continuous and avoid thermal bridges. Thermal bridges provide express lanes for heat energy to travel right through walls, floors, and roofs. When a conductive material (like metal) provides a pathway for heat to bypass the insulation, it lowers the overall capacity of the building envelope as a whole to resist heat transfer. Thermal bridges also pose a mold risk due to the potential for condensation to occur when warmer air comes in contact with a cooler surface within the wall, floor, or roof cavity. Passive Houses and buildings avoid thermal bridges to ensure energy-efficiency, thermal comfort, and building durability.
High Performance Windows and Doors
Even high quality code-minimum windows have a very low thermal performance, with an R-value of approximately 3. They tend to seal poorly and leak air, especially those that operate by sliding action. Depending on climate, windows for Passive houses and buildings will generally have R-values between 6 and 9, meaning they resist the transfer of heat two to three times better than the R-3 windows. The high-performance of Passive House windows comes from triple-glazing in thermally broken frames. Operable Passive House windows also tend to be casement or lift-and-slide types to minimize air leakage. Like the additional insulation, high-performance windows and doors make Passive Houses and buildings more comfortable and quiet too.
Airtight Construction
All homes and buildings need openings allowing people, pets, natural light, and ventilation air to enter or exit. These are intentional openings designed for specific purposes. Passive homes and buildings are carefully detailed and constructed to eliminate unintentional openings—small gaps, cracks, and holes that air can flow through. Even with continuous insulation, air infiltration can make a space feel drafty in winter or too warm in summer. But preventing air leakage is not only about thermal comfort; it is even more critical for building durability. Airtight construction prevents moisture (transported by air in the form of water vapor) from entering and causing damage to wall, floor, or roof assemblies. Certified Passive House buildings have verified airtightness to ensure long-term sustainability with lower energy and maintenance costs.
Continuous Ventilation
Because a portion of the fresh air supply in conventionally built homes and buildings comes from their leakiness, continuous mechanical ventilation is essential in an airtight Passive House building. Passive homes and buildings have exceptional indoor air quality because an air exchanger continuously delivers filtered fresh air to, and exhausts old stale air from, the interior. The amount of exhaust air is replaced by an equal amount of supply air, preventing an air pressure differential between interior and exterior which causes infiltration and air leakage. Technically an “active” system, continuous ventilation with heat recovery saves almost as much energy as it uses by harvesting heat/energy from the outgoing conditioned air and transferring it to the incoming outdoor air. Conditioned air also gets distributed between rooms, making energy-efficient ductless heat pumps more practical in homes and buildings with multiple rooms. Passive houses and buildings provide healthy indoor air with continuous and balanced ventilation.