Insulating on top of a slab is a fast way to warm up a slab-on-grade concrete floor. In the case of a house in central Missouri, it is also a way to eliminate the concrete slab from the equation.
A slabless slab may sound far-fetched, but peeling back the layers, we can see that the assembly begins with a compacted base over undisturbed ground.
Step by step:
- On top of that sits four inches of crushed gravel, which replaces the slab.
- On the gravel goes a sheet of 10-mil plastic with the edges folded up and sealed to the mudsills.
- Seams are overlapped six inches and taped.
- Over the plastic goes rigid insulation with sufficient compressive strength.
- Along the bottom of the wall, a ½ inch sub-base is placed so the spray foam installers can get a continuous air seal at the inside connection.
- The first layer of subflooring is laid without fasteners.
- The second layer is glued and screwed perpendicular to the first layer.
- The edges of the sheets are kept about ¼ inch from the walls to allow for movement in the future.
Exterior insulated sheathing makes this an airtight, thermally isolated, water-tight subfloor that is rock solid over a slabless slab that is not far-fetched.
—This detail comes from the drawing board of Steve Baczek in Reading MA and the jobsite of Jake Brutton, Aarow Building in Columbia, MO (climate zone 4/5)