What is a Passive House?
Wikipedia.org states that a Passive House (originally Passivhaus in German):
“…is a rigorous, voluntary standard for energy efficiency in a building, reducing its ecological footprint. It results in ultra-low energy buildings that require little energy for space heating or cooling. The standard is not confined to residential properties; several office buildings, schools, kindergartens and a supermarket have also been constructed to the standard. Passive design is not an attachment or supplement to architectural design, but a design process that is integrated with architectural design. Although it is mostly applied to new buildings, it has also been used for refurbishments.
The Passivhaus standard requires that the building fulfills the following requirements:
- The building must be designed to have an annual heating and cooling demand as calculated with the Passivhaus Planning Package of not more than 15 kWh/m2 (4,755 BTU/sq ft; 5.017 MJ/sq ft) per year in heating or cooling energy OR be designed with a peak heat load of 10 W/m2 (1.2 hp/1000 sq ft).
- Total primary energy (source energy for electricity, etc.) consumption (primary energy for heating, hot water and electricity) must not be more than 60 kWh/m2 (19,020 BTU/sq ft; 20.07 MJ/sq ft) per year.
- The building must not leak more air than 0.6 times the house volume per hour (n50 ≤ 0.6 / hour) at 50 Pa (0.0073 psi) as tested by a blower door, or alternatively when looked at the surface area of the enclosure, the leakage rate must be less than 0.05 cubic feet per minute.
Typically, passive houses feature:
- Fresh, clean air: Note that for the parameters tested, and provided the filters (minimum F6) are maintained, HEPA quality air is provided. 0.3 air changes per hour (ACH) are recommended, otherwise the air can become “stale” (excess CO2, flushing of indoor air pollutants) and any greater, excessively dry (less than 40% humidity). This implies careful selection of interior finishes and furnishings, to minimize indoor air pollution from VOC’s (e.g., formaldehyde). This can be counteracted somewhat by opening a window for a very brief time, by plants, and by indoor fountains.
- Because of the high resistance to heat flow (high R-value insulation), there are no “outside walls” which are colder than other walls.
- Homogeneous interior temperature: it is impossible to have single rooms (e.g. the sleeping rooms) at a different temperature from the rest of the house. Note that the relatively high temperature of the sleeping areas is physiologically not considered desirable by some building scientists. Bedroom windows can be cracked open slightly to alleviate this when necessary.
- Slow temperature changes: with ventilation and heating systems switched off, a passive house typically loses less than 0.5 °C (1 °F) per day (in winter), stabilizing at around 15 °C (59 °F) in the central European climate.
- Quick return to normal temperature: opening windows or doors for a short time has only a limited effect; after apertures are closed, the air very quickly returns to the “normal” temperature.
- Some have voiced concerns that Passivhaus is not a general approach as the occupant has to behave in a prescribed way, for example not opening windows too often. However modeling shows that such concerns are not valid.”
For a quick review of how Passive House works, see Hans-Jörn Eich’s excellent video:
These standards are much higher than houses built to most normal building codes. Let’s break these up into a few sections and discuss them one at a time:
Passive house buildings employ super insulation to significantly reduce the heat transfer through the walls, roof and floor compared to conventional buildings. Insulation keeps winter warmth inside the building and is also used to protect the interiors from the hot summer sun. A disadvantage resulting from the thickness of wall insulation required is that, unless the external dimensions of the building can be enlarged to compensate, the internal floor area of the building may be less compared to traditional construction.
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. Passive house is designed so that most of the air exchange with exterior is done by controlled ventilation through a heat-exchanger in order to minimize heat loss (or gain, depending on climate), so uncontrolled air leaks are best avoided.
It is essential to avoid all envelope thermal bridging to prevent condensation and heat losses. This involves careful attention to detailing during design stage and during installation. Every place where almost any material touches another, a thermal bridge is made. Most houses today are built with insulation between the studs. The studs would then be a thermal bridging letting the heat conduct through the wood. This makes the walls’ insulation much less efficient.
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 or 62°F on the coldest day of the year. This eliminates the need for perimeter heating below windows – a huge saving. For unobstructed equator-facing Passive house windows, the heat gains from the sun are, on average, greater than the heat losses, even in mid-winter.
Passive house relies on internal heat gain from passive solar heat gain during the heating season. Orientation of windows towards the equator is crucial, so long as adequate summer shading from large over hangs or foliage 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.
Use of passive natural ventilation is an integral component of passive house design where ambient temperature is conducive — either by singular or cross ventilation, by a simple opening or enhanced by the stack effect from smaller ingress with larger egress windows and/or clerestory-operable skylight. When ambient climate is not conducive, mechanical heat recovery ventilation systems, with a heat recovery rate of over 80% and high-efficiency electronically commutated motors, are employed to maintain air quality, and to recover sufficient heat to dispense with a conventional central heating system. Since passively designed buildings are essentially air-tight, the rate of air change can be optimized and carefully controlled at about 0.4 air changes per hour. All ventilation ducts are insulated and sealed against leakage.
Don’t think that you have to sell your house and build another just to get a Passive house. It would be easier to start fresh, but it can be done to your house that you are currently in now. It will definitely be a lot of work. Here is a video of someone who retrofit a house built in the 50’s and made their dream a reality:
Image Credit: Featured Image