Long time lurker, first time poster. I'm a first time home builder - 2/3 of our house is on concrete slab while the remaining 1/3 is on bearers and joists. We live in the NSW Southern Highlands so it gets very cool here in winter - plenty of frosts with average minimums hovering in the low single digits.
My two choices for the subfloor insulation is either Foilboard stapled to the bottom of the joists, or Concertina Foil Batts. The Foilboard seems to provide better R ratings for both summer and winter, whereas the Concertina batts seem to be better for winter and only moderately good for summer.
However, I've read here and there that people believe that too much underfloor insulation can actually be a bad thing in summer - that the heat remains trapped far too much and actually works against house cooling. Is there any truth to that? Or are maximum R values regardless of season always the way to go?
Just as an FYI, the Concertina batts are probably going to cost 15-20% less than the Foilboard, so there is that in their favour.