Adding underfloor insulation, help needed

If space is tight any of the fibreglass batts can be a real PITA to install due to the dreaded itch problem. If I was going to attempt it again I would use one of the polyester batt insulations.
Wind can rob heat or add it so I used breathable but wind resistant weed mat over the fibre stuff.
For me the hardest part of the actual install was the super hard 50 YO hardwood framing, the staples wouldn't go in.
Breathable heavy duty RFL wall wrap on it's own would also work to a degree.
Where I did manage to get the underfloor batts installed the house is more comfortable, warmer in winter and cooler in summer

If space is tight any of the fibreglass batts can be a real PITA to install due to the dreaded itch problem. If I was going to attempt it again I would use one of the polyester batt insulations.
Wind can rob heat or add it so I used breathable but wind resistant weed mat over the fibre stuff.
For me the hardest part of the actual install was the super hard 50 YO hardwood framing, the staples wouldn't go in.
Breathable heavy duty RFL wall wrap on it's own would also work to a degree.
Where I did manage to get the underfloor batts installed the house is more comfortable, warmer in winter and cooler in summer

I did my underfloor with the earthwool underfloor batts, I found them cheap online and then made bunnings price match. earthwool is far nicer to handle then the normal fibreglass stuff.

I then bought a long long roll of the blue plastic box strapping stuff. I shoved the batt into place between the joists, and then using a staple gun held it in place with the blue strapping tape over the batt.

Best bang for buck will come from fixing biggest problem, but not sure what is the most significant cause of heat loss with information posted. Thermal losses in winter will be likely be mainly convection, then conduction. Have you eliminated all drafts (around doors, windows, wall vents, ceiling penetrations like extractor fans, etc)? What is the wall construction and what insulation is already there? Are the windows double glazed? Do you get condensation on the inside of glass windows? You might get a better insight on where heat leaks with some thermal imaging of inside surfaces on a cold day.

Originally Posted by bootes

I had wondered how successful I could hope to be making changes blind, or whether a more measured approach might be more successful? I'm in a rural area with no equipment hire places local, so would have to purchase one - but just wasn't sure whether spending a few hundred dollars to buy one, would be more worthwhile than just slapping up some insulation regardless?

It's hard to know what to do when the mechanisms of heat loss are essentially invisible. However just doing SOMETHING isn't necessarily going to be a winner and may in fact lead to a false conclusion that nothing is worth doing. Here's a (poor) analogy: say you have a bucket that leaks water through cracks, small holes and large holes. Just doing something is like putting a bandage around the middle of the bucket. It may reduce the loss of water from the bucket if it inadvertently fully or partially covers some of the holes, but will it make any significant difference? Probably not. First fix the big holes, then the small holes, then the cracks. Surprisingly (or not!) once the big and small holes are fixed, filling the cracks seems to make a huge difference, but fixing them first leads to an imperceptible result.

Without an IR camera, I suggest drafts as #1 priority. The candle trick on a really windy day is a great tool. Then look at surface temperatures - hold your hand on walls, ceilings, doors, floors, windows etc. Anything that feels cold on a cold day is literally sucking out heat, more-so the colder it feels. Bear in mind that heat loss is a function of both leakage and area. A small loss over a large area might be more significant than a large loss over a small area. E.g. a large double glazed lounge window can lose way more heat than a single glazed toilet window.

To address some specifics you raised, aluminium frames are notorious for conduction heat out in winter. I have successfully used 4mm acrylic sheet to cover the glass and frame of a (non opening) window exposed to cold winds. I use simple clips to hold the acrylic in place so that it can be removed for cleaning. The sheet also blocks drafts through a poorly fitted window retaining rubber, which seems to be a common problem with aluminium framed windows. Remember to allow 1mm per meter minimum for expansion.

'Bubble glazing' is a cheap and effective retrofit to bathroom and other privacy windows, also skylights. Simply cut some good strength bubble wrap to fit over frosted glass; hold in place with sticky tape. I don't think anyone ever noticed the bubble glazing unless I pointed it out. I used five layers simply laid in sheets on the ceiling diffusor of a large skylight in a bedroom with no exterior window, and it made a quite noticeable difference to warmth without affect the light.

Penetrations through the ceiling can be quite significant heat losses, e.g. exhaust fans, range hoods, chimneys, poorly fitted access covers, etc. Range hoods should have an internal damper; a thing called a draft stopper can be added to some exhaust fans (https://www.draftstoppa.com.au/draftstoppa/), chimneys or flues should have a damper that can be closed. Also wall vents in older houses which were necessary with open gas fires are redundant today and should be blocked.