I live in Melbourne, Australia, and recently bought my first house. It's an 1880s brick Victorian with a slate roof. The house currently has no insulation at all in the ceiling/attic/loft (I'm an American living in Australia, please excuse any strange word or spelling choices!), so we're getting quotes to have some put in. The maximum R value available seems to be R6, so I had initially planned to just go with that, and some installers have recommended it. But one that I talked to asked first if we are planning any other significant renovations or insulation anywhere else. I said no, and he said that in that case the R6 insulation will likely only perform at an R4 level, so it may not be worth the extra cost vs. just putting in R4.

As I mentioned, the house is brick, with no insulation either in the walls or floor (suspended timber with no access underneath, therefore no way to insulate there unless we pull up the old floor). I don't yet know if the walls are solid or cavity, so at best if they are cavity we could have bulk insulation blown in. The windows are all single-glazed, and I don't intend on replacing any of them in the near future, because they are in good working order and the old ones have been modified with draught-proofing so they seal tightly. After we get the insulation done, I plan to go through and seal up gaps everywhere that I can. I know there are a lot around the skirting boards and doors that need attention.

So my question is, if the walls and floors may never have any insulation added to them, is it worth doing the R5 or R6 insulation in the ceiling? Or should we just do the R4 as this one place suggested?

That last question is vital - makes a great difference. Also sub- tropical humidity, or not - excuse my ignorance
Surely there are Australian Green Building website/forums where all this is well rehearsed for each region?

omehegan - Given that your winters are not that cold (relative term!) I would suggest insulating the loft with 200mm or a bit more with glasswool or equivalent and fixing all the draughts. Then perhaps secondary glazing. These will probably be the cost effective things to do. After that unless you can get grant support insulating walls and floors will never give a return on the investment. Even here where winter goes down to -18 C and can be below 0 for a few weeks the ROI on wall insulation makes it difficult to justify.
Use the funds that would have insulated the walls to put PV on the roof, it will give a better ROI and reduce your carbon footprint

Thanks for all the input so far! I should have mentioned that I definitely will be doing PV in the next year or so, once other more pressing improvements are taken care of. Sounds like I should go for the R6 ceiling insulation and focus on the drafts.

On the R values, this is the standard measure used in Australia for insulation comparison. We use the SI/metric version. I know the US uses an inch-pound/imperial measure with the same name but much higher relative values. See https://en.wikipedia.org/wiki/R-value_(insulation)

Apparently U value measures heat transfer through a building element, such as a built wall with all materials included. Higher means more conductive, so worse insulation. R value is essentially the reciprocal, it's how well the material resists heat transfer. To convert between them, you just divide the value into 1. So my proposed R6 insulation would have a U value of .16. In Australia we seem to use U value to describe the performance of windows, but R value for all types of insulation.