Gravel + Perlite insulating under slab

Hi there.

I'm looking at doing a radiant heated slab on grade for a circular structure. I read in another thread that people advised against using pea gravel in a RMH due to insulative value.

I'm wondering if it would make sense to mix small sized gravel + perlite and compact to 4-6inches as a base for insulating a 3.5" concrete slab w/radiant heat. I'm trying to avoid the expense of the rigid foam insulation.

The other idea I'm playing with is collecting used garage door panels, placing them over compacted gravel and pouring a slab. Thoughts?

Pretty radical ideas, but thought I'd share them with the experienced folks around here.

I think subsoil moisture would be another factor in the matrix. Rigid foam insulation also provides a moisture barrier. I'm pretty sure Perlite et. al. only insulates when dry.

Douglas makes a good point. I'm all for avoiding rigid foam but you don't want something that's wicking subterranean water into itself. What about a poly film barrier underneath the gravel and perlite mix? I'll also put in a plug for biochar as a possible ingredient, as it has similar properties to perlite and you can make it yourself.

Have a look at my mate Blue Forsyth's article in our society newsletter to see how he used biochar as underfloor insulation.