Five Passive Design Criteria

Provided a design meets the performance criteria and is modeled in PHPP, the designer has a high degree of flexibility in designing a Passive Energy House as they wish. Such an highly energy efficient building does not need to look any different than any other traditionally built house.

However all projects share some characteristics and typically the performance criteria can be delivered by following these five steps:

  1. Optimise the building's form and orientation to reduce heat loss and maximise solar gain.
  2. Super insulation with no thermal bridging (0.15W/m2K and triple glazing (<0.8 W/ m2K)
  3. Airtight fabric <0.6 air exchanges ~ 1.6 m2/h/m3 at 50 Pa
  4. Mechanical ventilation and heat recovery >75% efficiency.
  5. Maximum primary energy demand: 120kWh/m2/yr

This results in buildings whose heating load is reduced by 80-90% compared to a modern standard building - a few candles could heat a room! Therefore a traditional heating system is not necessary any more. The remaining heating demand can be met with, for example, a biomass wood burning stove, a small underfloor heating system (only in a few rooms), or air source heating in the ventilation duct.

1. Optimising a building's shape and orientation to reduce heat loss and maximise solar gain

  • Designing compact forms with minimal surface to volume ratios
  • Sizing south facing glazing to capture solar heat gains
  • Minimize over shading through other buildings or objects
  • Consider forms of shading to avoiding summer overheating
  • Huddling buildings together to reduce exposed areas

2. Super insulating the building fabric

  • Low wall, roof and floor U-values, typically 0.15 W/m2K or lower, about 300mm of insulation
  • Thermal bridge-free construction
  • The thermal envelope has to be designed and build to highest specifications
  • High performance glazing with whole window U-values, typically 0.8 W/m2K or less (triple glazing)

3. Reducing ventilation heat loss with an airtight fabric

  • Reduce air permeability below 0.6 ach-1 @ 50Pa (6-10 times better than standard UK construction)
  • Air tightness foil for all joints of building components
  • Performance guarantee by air permeability testing

4. Providing continuous fresh air with mechanical ventilation and heat recovery

  • An 80-90% efficient heat exchanger and mechanical ventilation ensure that the home is superbly ventilated without increased heat loss. Correct design and installation of the low power fans and ducting ensures the system runs silently
  • Some MVHR systems have a summer bypass for bypassing the heat exchanger in summer
  • Occupants can always choose to additionally ventilate the house by opening windows

5. Reducing primary energy demand to less than 120kWh/m2 /year and applying renewable energy systems when appropriate...

  • Specify low energy light fittings and appliances
  • Insulate domestic hot water pipes and cylinders
  • Provide hot water with solar thermal panels and storage
  • Further reduce CO2 emissions with renewable energy systems