Windows play an essential role in the overall energy efficiency of a home. Up to 40% of heat is lost and as much as 50% of unwanted heat is gained through windows. This heating effect is known as ‘solar gain’. The position of windows and the kind of window protection used (e.g. glazing) are important factors in determining the heating and cooling energy bill of households each year.
Preventing Heat Gain in Summer
It is better to prevent the heat from the sun from reaching the window instead of dealing with the problem after the heat has entered into your home. External shading is much more efficient in summer at keeping your home cool than using curtains and internal blinds.
Preventing Heat Loss in Winter
Single sheets of unprotected glass generate three major concerns during winter: discomfort, heat loss and condensation. Unprotected glass can lose nearly ten times more heat than an insulated wall with the same area. Choosing the right window and protection can cut down on the amount of heat loss during winter.
Methods to prevent heat loss through windows include:
- Double glazed windows
- Low emissivity glass
- Toned glass, reflective glass and reflective films
- Curtains and blinds
- Insulated shutters
Double Glazed Windows
Double glazed windows are composed of two sheets of glass separated by a sealed air space, normally between 6 mm and 20 mm in width.
Double glazing is designed to reduce the heat loss associated with single sheets of glass while at the same time permitting natural light to pass through. The performance of any window is improved by double glazing. Double glazing also reduces the amount of noise transmitted through the window. Glass bricks or glass blocks also use double glazing to provide thermal insulation and reduce heat lost during winter.
Low Emissivity Glass
Low emissivity glass, or Low-E glass, has an invisible coating which reflects radiant heat back into the room. Low-E glass is typically available only in double glazed windows. Whereas this layer is applied to the interior layer of glass in cooler climates, in warmer climates, a Low-E coating is applied to the outside layer of glass, as incoming solar radiation is then reflected back outside, away from the room.
Toned Glass, Reflective Glass and Reflective Films
Toned glass, reflective glass and reflective films are forms of glass treatments which are designed to reduce the amount of heat gain into the home by absorbing and reflecting more heat than clear glass. By properly selecting the toned or reflective glass, the amount of summer heat gain will be reduced, while retaining light transmissions when compared to ordinary clear glass.
Roof Windows, Clerestory Windows and Skylights
Daylight is a rich and complicated source of light. The ideal use of daylight requires heat flow management that can significantly improve the quality of life within the building or structure. Skylights, roof windows and clerestory windows are an effective way of letting daylight in; however, careful consideration of the materials used is required to achieve a comfortable temperature in the room below.
There are a number of products currently available which minimize the penetration of heat and ultra-violet rays. Properly selecting the right window and protection can reduce heat loss from skylights or roof windows than compared to ordinary clear glass.
Self-cleaning coatings can also be added to the outer layer of a window, particularly in the case of skylights. This makes use of ultraviolet light to decompose organic matter that might collect on a roof window, and the decomposed matter can then be washed off naturally by rainwater.
Choosing the Right Frame
It is also important to consider the material of the window frame in order to have a fully energy-efficient window. Heat loss or heat retention can occur, depending on the material used.
PVC may have desirable thermal properties but can crack and be brittle after extensive wear, together with being difficult to dispose of at the end of its life cycle. While metal is an option in some climates, it is inappropriate in general due to conducting heat outside of the building being an undesirable property, and that it can potentially expand under significant heating and place stresses on the window. Wood is a better option for most climates, particularly treated softwood or hardwood from a sustainable source.
Last update 17th February 2008