Some rough calculations on heat loss through ceiling, walls and windows. The emphasis is on "rough".

Heating Degree Days (HDD) from the Bureau of Meteorology website gives an idea of the average temperature difference between 18C and the outside temperature, for days when the temperature is below 18C.

http://reg.bom.gov.au/jsp/ncc/climate_averages/degree-days/index.jsp?maptype=1&period=an&product=hdd18#maps

For Melbourne, HDD is about 1500 on an annual basis.

For Melbourne in July, the figure is between 200 and 300, so divide that by 31 and you get the average temperature difference between inside and outside as 7 to 10C.

So working out the annual heat loss through a 220m2 ceiling with R4 insulation on days when heating is required

220m2 / R4 * 1500 HDD * 3600 seconds/hour * 24 hours/day = 7129MJ per year

Heat loss through 122m2 of walls with R2 insulation

122 / R2 * 1500 * 3600 * 24 = 7906MJ per year

Heat loss through 58m2 of windows with U value 4.2 (double glazed, aluminium frame)

58 * 4.2 * 1500 * 3600 * 24 = 31571MJ per year.

Total 46604MJ, compared with our actual gas usage of 30000MJ for a year.

Expressed as percentages

Ceiling 15%

Walls 17%

Windows 68%

With single glazing it would have been even worse (U value 6.5). The best windows have a U value of about 2, or half the heat loss of the double glazing calculation.

The above doesn't take into account window covering. Heavy drapes + pelmets reduces heat loss by a factor of about 2/3, so window calculation above should be more like 21047MJ, and the total 36082MJ, which is closer to our actually gas usage.

I suspect that the above calculation is similar to that used by FirstRate 4, which is not as accurate as the modelling used in FirstRate 5.

Things that aren't right in the above calculations:

Heating set temperature is lower at night.

Ignores heat loss through floor. To calculate this you need to know the sub-floor temperature.

Heat loss through ceiling should be increased relative to walls because room air temperature next the ceiling is higher than at the floor.

Ignores solar heat gain through windows and heat from appliances and people.

Ignores air leakage around windows and doors.

Although the calculations are rough, they do indicate that window treatment will have the greatest impact, and that improving ceiling or wall insulation will have little effect.

Do the newer energy rating programs give you the relative figures for heat loss through ceiling, walls, windows and floors?