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Community and Q&A

Sub-Slab Assembly

Jeremiah_Sommer | Posted in Energy Efficiency and Durability on

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.

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  1. Expert Member


    No problem I can think of. If you had a very high water table you might want to add a drain at the foam level.

    I'm sorry I can't find it, but about a year ago GBA published a story on project that used an extreme version of that, where they spray-foamed the interior of the stem-walls and bottom of the (rocky) excavation before adding sub slab fill. I would imagine the slab won't respond to temperature swings as quickly, but maybe that's a good thing?

    1. Jeremiah_Sommer | | #2

      Thanks Malcolm, I should have indicated that a perimeter drainage system would be in play if necessary.

  2. bob_swinburne | | #3

    I wonder if this is a job for Glavel?

    1. Jeremiah_Sommer | | #4

      It’s a perfect job for Glavel! Sadly here in South Western On, Glavel is not very accessible and transport is pricy.

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