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You will probably find 5.0 wont fit into the roof space around the edges as it is very thick. Discuss your requirements with one of the insulation companies and go from there. We were originally looking at the 5.0 but it wouldn't fit into our roof cavity space and we were told would be totally unneccessary in Sydney. We went with 4.1 which is apparently also an overkill.

In terms of thermal performance, the more insulation the better, but be aware there are diminishing returns in putting more ceiling insulation in beyond around R3.5, and there are drawbacks in terms of the height and weight of the batts. So it depends on cost and what the builder is happy to do. If it's a metal roof, you can also get more protection from the heat by adding insulation under the roof sheets with products like Bradford Anticon, and if here's ducted aircon, that also helps reduce heat load into the ducts.

To estimate the effect of that extra R1.5, for a typical dark coloured roof, you might have 60 degree roof space temperature, and 25 degrees inside, giving a 35 degree differential. Assuming perfect installation. for 200m2 of living space, the heat flow through the ceiling with R3.5 would be 2kW, and for R5 that would reduce to 1.4kW, for a saving of 0.6kW heat flow. For a light coloured roof the figures might be more like 1.3kW and 0.7kW respectively, for a 0.4kW heat flow saving. Meanwhile, you might have another 4kW of heat coming in through the windows and walls.

Min recommended for Sydney is roof/ceiling R 4.1, walls R 2.8
Minimum insulation level by climate — Warm temperate; Mild temperate; Warm humid - about 1/3rd the way down this page...
http://www.yourhome.gov.au/passive-design/insulation

Stewie

I wonder if those recommendations include the insulation effect of the plasterboard, as I thought the R2.5 was as high as your could currently get with insulation batts in a standard brick veneer wall.

They also have a note to say that the ceiling insulation should be higher than that when the roof upper surface absorbance is greater than 0.4 - which is pretty much anything except a white roof.

I wouldn't worry about the plasterboard oneJohn. Standard 10mm only has an R-value of 0.06 - hardly worth mentioning.
You can also increase the total R-rating for a BV wall by opting for R 2.5HD batts which fit into a standard 90mm timber frame which would boost you up to around R 3.0 for that wall. But as with all things you pay more - $8.59 per sq m for the 2.5HD as opposed to something like $5 per sq m for R 1.5 batts.
Or you can go the whole hog like we're doing and go up the next level and get the wall up to just a bit below R 5. Basically an improved BV wall that is a lot better thermally and only another 30mm thicker.

http://i1212.photobucket.com/albums/cc455/stewiesno1/Picture%201_zpsvcvonlhp.png



Yes I agree. For a city like Sydney in the warm/mild temperate zone R 4 is probably all you need especially where we are on the coast. Summer temps around 28-30ºC, winter 10-12ºC min. If I lived further West though where the temps are 5º hotter and frosts in Winter I'd want to boost those walls and ceilings to a higher rating.

Stewie