Summer heat gain through Windows

heat transfer

It is important to protect windows with external shading devices, through appropriate window sizing and location, in order to minimise heat gain in summer.

Comparison of heat gains through different treatments for windows in summer

(According to Sustainable Energy Authority Victoria 2002)

  • Unshaded single-glazed window: 100%
  • Standard double glazing as available in Australia: 90%
  • Vertical blinds/open weave drapes: 76%
  • Internal venetian blinds: 55-85% (Effectiveness is reduced as the colour darkens)
  • Internal drapes or Holland blinds: 55-65%
  • Tinted glass: 46-65%
  • Solar control film/reflective glass: 20-60% (Available in different kind of configuration with varying effectiveness)
  • Trees, full shade: 20-60%
  • 1 metre eave over north wall: 30%
  • Roller shutters: 30%
  • External awnings: 25-30%
  • 2m pergola over north wall covered with deciduous vines or shade cloth: 20%
  • Outside metal blind or miniature louvers, parallel and close to window: 15-20%

External shading devices are an effective way to minimise heat gain through glass in summer and keep a building cool. They provide far better protection from heat gain than internal window covering. However, if external shading is not possible, internal coverings can at least reduce the unwanted heat gains. Shading devices should always enable ventilation outside the window, as shading fitted too closely to a window can trap warm air which can be conducted into the house.

Eaves, verandas or pergolas are commonly a part of the building structure, they are durable and do not require ongoing adjustments. It is essential to have a certain distance between the underside of the shading devise and the top of the window. But these fixed shading devises should only be used over north-facing windows, as they lack flexibility and aren’t adjustable. East and west-facing windows need a flexible shading devise that can be completely retracted in order to let the valuable sun through in winter, but to protect from the harsh summer sun. Adjustable shading includes amongst other things canvas blinds, different types of shutters, angled metal slats, louvers or shadecloth over pergolas. Adjustable shading requires action from the occupants, as they have to respond to climatic conditions.