insulating a structural slab

[BobNeng]

Greetings all,

I'm new to these forums although I've checked out a few threads over the years and the time has come for me to jump in. This isn't technically a structural question, but I think this is probably the place to get some good feedback.

I have a project that involves a structural slab below a bluestone paver and dust patio with a subgrade conditioned space below. At present, there will be a minimum of 6" of cover (soil, gravel dust, pavers) over the slab (with all the requisite membranes, drainage boards, protection board, etc.). Code mandates R-10 insulation for foundation walls in climate zone 4 (Virginia) which is typically 2" rigid insulation, but the nearest requirement to my particular situation seems to be R-38 for ceilings with attic above, but that's 7" of rigid.

I know that soil and concrete aren't good insulators, but I suspect there's some heat transfer at work so my question is.....

How much rigid do I really need? I'm certainly not going to ask code officials because they'll start flipping pages and I'll end up with R-38. If that's really the case, fine, but my experience is that people look to us (the guys with the stamps) for the final word/reality check.

Thoughts?

Thanks

 

[dik]

If it is loaded, you might check what R value you get with 2" of DOW Hi-40 or something of that ilk.

Dik

 

[BobNeng]

Thanks for the reply.

I think High 40 and it's cousins give a higher compressive strength (which we'll probably want), but not a higher R-value.

 

[dik]

They are extruded polystyrene with a high compressive strength and suitable for underslab/wet conditions. Because they are more dense, you can check with DOW on the R value. They come in strenghts up to HY-100, and maybe higher. They have been used for years, with good results.

Dik