The first time I detailed a “slabless slab” was in 2013. I was working with homeowners who disliked the look and feel of concrete and wanted to install elm flooring they had salvaged. The design plans called for a robust insulation and air-sealing package, which meant an expensive building envelope. To reduce costs—and the overall square footage of heated space—we excluded a basement from the project. That meant we would be doing a slab on grade. But after learning about their flooring preference, the question came up: Why did we need a concrete slab at all? Why not skip the slab and the embodied carbon that comes with it?
This seems to be how many green building pros ended up experimenting with a concrete-free assembly. Today, there are several variations on the concrete-free slab being used in high-performance and Passive houses but 10 years ago, information on the method was limited so there was a learning curve.
A simple system
First, a few quick notes on this assembly: It works in any climate zone, though insulation levels will vary. Depending on the current cost of floor sheathing, wood flooring, and labor, it could offer some savings. I’ve never had a code issue with this system; if it can support a live load of 40 lb. per sq. ft., it’s fine.
The assembly itself is relatively straightforward, but below are some key considerations I learned about from first-hand experience.
Foam density is critical. Everything inside the house will be bearing down on this foam including interior walls. I typically spec high-density (15 psi min.) EPS for below-slab insulation; it works well in this application and is readily available in varying thicknesses and densities. For interior structural loads, I avoid load-bearing walls to avoid a strip footing, which creates a thermal…
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