13 Sep Blog

The 5 principles of Passive House design

passive house image-1

Australia’s largest Passive House was recently completed in Brisbane’s inner-west, putting this type of sustainable construction into the spotlight.

Knauf Australia also recently supplied plasterboard to Sapphire House, one of the display houses on Sustainable House Day 2019. Sapphire House is the first Passive House in Australia to be built to Bushfire Attack Level Flame Zone (BAL-FZ) regulations, the highest bushfire risk rating.

The first Passive House was built in 1991 by Austrian physicist Wolfgang Feist as a solution to more efficiently heat European homes in winter, but this building concept has been proven to be just as effective to cool down a home in the sweltering Queensland summers as keeping European houses cozy in their harsh winters.

In Australia the demand for heating and cooling is mixed. In southern Australia the demand is mixed between space heating and cooling, while it is mostly cooling in the central and northern parts of the country. Passive House design can successfully be implemented in these different climates.

What is Passive House?

Passive House (from the German Passivhaus) originated in Europe. The concept has been around for approximately 30 years, and is based upon the premise that adopting a few key design principles can enable the creation of buildings with healthy, dry indoor spaces that require little or no heating or cooling.

Passive House buildings are designed to be resilient to temperature peaks and be comfortable all year round.

The term can be misleading because the concept is not only applied to houses, but all dwellings. Across the spectrum, certified Passive House buildings have one thing in common: an exceptionally high level of energy efficiency and comfort.

By implementing the correct design strategies, Passive House buildings are able to achieve an extremely high level of energy efficiency. To gain full accreditation from the International Passive House Institute the total heating and cooling demands of a new build must remain under 15kWh/m²/year. This is only about 10% of the energy used in conventional buildings. 

NatHERS

In Australia the Nationwide House Energy Rating Scheme (NatHERS) rates the energy efficiency of a home, based on its design using a star rating system (out of ten).

The National Construction Code (NCC) references these star ratings specifying a minimum number of star levels for all new houses built in Australia. In most parts of the country, the 6 star standard rating is the minimum to be approved for the construction of stand-alone (Class 1) dwellings.

The standard energy requirement for Passive House accreditation is said to be equivalent to a 7.3 star design in Canberra, and a 6.4 star design in Melbourne.

The 5 principles

According to the Australian Passive House Association, a Passive House is designed and built in accordance with five simple building-science principles:

  • Airtightness
  • Thermal insulation
  • Mechanical ventilation heat recovery
  • High performance windows
  • Thermal bridge free construction

1. Airtightness

For Passive House accreditation, the building envelope must achieve an extremely low air leakage performance (less than 0.6 air changes per hour at 50 pascal).

Airtightness is the first line of strategy in achieving energy efficiency, comfort and affordability. It involves the construction of highly insulated exterior walls, roof, windows and floor slab.

Airtightness is often seen as a challenge in the Australian climate with our tendency to fling open doors and windows to let the breeze in, but Passive House design does not mean that you need to live in an airtight box. The difference is that when you close the doors and windows, there are no gaps for warm or cold air to escape, maintaining a comfortable indoor environment.

Australian building codes do not currently require mandatory air leakage testing, but such testing (e.g. a blower door test) is required at various stages of construction for full Passive House accreditation.

2. Thermal insulation

Thick and continuous insulation provides proper thermal separation between the heated or cooled inside environment and the outdoors. This improves thermal comfort and reduces the risk of condensation.

3. Mechanical ventilation heat recovery

Mechanical Ventilation with Heat Recovery (MVHR) is the process of improving the indoor air quality without opening windows or doors. It does not mean that windows and doors cannot be opened, only that they don’t need to be opened to achieve fresh air quality.

MVHR is for indoor air quality control and not for heating or cooling buildings, but MVHR systems do recover warm and cool air that would otherwise have gone to waste. They also clean the air from pollution and help to regulate humidity.

4. High performance windows

Windows play an important role in Passive House design to allow solar radiation to warm up the interior in winter, but minimise radiation in the warmer months. Windows are typically designed to be airtight and double or triple glazed.

5. Thermal bridge free construction

Good insulation will not be of any value if it is not continuous. This means keeping penetrations through the insulation to a minimum, and where it can’t be avoided, using low or non-conducting materials. The aim is to avoid thermal bridges along which heat can escape. Thermal bridges can also increase the condensation risk.

Thermal mass

Of course, designing an accredited Passive House is more complicated than adhering to the five design principles mentioned above. Other factors such as thermal mass also need to be taken into consideration.

Thermal mass is the ability of a material to absorb and store heat energy. It takes a large amount of energy to change the temperature of high density materials with a high thermal mass, such as concrete, bricks and tiles. When the temperature goes down, the absorbed heat will radiate back into the space.

According to the New Zealand architect and Passive House designer Elrond Burrell the amount of thermal mass needed isn’t significant though, and is easily provided in lightweight construction.

Further resources

  • Knauf Australia provides a range of wall and ceiling systems that are ideal for Passive House design. Download our Blueprint Technical Manual for more information.
  • Use Knauf’s Wall Thermal Wall Calculator to estimate the Total R-Value of a wall of Class 1 and 10 buildings. The thermal resistance of an external wall has significant influence on the energy efficiency of a building. The total R-Value, as per 2019 NCC Volume Two, is the sum of the thermal resistances (R-values) of the individual component layers in a composite element including any building material, insulating material, airspace and associated surface resistances.