I'm going to be insulating under our existing timber floor using polystyrene batts / sheets (inserted between joists). I will also use silverfloor sarking under the joists to create an air seal. The joist height is 100mm so looking to use 25-40mm poly batts against timer floor which leaves a 60-75mm air gap to the silverfloor sarking.
There are the Expol batts which are around $11 / sqm; Foilboard 10mm is around $9 / sqm (requires cutting & fiddling of course)....but I have found some polystyrene foam sheets for around $1/sqm - quite a significant price difference.
For example: https://buybuildingsupplies.com.au/10mm-grade-polystyrene-foam-sheet-2400x1200mm-p-28557.html. Even a 40mm thick sheet is $10.81 = $3.75 / sqm.
Am I missing the fact that the cheaper foam sheets are a more basic form of polystyrene, that is not EPS, so they dont have a comparable r-value to the Expol or Foilboard?
Also I appreciate the Foilboard will be better at keeping summer heat out with the reflective layer but I am using silverfloor as well so this shouldn't matter.