At Energy Rating Perth we always inform designers and our clients that the key factor for energy efficient glazing is the window size and correct orientation. A designer must be mindful to ensure the window sizes are not too large to allow unnecessary heat gain and neither too small to allow not enough heat gain and light entry. It is recommended to place larger windows on the northern façade for passive solar design while smaller ones on the eastern and western façade to avoid excessive heat gain in the summers. Our energy assessors urge our Energy Rating Perth clients to include this early into their design process rather than implementing it reluctantly once they have their hearts set on their new home designs.


While the performance of the glazing is very important, it is the overall glazing assembly that denotes the glazing performance and the natural cross ventilation in your home. Common window types are given below:

  1. Casement: These are an older style of window usually with a timber frame. This is made of one or two panels hinged on the sides onto a frame. The panels open up fully and allow full air entry. Although these windows aid in cross ventilation, they are harder to seal so cause air infiltration into the building. Traditionally these windows cannot be screened therefore they cannot be left open at all times as they allow dust, rain, and insects in.
  2. Side-hung and double hung windows: These are windows with panels that operate by sliding up or down. In a side hung, only one panel slides (upper is fixed) while in double hung, both the panels slide up or down. Unlike casement windows, these can be screened and therefore can be easily opened to allow ventilation but only 50% of the window can be open at a given time. The air and moisture leaks can be minimised as they can be sealed with weather-stripping but are not completely airtight due to the opening mechanism.
  3. Awning and Hopper Windows: These are windows hinged at the top (Awning) or bottom (Hopper) and only open about 30-40% of the total window opening. If they are windows which are used intelligently, they allow for efficient ventilation especially when used as highlights as they allow the risen hot air to escape allow a cooler breeze inside the space. Due to their opening size, in comparison to a traditional casement window, they help in protecting from air infiltration as they can be tightly sealed. Unlike hopper windows, awning windows are also weather tight as well and a good choice for maintaining thermal comfort in hot and/or humid/rainy conditions.
  4. Sliding window: This is an easy operating window in which 50% of the window slides and opens. They can be screened therefore allow efficient ventilation. Due to the lack of complicated moving parts, the window closes tightly and can be sealed quite efficiently blocking air infiltration. Changing the glazing type in a sliding window is simpler than other types and the overall glazing system can function better.


Window frames are also an important aspect of window assembly. They reduce the area of uninterrupted glazing and hence reducing the total amount of solar radiation that enters through the glazing. Although the glass may have the same U-value/SHGC, varying the ratio between the window frame and glazing area significantly alters the overall thermal performance of the window assembly. Another important factor is the conductance of the window frame materials. A frame of a high conductance such as aluminium (especially dark ones) will allow more solar radiation to enter the building. Aluminium frames can greatly reduce the thermal resistance (or increase the U-value) unless a thermal break is used between the interior and exterior using a low conductivity component to avoid thermal bridging and in that case the U-value is greatly improved. On the other hand, materials like timber and uPVC offer a comparatively low U-value in the window system due to their insulating abilities.

The energy assessors at Energy Rating Perth help you choose the window type and the overall glazing assembly so the energy rating of your house is at an optimum level in order to ensure minimum cost and savings not only at time of construction but for years to come.

Contact us today for more information!