Passive
Passive houses are houses in which a comfortable year round indoor climate can be maintained without resort to a conventional active heating system. Instead, the house is heated from a combination of passive sources including solar gain, the background heat emissions from appliances and even the occupants of the building. The solar gain refers to the solar radiation which is absorbed by the building itself. This can be maximised through the generous deployment of south facing glazing. It should be noted however, that heat losses through glazing are much higher than through a well insulated wall and that too much glazing in the wrong place can be counter productive and lead both to excessive heat losses in the winter and overheating during the summertime.
Additional features typically include solar thermal panels, heat recovery ventilation systems and micro heat pumps used to extract low density heat from exhaust air or from the building sub-floor.
Under the usually accepted definition of ‘passive’ an active heat source is also permitted, providing the total energy provided for space heating does not exceed 15 kWh/m²/yr. In addition, the total energy input for space heating, domestic hot water supply, cooking and the use of electrical appliances should not exceed 42 kWh/m²/yr. While the heat energy requirements can be accurately calculated from the design and specifications of the building and are relatively easy to achieve, the second requirement is more of a lifestyle issue and can only be assessed after the building is in use.
Given that an average household in Ireland uses an equivalent amount of energy (42 kWh/m²/yr) for electrical appliances alone, it can be seen how difficult it is for houses to meet this second criteria. Indeed, many houses which are described as ‘passive’ may have higher than average electrical energy requirements owing to the use of immersion heaters for back up water heating, heat recovery ventilation systems and micro heat pumps. This electricity nearly always comes from non-renewable sources.
Some Other Specifications
U values: The average U value of the shell of a passive building is usually < 0.1 Wm²K.
Airtightness: There must be under 0.6 air changes per hour from natural ventilation. While in principle this is a good idea, it must be noted that houses which are extrememly air tight may require some form of assisted ventilation in order to maintain a healthy indoor environment.
Passive solar gain: Often stated as being in the region of 40% of total space heating requirements though in practise this is extremely difficult to achieve.
Embodied Energy
The reliance of passive houses on high-tech equipment and products, often manufactured thousands of miles away, suggests that most passive houses have a higher than average embodied energy. In some cases, the energy used to manufacture and transport all the materials used in the house may exceed the total energy used to heat the building over its entire lifetime. Unfortunately, very little research has been carried out on accurately assessing the embodied energy of passive houses and claims that these houses are of very low environmental impact have yet to be proven.
See: Low Environmental Impact Housing , Life Cycle Analysis
