Energy efficiency which embraces a range of strategies, from basic draughtproofing to using energy-efficient appliances and heating systems, is key to eco-design. Of these strategies one of the most critical is insulation. The colder the climate, the more imporant it is for a house to be well insulated. Heat is lost primarily through walls, windows, the roof and the basement. Just by insulating walls and the loft space, heat loss could be reduced by half.
Materials vary widely in their ability to conduct heat, with metals, for example, being very efficient heat conductors and hence poor insulators, and light porous materials, like wool, very poor heat conductors and consequently good insulators. Air is also a very poor heat conductor which is why materials that are honeycombed with air pockets are good insulators. Increasing the thickness of a material also increases its insulating properties.
How well a structual element, such as a roof or wall, performs as an insulator is expressed as its U-value, a figure that is derived according to a formula that takes into account the thermal conductivity of each of the components that makes up that element. For example, in the case of a standard cavity wall, the calculation is based on the conductivity of the exterior brick, the airspace, the insulating material, the interior brickwork, and the plaster or other finish. The lower the U-value, the higher the degree of insulation provided.
The standard way of insulating is to line roof spaces and walls and fill cavities with a baulky insulation product. This may be made of a number of different materials, including cellulose, mineral wool, glass fibre and extruded polystyrene, some of which are more environmentally friendly than others. In most cases, grond and basement floors also need insulation, as dowater tanks and pipes. Heat loss can also be reduced by using a highly reflective material, such as foil , to bounce the heat across a void or cavity instead of the heat being absorbed by a wall - a conventional example is placing foil behind a radiator on an external wall so that heat is directed back into the room.