Passive Houses rely on an airtight envelope, lots of insulation, thermal mass, heat-recovery ventilation systems, and a thorough approach to slowing heat transfer through the walls.
Wall assemblies tend to be thicker. It’s like wrapping an additional blanket around the existing house. But, the most unusual thing about the walls apart from air tightness is the extreme attention paid to eliminating thermal bridging. A typical home can lose 25% of its heat from thermal bridging, Wood frame studs have a lower insulating value than the insulation between them, and if the studs are directly in contact with the inside and outside of the wall, they can act to conduct heat out on cold days, resulting in unwanted heat loss.
The idea with a Passive House is to stabilize temperatures by making the thermal mass of the house work for you like a giant hearthstone. You don’t need a conventional furnace. Once the house is at the desired temperature, it takes very little energy to keep it there. Desert climates of course are more concerned with keeping cool by managing solar heat gain, whereas cold-winter areas are more concerned with staying warm. Tailoring the design for the specific climate and site conditions is of paramount importance.
Humidity also has to be managed, as with other tight-envelope buildings. Placement of vapour barriers is dependent on climate, similar to other types of construction. The wall assemblies can have extensive attention given to waterproofing as well as the placement of air and vapour retarders/barriers.
From inside to outside:
- Furred-out electrical & plumbing chase
- Oriented strand board layer for air-barrier & structural sheathing & water vapor retarder/barrier
- Cellulose/rockwool between the structural framing
- Exterior insulation board
- Rain-screen furring
- Exterior siding
Thermal protection is achieved through the use of super insulated assemblies with an external insulation stopping thermal bridging effects.
Heat protection is achieved through phase change material (PCM), placed in an exterior wall, that alters the temperature profile within the wall and thus influences the heat transport through the wall. This may reduce the net energy transport through the wall via interactions with diurnal temperature swings in the external environment or reduce the electricity needed to meet the net load through the wall by shifting the time of the peak load to a time when the cooling system operates more efficiently.
Moisture protection is achieved through the use of the right composition of water vapour permeable products. The dew point is distanced to the outside and leaves the wall assembly without any condensation; hence no mould or structural issues which adds to an improvement in indoor air climate.