Earlier this week we wrote about Habitat for Humanity’s first passive house being built in Vermont. While many readers are perhaps familiar with some of the building standards for LEED or Energy Star certification, we’re also interested in the exact criteria that go into building a passive home.
Passive house certification is certainly the highest energy standard for home building. As the New York Times notes, a LEED-certified home can be certified as such with only a 15% improvement in efficiency over conventional construction; passive homes are capable of achieving 90% efficiency over conventional homes, and some are even able to return electricity into the grid, netting a negative energy consumption rate.
These homes are well-insulated (nearly air-tight) and heated by passive solar energy – like a greenhouse – and internal energy gains from human body heat or, for example, the heating of a tea kettle. Achieving this level of efficiency requires exacting design, including the very specific angle of construction towards the sun. Achieving passive house certification from the Passive House Institute U.S.A. requires a very rigorous evaluation of the home’s energy consumption and insulation. For a home to be passive, it must be determined to have:
1. a maximum annual heating requirement less than or equal to 15 kWh/(m²a)
2. a pressurization test result with a maximum of 0.6 h^-1
3. an entire specific primary energy demand less than or equal to 120 kWh/(m²a) including domestic energy consumption.
What this means is that the house cannot require heating or cooling demands beyond the specific thresholds detailed above. Accomplishing this requires super-tight insulation of the home. To be more specific, the pressure envelope of the home cannot exceed a loss of pressure of 0.6 air changes per hour (the number of times per hour that a room’s total air volume is exchanged with fresh air at 50 pascals), measured by blower-door test – only a minimal amount of air (heated air in the winter) is allowed to escape the home. This level of insulating efficiency reduces the heating requirement to below the aforementioned threshold, and reduces the level of electrical consumption needed to cool or heat the home.
To read more about passive house certification, you can read Passive House Institute U.S.’s performance characteristics, or read Passiv Haus Insititut’s residential criteria (PDF).