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Welcome to the Bunnings Workshop community @RKH. It's wonderful that you've joined us, and many thanks for your question on insulation.
The R-value listed on insulation products is their resistance to heat flow. The higher the R score, the better insulating properties it has.
Typically, foil products would contribute less than R2.0, though they effectively block air movement if sealed correctly. You might like to read through this recent discussion about a cheap and easy way to insulate underfloor of weatherboard house from @TimD.
The R-value of batts start at R1.5 and go all the way up to R6.0. The insulating properties of batts far exceed those of foil alone. The higher the R-value the thicker the batt is, so you'll need to work out what the thickest batt is you can install. Generally, this would be determined by the height of your joist.
Covering the batts with foil could be considered and would be a good idea if the underside of the house is exposed to the elements.
Please let me know if you have any questions.
Mitchell
@RKH,
Batts can be cut down to size with a Stanley knife. If the space between your joists is 455mm, then you would cut the next size up batt down to 475mm. This means when you compress it into the gap, it's a nice snug fit. I believe you'll find the 565mm batt slightly too wide if you try to fit it into your gap. If they are too wide, they'll spring out of the gap. You'll be using string stapled into the underside of the joist to hold them in place, so you could attempt to use the full 565mm batt and see how it goes. However, you don't want to compress Batts too much as they actually start to lose R-value.
I've actually just noticed the Earthwool R2.0 90mm x 470mm x 2700m 16.5m² Insulation Quilted Underfloor Batt - Pack of 13, which would be perfect for your 455mm gap.
Mitchell
