Data Insights – Are we sacrificing our windows for greater energy efficiency?

December 11, 2018

Windows are essential for allowing in natural daylight and providing views to the outside world. However, windows can be a challenge when it comes to designing high star rating buildings. In winter, heat loss from the inside to the outside can be up to 40% through windows, while in summer heat gains can be up to 80%. High performance glazing and window frames can help reduce this heat transfer, but reducing window area can also help improve a dwelling’s rating.

Analysis of CSIRO’s HStar data reveals that designers are reducing total window area in high star rated buildings. The chart below shows the average total window area for new class 1 dwellings arranged by star rating bands. It shows that as star ratings increase, the total window area decreases. Indeed, on average a 6 star dwelling has 50% more window area (36m²) than an 8 star dwelling (24m²). However, the chart also shows that the average floor area (excluding the garage) also decreases with star rating and that a 6 star dwelling on average is 35% bigger than a 8 star dwelling (158m² vs 117m²). It is the ratio between window area and floor area that gives us a better understanding of whether we are sacrificing our windows for star rating gains. The percentage numbers on the chart indicate this ratio and show that the window to floor area ratio has decreased by around 11% from 6 star houses to 8 star houses. Although this is a decrease, it is not as dramatic as the window area values alone would indicate.

Although this reduction in window area may achieve a desired star rating result, it is worth considering if this is the best way to achieve energy efficiency and what other unintended consequences may occur through reduction in glazing area. What is the impact on natural daylighting, on building amenity and the comfort and wellbeing of the occupants?
There are provisions in the building code that address requirements for natural daylighting. The code requires that habitable rooms have a light transmitting area of not less than 10% of the floor area. Effectively this is a floor to window area ratio of 10% meaning that our current dwellings are well above this minimum requirement. What the optimal ratio should be is very dependent on climate location, but a general rule is that a ratio of no more than 25% is best with the majority of windows facing north and minimal amounts facing east and west. Southern facing windows in most parts of Australia will receive no direct sunlight, but do provide for crossflow ventilation, so can be strategically used.
Window areas in high star rated houses have reduced, but so has the floor areas of these dwellings. Consequently, window to floor area ratios in high star rated dwellings have remained relatively high and are well above the minimum requirements set out in the building code, but remain below the 25% mark which is considered the best practice maximum. Analysis of the HStar data would seem to suggest that high rating houses (>7 stars) have achieved an optimal solution, with a good window to floor area ratio and a high energy efficiency rating, demonstrating that we can have windows and high star ratings too.

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Nationwide House Energy Rating Scheme
Supported by data from the Nationwide House Energy Rating Scheme (NatHERS)