How to control heat gain through windows

Air conditioners are designed to cool a room. Windows are designed to let sunlight and air in - but an unfortunate side effect of most windows is that they also let a lot of heat in and out of a room. If you're getting an air conditioner, consider how the windows in your room will affect the air conditioner's efficiency.

Thermal curtains  
Thermal curtains can help reduce heat gain.

According to the Building Code of Australia (also called the BCA), windows are now legally required in all 'habitable' rooms, and their size must be equivalent to at least 10% of the floor area. In some rooms, this will mean a lot of window space, which in turn will mean a lot of opportunity for unwanted heat to enter the room.


Window orientation and glazing options

When it comes to selecting an air conditioning unit, window size and direction both need to be taken into account. Windows that are west facing will be exposed to hot afternoon sun, and will therefore allow more heat to enter the room than windows that face in other directions. Rooms with large west facing windows normally require a larger capacity air conditioner than the same sized room with south facing windows.

Controlling the heat that comes through windows can make a big difference to your needs. Some options to do so include installing double glazing (especially with low-e glass), and using solar films. These systems aren't necessarily expensive, are normally quite effective, and don't affect the amount of light coming into the room too badly.


Awnings and external shading

Another (sometimes cheaper) way to control the heat that travels through windows is to installing awnings or other forms of external window shading. By preventing direct sunlight from hitting a window, these options offer a simple and effective way to reduce the amount of heat through windows. External window shading is an important part of passive house design, and making sure that the amount of light isn't too badly reduced is a great way to help keep heat at bay.



Curtains, blinds and internal shading

Heat gain and loss can also be controlled (to a degree) through the use of blinds and curtains. In fact, the introduction of thermal curtains to an existing property, or a new property that has east or west facing windows, is probably the cheapest and simplest way to control heat gain and heat loss - although it's not the most effective. The best way to look at thermal curtains is as another form of insulation. In a similar fashion to wall and ceiling insulation batts, thermal curtains also come with an R-value.

An R-value is the measurement used to figure out the insulation’s thermal resistance, which shows how good of an insulator it is. The higher the value, the better the insulator will be. Typically the thicker a material is, or the more ‘layers’ a material has, means it is a better insulator and prevents more transfer of heat.

  • Awnings and other ways to shade windows from the outside are more effective than curtains and blinds, because they prevent heat from being transferred through window glass.