By employing a super-insulated thermal envelope and super-airtight construction while using free solar energy, Passive Houses require only 10% of the energy used in a standard new building.
Passive house buildings provide a comfortable indoor climate, being both cool in summer and warm in winter, without the need for a conventional heating system. The savings gained from not requiring a furnace or air conditioner enable Passive Houses to be built for little or no additional cost compared to ordinary buildings. Driving down the cost of passivhaus buildings is a major focus of research at Local Impact Design.
The intensive monitoring of tens of thousands of Passive House buildings in Europe and worldwide by the Passivhaus Institute over the last twenty years has clearly demonstrated and validated the performance of these buildings. A growing number of stakeholders within the construction industry are realising the benefits of aiming towards and achieving the Passive House standard including breakthroughs in affordable housing.
Passive House Design Basics
The Passivhaus approach can be explained briefly under the following headings:
Passive house relies on internal heat gain from passive solar heat gain during the heating season. A southerly orientation of windows is crucial, so long as adequate summer shading is provided to avoid overheating. Due to low east/west sun angle, effective shading is difficult and glazing should be reduced on these facades. Buildings with long E-W axis are ideal.
A simple, compact shape results in less heat loss through the roof and walls. In colder climates, reaching the Passive House standard is economically unattractive with designs where the area to volume ratio (A/V ratio) is over 0.7:1. As a bonus, a general rule is that the simpler the shape of a building, the lower will be its unit cost.
Passive House concentrates on a fabric-first approach to reduce energy consumption, utilizing a high level of insulation and a draught-free construction. Insulation keeps winter warmth inside the building and is also used to protect the interiors from the hot summer sun.
Thermal-bridge free design
It is essential to avoid all envelope thermal bridging to prevent condensation and heat losses. This involves careful attention to detailing during the design stage and during installation. Modeling using THERM is used to determine thermal bridging coefficients for a chosen design.
Air-tight construction is achieved by implementing a continuous air barrier, careful sealing of every construction joint and sealing of all service penetrations. Tightness is tested using a blower door test. A Passive House exhibits air leakage of 0.6 air changes per hour, up to 5 times better than a typical new home.
Windows in a Passive House have exceptionally high R-values based on triple-pane insulated glazing with air-seals and thermally-broken window frames. These high-performance windows allow solar heat gain and daylight without losing undue amounts of heat such that windows are no longer the weakest part of the envelope but actually represent a net gain! These windows are so well insulated that the internal surface temperature of the windows will not fall below 17°C on the coldest day of the year. This eliminates the need for perimeter heating below windows – a huge saving.
A high-performance vapour control layer is essential to avoid ‘Interstitial condensation’ in the wall or roof cavity, which leads to reduced building life, health issues from toxic mould, as well as wasted energy.
Heating from internal heat gains
Passive houses make extensive use of heat gain from passive solar (through windows) and internal heat sources such as waste heat from lighting, stoves and other appliances, as well as occupant body heat. The combined effect of internal heat gains, together with thermal bridge-free design, super-insulation and air tightness, means that heat loads are reduced by such a degree that a conventional heating system can be eliminated.
Low-energy heat recovery ventilation system
Low-energy heat recovery ventilation (HRV) systems provide a plentiful supply of fresh air even when windows are mostly closed in the cold winter months. In addition to maintaining a flow of fresh air, efficient heat recovery ventilation systems provide perfect air humidity levels for health conditions all year round without wasting heat. With high-efficiency heat exchangers and electronically commutated motors (ECM), a Passive House-approved HRV can save 10 times more energy than it uses.
The extremely low energy consumption of a Passivhaus often results in cost savings to the occupier of a Passivhaus building of more than 90% per year compared to ordinary buildings. Such low energy requirement means that zero-energy is relatively easily achievable since less renewable energy is required to replace the energy that would otherwise come from fossil fuel.
In addition to insulating the interiors of a building from solar heat gains, automated solar shading will help avoid overheating in the summer. Night time natural fresh air circulation to purge-cool a little thermal mass will further improve comfort in summer months.
Passive House Basics in a Nutshell
A 90-second review of how Passive House works: