Off the cuff back of brain calculations and minimal practical experience
1st thoughts are that:

Polystyrene slab insulation designed for underfloor post construction
insertion (or almost any polystyrene that will fit) should be able to
provide whatever insulation level that calculations and available $ allow.
Your cited PIRMAX looks as good as any (better than most). Presumably
you've chosen it on substantially more than just $ per thermal
effectiveness.

Brochure here with R values
http://reflexinsulation.com.au/wp-content/uploads/Reflex-PIRMAX-HR-PANEL-Te=
chnical-Brochure.pdf
Even the 20 mm , R =3D 1 sounds as though it would require heating/cooling
loads that are useful.
[ 0.02 W/m/K.  20 mm =3D 1/50 m so energy flow =3D 0.02 x 50 W/k/s/m^2. =3D=
 1
Watt/m^2/K
200 m^2 =3D 200 Watts/K
2 kW / 10 K improvement.
Increase thickness to improve linearly.
E&OE.

Photos would be useful and interesting but there should be some means of
fixing that addresses the stated hazards. (I didn't see the 100 year
outback flood in there).

Drainage for condensation should be able to be addressed by design of upper
surface profile and weep holes.
(Or maybe just weep holes).



             Russell
--=20
http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.