Steve Baczek and Jake Brutton | November 06, 2018

details // Concrete & Masonry, Framing, Insulation & Air Sealing

Animation: Floating Insulated Subfloor on a Slabless Slab


Wait, what? 


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 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)



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