A Passive House is designed to retain more heat over time meaning less energy is needed to maintain temperature – much like a thermos flask. Careful design & detailing combined with a comprehensive understanding of the Passive House principles means it is possible to construct a house which uses almost no heating and has a fantastic indoor climate. It is then very easy to create a Net Zero Energy House, as your energy requirements are so low.
In a traditional New Zealand house a substantial proportion of the space heating is needed to replace heat energy that has been lost through the walls, floors and roof. In a Passive House the loss through the thermal envelope is minimal, so that a combination of passive heating and heat recovery ventilation provides almost all of the heating.
Energy Use in New Zealand Households. Report on the 10 year analysis for the Household Energy Use Project (HEEP). BRANZ Study report No. SR 155 (2006).
Specific Energy Demand
The Passive House standard has a Specific Energy Demand (heating & cooling) of 15kWh/(m2a). Team Green Architects has a company minimum standard of 50kWh/(m2a) (see the “The Comfort Equation” for more information and typical Building Code minimum energy demands). If you have achieved Passive House or are coming close to this reduced Heat Energy Demand other areas of reducing energy become much more significant.
Primary Energy Use
The Passive House standard has a maximum annual Primary Energy Use of 120kWh/(m2a). This is the total energy of all household appliances, and is achieved by reducing the requirement for grid electricity. This can be done by utilising highly efficient Hot Water Heating, LED for lighting, and the highest Energy Star rated appliances. This will further reduce your overall energy usage and therefore energy bill. You can then look at alternative forms of renewable on-site energy production. This will much more easily allow you to have a Net Zero Energy Building/House bringing your annual energy bill to Zero!
PASSIVE HOUSE COMFORT EXPECTATIONS:
- Operative temperatures in Winter of at least 20°C (throughout the house)
- Operative temperature in summer does not exceed 26°C at 60% Relative Humidity (or equivalent) for more than 5% of the time
- Relative Humidity is in a range of 30-70%
- Air Velocity is below 0.2m/s
- No thermal draft; operative temperatures close to air temperatures. (Operative temperature is the mean of the surface temperature and radiant room temperature)
- Ability to adjust hygrothermal conditions to a degree
- Frequency of Overheating ≤ 10% of the time
PASSIVE HOUSE CRITERIA:
- Maximum Heat Load ≤ 10W/m2
- Specific Energy Demand (heating & cooling) ≤ 15kWh/(m2a) (Energy required for space heating or cooling)
- Maximum Annual Primary Energy ≤ 120kWh/(m2a)
- Airtightness (under pressure of 50 Pascal) n50 ≤ 0.6h-1 (Air changes per hour)
- Internal Surface Temperatures ≤ 3°C Below mean Ambient Temperature
- Thermal Bridges (See ‘Thermal Discomfort & Unhealthy Buildings’) ≤ 0.01 W/(mK)
- Ventilation System ≥ 75% efficient
PASSIVE HOUSE FUNCTIONAL DEFINITION:
"A Passive House is a building for which thermal comfort can be achieved solely by heating or cooling of the fresh air volume required to provide good indoor air quality anyway."
Downloads and further recommended reading on Passive House:
- IPHA Passivhaus brochure
- UK guide to Passive House - BRE Passivhaus primer
- Proclima NZ Study on airtightness and moisture control
- Why Ventilate? Fantech NZ
- Zehnder Ventilation brochure