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A low-energy house is any type of house that from design, technologies and building products uses less energy, from any source, than a traditional or average contemporary house. In the practice of sustainable design, sustainable architecture, low-energy building, energy-efficient landscaping low-energy houses often use active solar and passive solar building design techniques and components to reduce their energy expenditure.
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General usage
The meaning of the term 'low-energy house' has changed over time, but in Europe it generally refers to a house that uses around half of the German or Swiss low-energy standards referred to below for space heating, typically in the range from 30 kWh/m²a to 20 kWh/m²a (9,500 Btu/ft²/yr to 6,300 Btu/ft²/yr). Below this the term 'Ultra-low-energy building' is often used.
The term can also refer to any dwelling whose energy use is below the standards demanded by current building codes. Because national standards vary considerably around the world, 'low-energy' developments in one country may not meet 'normal practice' in another.
National standards
In some countries the term relates to a specific building standard. In particular, these seek to limit the energy used for space heating, since in many climate zones it represents the largest energy use. Other energy use may also be regulated. The history of passive solar building design gives an international look at one form of low-energy building development and standards.
Europe
In Germany a low-energy house (Niedrigenergiehaus) has a limit equivalent to 7 litres of heating oil for each square metre of room for space heating annually (50 kWh/m²a or 15,850 Btu/ft²/yr). In Switzerland, the term is used in connection with the MINERGIE standard (42 kWh/m²a or 13,300 Btu/ft²/yr) or the Minergie-P (equivalent to the Passivhaus).
In comparison, the German Passivhaus ultra-low-energy standard, currently undergoing adoption in some other European countries, has a maximum space heating requirement of 15 kWh/m²a or 4,755 Btu/ft²/yr.
A "sub-10 passive house" is under construction in Ireland that has an independently evaluated PHPP (Passive House) rating of 9.5 kW/m2/year. Its form of construction also tackles the issue of embodied energy, which can significantly distort the lifecycle CO2 emissions associated with even low energy use houses.
North America
In the United States, the ENERGY STAR program is the largest program defining low-energy homes and consumer products. Homes earning ENERGY STAR certification use at least 15% less energy than standard new homes built to the International Residential Code, although homes typically achieve 20%–30% savings.
In addition, the US Department of Energy launched a program in 2008 with the goal of spreading zero-energy housing over the US. Currently, participating builders commit to constructing new homes that achieve 30% savings on a home energy rating scale.
Zero-energy and energy-plus buildings
Beyond ultra-low-energy buildings are those that use, on average over the course of a year, no imported energy - zero-energy buildings – or even those that generate a surplus - energy-plus houses – both of which have been and are being successfully built.
This can be achieved by a mixture of energy conservation technologies and the use of renewable energy sources. However, in the absence of recognized standards, the mix between these – and consequently the energy-use profile and environmental impact of the building – can vary significantly.
At one end of the spectrum are buildings with an ultra-low space heating requirement that therefore require low levels of imported energy, even in winter, approaching the concept of an autonomous building.
At the opposite end of the spectrum are buildings where few attempts are made to reduce the space heating requirement and which therefore use high levels of imported energy in winter. While this can be balanced by high levels of renewable energy generation throughout the year, it imposes greater demands on the traditional national energy infrastructure during the peak winter season.
Introduction
Low-energy buildings typically use high levels of insulation, energy efficient windows, low levels of air infiltration and heat recovery ventilation to lower heating and cooling energy. They may also use passive solar building design techniques or active solar technologies. These homes may use hot water heat recycling technologies to recover heat from showers and dishwashers. Lighting and miscellaneous energy use is allieviated with fluorescent lighting and efficient appliances. Weatherization provides more information on increasing building energy efficiency.
Passive Houses are required to achieve a whole building air change rate of no more than 0.6 ac/hr under forced pressurisation and depressurisation testing at 50Pa minimum. On site blower door testing by certified testers is used to prove compliance.
A significant feature of ultra-low-energy buildings is the increasing importance of heat loss through linear thermal bridging within the construction. Failure to eliminate thermal pathways from warm to cold surfaces ("bridges") creates the conditions for interstitial condensation forming deep within the construction and lead to potentially serious issues of mould growth and rot. With near zero filtration losses through the fabric of the dwelling, air movement cannot be relied upon to dry out the construction and a comprehensive condensation risk analysis of every abutment detail is recommended.
Passive solar design and landscape
Passive solar building design and energy-efficient landscaping support the low-energy house in conservation and can integrate them into a neighborhood and environment. Following passive solar building techniques, where possible buildings are compact in shape to reduce their surface area, with principal windows oriented towards the equator - south in the northern hemisphere and north in the southern hemisphere - to maximize passive solar gain. However, the use of solar gain, especially in temperate climate regions, is secondary to minimizing the overall house energy requirements. On the other hand in hot climates temperatures excess heat can create uncomfortable indoor conditions. Passive alternatives to air conditioning systems such as temperature-dependent venting have been shown to be effective in regions with cooling needs. Other techniques to combat excessive solar heat gains include Brise soleils, trees, attached pergolas with vines, vertical gardens, green roofs among others.
Low-energy 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 overheating in spring or autumn before the higher sun angle "shades" mid-day wall exposure and window penetration. Exterior wall color, when the surface allows choice, for reflection or absorption insolation qualities depends on the predominant year-round ambient outdoor temperature. The use of deciduous trees and wall trellised or self attaching vines can assist in climates not at the temperature extremes.
Lighting and electrical appliances
To minimize the total primary energy consumption, the many passive and active daylighting techniques are the first daytime solution to employ. For low light level days, non-daylighted spaces, and nighttime; the use of creative-sustainable lighting design using low-energy sources such as 'standard voltage' compact fluorescent lamps and solid-state lighting with Light-emitting diode-LED lamps, organic light-emitting diodes, and PLED - polymer light-emitting diodes; and 'low voltage' electrical filament-Incandescent light bulbs, and compact Metal halide, Xenon and Halogen lamps, can be used.
Solar powered exterior circulation, security, and landscape lighting - with photovoltaic cells on each fixture or connecting to a central Solar panel system, are available for gardens and outdoor needs. Low voltage systems can be used for more controlled or independent illumination, while still using less electricity than conventional fixtures and lamps. Timers, motion detection and natural light operation sensors reduce energy consumption, and light pollution even further for a Low-energy house setting.
Appliance consumer products meeting independent energy efficiency testing and receiving Ecolabel certification marks for reduced electrical-'natural-gas' consumption and product manufacturing carbon emission labels are preferred for use in Low-energy houses. The ecolabel certification marks of Energy Star and EKOenergy are examples.