Passive Home
From Antivist
The term Passive house (Passivhaus in German) refers to the rigorous, voluntary, Passivhaus standard for energy use in buildings. It results in ultra-low energy buildings that require little energy for space heating. A similar standard, MINERGIE-P®, is used in Switzerland.
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Space heating requirement
By achieving the Passivhaus standards, Passivhaus buildings are able to dispense with conventional heating systems. The ability to do this is the underlying Passivhaus objective. However this does not mean that no heating is required, and most Passivhaus buildings do include a system to provide low levels of supplemental space heating. This is normally distributed through the low-volume heat recovery ventilation system that is required to maintain air quality, rather than by a conventional hydronic or high-volume forced-air heating system, as described in the space heating section below.
Design and construction
Passive solar design
Following passive solar building design techniques, where possible buildings are compact in shape to reduce their surface area, with windows oriented towards the south (in the northern hemisphere) to maximise passive solar gain. However, the use of solar gain is secondary to minimising the overall energy requirements.
Passive houses can be constructed from dense or lightweight materials, but some internal thermal mass is normally incorporated to reduce summer peak temperatures, maintain stable winter temperatures, and prevent possible over-heating in spring or autumn before normal solar shading becomes effective.
Superinsulation
Passivhaus buildings employ superinsulation to significantly reduce the heat leakage through the walls, roof and floor compared to conventional buildings. A wide range of thermal insulation materials can be used. Special attention is given to eliminating thermal bridges.
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.
Advanced window technology
To meet the requirements of the Passivhaus standard windows are manufactured with exceptionally high R-values (low U-values, typically 0.85 to 0.70 W/m²K for the entire window including the frame). These normally combine triple-pane insulated glazing (with a good solar heat-gain coefficient, low-emissivity coatings, argon or krypton gas fill, and 'warm edge' insulating glass spacers) with air-seals and specially developed thermally-broken window frames.
In Central Europe, for unobstructed south-facing Passivhaus windows, the heat gains from the sun are, on average, greater than the heat losses, even in mid-winter.
Airtightness
The standard requires the building to achieve very high levels of airtightness, much higher than are normally achieved in conventional construction. Air barriers, careful sealing of every construction joint in the building envelope, and sealing of all service penetrations through it are all used to achieve this.
Airtightness minimises the amount of warm (or cool) air that can pass through the structure, enabling the mechanical ventilation system to recover the heat before discharging the air externally.
Ventilation
Mechanical heat recovery ventilation systems, with a heat recovery rate of over 80% and high efficiency ECM motors, are employed to maintain air quality. Since the building is essentially airtight, the rate of air change can be optimised and carefully controlled at about 0.4 air-changes per hour. All ventilation ducts are insulated and sealed for air tightness.
Although not compulsory, earth warming tubes (typically �20cm diameter, �40 m long at a depth of �1.5 m) are often buried in the soil to act as earth-to-air heat exchangers and pre-heat (or pre-cool) the intake air for the ventilation system. In cold weather the warmed air also prevents ice formation in the heat recovery system's heat exchanger.
Space heating
In addition to using passive solar gain, Passivhaus buildings make extensive use of their intrinsic heat from internal sources � such as waste heat from lighting, white goods (major appliances) and other electrical devices (but not dedicated heaters) � as well as body heat from the people and animals inside the building. Together with the comprehensive energy conservation measures taken this means that a conventional central heating system is not necessary, although they are sometimes used due to client preference.
Instead, Passive houses typically have a dual purpose 800 to 1,500 Watt heating and/or cooling element integrated with the supply air duct of the ventilation system. It is fundamental to the design that all the heat required can be transported by the normal low air volume required for ventilation. A maximum air temperature of 50°C (122°F) is applied to prevent any possible smell of scorching from dust that escapes the filters in the system.
The air-heating element can be heated by a small heat pump, by solar thermal energy, or simply by a natural gas or oil burner. In some cases a micro-heat pump is used to extract additional heat from the exhaust ventilation air, using it to heat either the incoming air or the hot water storage tank. Small wood-burning stoves can also be used to heat the water tank, although care is required to ensure that the room in which stove is located does not overheat.
Because the heating capacity and the heating energy required by a passive house both are very low, the particular energy source selected has fewer financial implications than in a traditional building, although renewable energy sources are well suited to such low loads.
Lighting and electrical appliances
To minimise the total primary energy consumption, low-energy lighting (such as compact fluorescent lamps), and high efficiency electrical appliances are normally used.
