Insulation under gravel in slab?
I’m looking at an engineer’s drawing (research, not from my own house), and I noted an interesting detail for a frost protected monolithic slab. Working up from the bottom, there’s a flat layer of crushed gravel and then a flat layer of EPS. The turn-down footing sits directly on the EPS, and the slab sits on a thick layer of compacted stone over the EPS. The vapor barrier is located in contact with the concrete.
Why don’t I see this detail more frequently? It seems well suited to locales with thick topsoil, where you have to remove 12+ inches of dirt anyway. There’s no thermal bridge, and the insulation is placed flat and weighed down naturally by the gravel.
Any thoughts?
–John
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Replies
John,
As long as the builder remembers that the layer of polyethylene belongs directly under the concrete, the system you describe can work.
When I built my garage, which has a thickened-edge slab, I found it easy to slope the crushed stone to create the thickened edge, and then to install the rigid foam directly under the poly and concrete. Sloping the rigid foam wasn't that tricky -- you can always use a little bit of housewrap tape to hold the rigid foam in position.
For more information, see Polyethylene Under Concrete Slabs.
John,
One example of your approach is the foundation built by Rob Myers. Here is a link to a GBA article describing his experience: A Timber-Frame House for a Cold Climate — Part 1.
Below, I will attach a relevant photo.
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