What goes under the concrete in a slab-on-grade home? In the old days, not much — just dirt. Eventually, contractors discovered that it made sense to include a 4-inch-thick layer of crushed stone under the concrete. The crushed stone provides a capillary break that reduces the amount of moisture flowing upward from the damp soil to the permeable concrete.
Since the crushed stone layer provides a fairly uniform substrate, it also may also reduce the chance that a concrete slab will be poorly supported by random pockets of soft, easily compressible soil.
Eventually, polyethylene was invented. Concrete contractors learned that a layer of poly helps to keep a slab dry, because it stops upward vapor diffusion from the soil.
Finally, some contractors in cold climates began installing a continuous horizontal layer of rigid foam insulation under their concrete slabs. The foam layer isolates the room-temperature slab from the cold soil under the slab.
At this point, we’ve got a sandwich with three or four layers, and the question arises: does the order of the different layers matter? What goes down first, and what goes down last?
According to most building scientists, here’s how the layers should go, from the bottom up: crushed stone; rigid foam; polyethylene; concrete.
Some contractors may ask: Is it a mistake to put the polyethylene lower down in the sandwich? The answer is yes. To understand why, it’s useful to study the history of blotter sand.
Beginning in 1989, the American Concrete Institute (ACI) recommended the installation of a 4-inch layer of granular material between a sub-slab vapor retarder and a concrete slab. ACI standard 302.1 R-96, Guide for Concrete Floor and Slab Construction, included this recommendation in Section 4.1.5: “If a vapor barrier or retarder is required due to local conditions, these products…