Either I haven’t looked hard enough or this idea is so bad that no one has brought it up. 

We have used a stem wall / slab foundation detail many times and our typical assembly looks like this…

1. Footing/wall
2. Compacted fill inside up to about 18″ from top of wall
3. 8-10″ washed stone fill that doesn’t require compaction to make levelling and plumbing trenching easier. 
4. 2.5″ R10 EPS
5. 10-15mil vapour barrier
6. 4″ concrete slab

While this works very well especially with only a couple inches of insulation and a municipal sewer system where we have plenty of fall.

We have a project coming up with considerably more sub-slab insulation 7-8″ (non cert PH) and a septic system. Ideally we would prefer to keep all of our horizontal plumbing drainage below our insulation, however this forces us to raise our top of wall another 8+” to accommodate fall to our septic system or use a pumping system which we would prefer to avoid. 

Has anyone done an “inverted” version of this assembly

1. Footing/wall
2. Compacted fill inside 
3. Insulation
4. Washed stone fill that doesn’t require compaction to make levelling and plumbing trenching easier. 
5. 10-15mil vapour barrier
6. 4″ concrete slab

I feel as though I may simply be missing something that rules this idea out, perhaps a thermal loss with the concrete not being placed directly above the insulation layer….. Anyway, I’m interested to hear from the GBA community.