Same scenario as the windows... looking at your ceiling insulation....
Example...
let's say you upgraded from R3.5 to R5.5 (I really have no idea).
- Energy usage using R3.5 = (1 ÷ 3.5) watts/m2/°K = 0.286 watts/m2/°K
Energy usage using R5.5 = (1 ÷ 5.5) watts/m2/°K = 0.182 watts/m2/°K
Savings by changing to R 5.5 = 0.286 - 0.182 = 0.104 watts/m2/°K
Using as an example 100m2 of ceiling and 10 °K as the temperature difference and $0.15/kWh. You would save in one year at 24 hours x 7 days x 52 weeks... 0.104 watts x 100 m2 x 10 °K x 24 x 365 ÷ 1000 *$0.15 = $136 per annum.
That calculation makes the same assumptions that you criticise in my window examples (10 degrees, 24 hours, 365 days per year). So if we assume the same error, your savings would be $2,300 (your energy total usage) ÷ $8,800 (single glazed estimate) x $136 (the example above) which equals $35.55 savings per annum for the $600 outlay... so my guess is your economic reason for the upgrade is also flawed... spending $600 to gain an annual $36 dollar benefit. That's 16 years not counting interest.
People are attracted to R values because they look like huge changes, but R value differences converted to conductance are tiny, hence the small improvement...
Ed