Area correction factor

Why is it needed?

The energy rating is calculated on the basis of 'energy loads' per square metre of the floor area. This allows homes of different sizes to be compared within a climate zone.

Smaller homes have a greater surface area (walls and ceiling combined) for their floor area when compared to larger homes. This is important because heat flow through building material depends on the size of the home’s surface area. Without an area correction factor, smaller dwellings would have more difficulty meeting each star level than larger ones.

The difference in surface area between different sized dwellings is further exacerbated by the higher proportion of window areas per wall area in smaller dwellings. This is because heat flow through windows is generally larger than through walls.

All NatHERS Accredited Software includes an area correction factor.

Different Size Houses in Row on Wooden Table

Shutterstock.com

How big is the effect?

The table below shows how a 4 star rating, without the area correction applied, is changed once the area correction factor is applied in four different locations for four different dwellings sizes:

 

Location

50m2

150m2

250m2

350m2

Hobart

5.2 stars

4.3 stars

3.6 stars

3.2 stars

Perth

5.2 stars

4.3 stars

3.7 stars

3.2 stars

Brisbane

5.5 stars

4.3 stars

3.6 stars

3.2 stars

Darwin

5.7 stars

4.2 stars

3.6 stars

3.2 stars

 

In other words a very small 50 m2 home which would achieve 4 stars in Hobart without the area correction would achieve 5.2 stars after the area correction is applied. Similarly a very large 350 m2 home in Brisbane which achieved 4 stars before the area correction was applied would afterwards achieve 3.2 stars.