It seems to me that if you can build with recycled plastic crap.. why not. at least it won't go to land fill and just take up space.  We have this thing about being all natural.
if youwere out in the boonies and had no choice fine.
But you could fill up plastic bottles for water mass or glass bottles for water mass in the floor with the radiant heat. you could put the matrix under the glass bottles and put the floor over the filled bottles.  these would spread the heat out.
you can fill up plastic bottles with loose insulation and milk weed what ever. keep them dry and put them in walls with regular insulation  or pulverized paper.
 

