Can the granular fill used for soil gas mitigation be compacted or is it self-compacting?
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In order to maintain a complete thermal break I want to place 1 1/2″ of xps under my slab. The 5″ slab acts as footings for interior bearing walls ( floor load only). The XPS easily exceeds the required compressive strength (even for foundation footings) but normally fill under any footing has to be compacted. I suspect that the granular fill (less than 10% fines) is considered self compacting. If compacting is required it could compromise the permeability? Alternatively I could place concrete footings below the foam layer but that would be a lot of work and expense if not required!
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Dave, according to the IRC, footings under structural loads need to be at least 6" thick. Partition walls can bear on a slab only, but if you're talking about supporting the floor above, you should put in a proper footing--either posts on pad footings, a continuous strip footing, or a thickened slab sized for the load. An engineer might sign off on a reinforced 5" slab but it would depend on the size of the load.
On high performance homes I like to put the footing under the foam--just form the footings to the same height as the perimeter footings, and run the foam over the top of all of the footings. Then use a double mud sill to make up for the lost 1 1/2". Or sometimes I encase the bottom and sides in foam, but there is not a huge amount lot of heat loss at those locations.
Like most environmentally conscious designers and builders, I don't use XPS, because the blowing agents are a potent source of carbon pollution (the new term for greenhouse gas emissions). It may require a special order but 15 psi EPS is fine under most slabs, and 25 psi EPS is fine under most load-bearing walls, with a much lower global warming potential than XPS.
I would not put a slab on soil that has not been compacted. 3/4" crushed stone is the typical material for the sub-slab layer in my area, but it varies around the country, due in part to different types of rock locally available. Where rounded stone is available it flows freely and self-compacts, but angular stone will compact (and should be compacted). If it includes fines it will compact better, but those fines are filling up the air spaces you want for air or water flow. As long as there aren't too many fines it will still allow for air flow, but if you're concerned about radon, it's a good idea to run a perforated pipe around the inside foundation perimeter.
Dave,
Fill that can't be compacted still benefits form being consolidated. Think of how you get the last few peanuts you bought at the bulk bins into a jar. Even though they don't compact, if you tap the jar they consolidate - sometimes to a remarkable extent. Using a vibrating compactor on any fill is a good idea.