Area Correction Factor
Why is it needed?
To enable houses of different sizes to be compared within a single climate zone, the energy rating is calculated on the basis of energy loads per square metre of house floor area.
Smaller houses have a greater surface area compared to their floor area than larger houses. Because heat flow through the building fabric depends on the surface area, without this correction factor smaller houses have greater difficulty meeting each star level. This impact is exacerbated because often window areas as a percentage of floor area will be relatively larger in small buildings compared to larger homes, and heat flows through windows are generally the largest.
How was it calculated?
The create the mathematical equation used for the Area Correction Factor unique to each climate zone, a simple house is changed in size from 25 m2 to 625 m2 in seven steps. These houses are simulated using the 2nd generation NatHERS calculation engine, and the area correction is calculated by noting the percentage change in energy use relative to the house at 200 m2. Houses smaller than 200 m2 benefit from the correction factor and larger houses are corrected to eliminate the mathematical surface to floor area and heat flow anomaly.How big is the effect?
The table below shows the change to the star rating of a house which achieves 4 stars without the area correction in four locations for four houses sizes:
| Location | 50m2 | 150m2 | 250m2 | 350m2 |
|---|---|---|---|---|
| Hobart | 1.2 stars | 0.3 stars | -0.4 stars | -0.8 stars |
| Perth | 1.2 | 0.3 | -0.3 | -0.8 |
| Brisbane | 1.5 | 0.3 | -0.4 | -0.8 |
| Darwin | 1.7 | 0.2 | -0.4 | -0.8 |
In other words a very small 50 m2 house which would achieve 4 stars in Hobart without the area correction would achieve 5.2 stars after the area correction has been allowed for. Similarly a very large 350 m2 house in Brisbane which achieved 4 stars before the area correction was applied would achieve 3.2 stars after this mathematical anomaly was corrected.
All 2nd generation software tools are required to use the same standard area correction factors.