Much of the heat lost out of a room is radiant energy. Therefore insulation with a reflective inward facing surface (with an airspace or gap above the reflective layer) will reflect most of this energy back in to the room. Reflective building paper or sarking (eg. Bradfords sislation, Silverwrap, Bradford's Enviroseal) will achieve this in the construction, however there are also products manufactured specifically for this purpose.
Silverbatts Silverfloor500 is a reflective wrap manufactured to fit between a standard 450 joist.
Silverbatts are available as:
Silverpleated battsBunnings stock Silverbatts in both 430 and 600 mm stud widths.
Silver Wave Core batt is a pop up batt that fits between joists incorporating an air cell and reflective surface in one batt.
Concertina foil batts from Wren industries are a similar pleated batt useful as ceiling, floor or wall insualtion.
These products are manufactured from craft cardboard with reflective aluminium foil adhered to the surfaces. The boards are then folded like a concertina, into batts, which allows easy fitting to variable joist widths. For underfloor use they are stapled onto the joists on one side, stretched acrooss the gap and stapled to the facing joist.
Pleated SIlverbatts and Silverfloor are available at Bunnings.