What Type of Insulation Goes Under a Concrete Slab?

Rigid foam board rated for ground contact — extruded polystyrene, expanded polystyrene, or rated rigid mineral fiber — is the standard insulation under a concrete slab.

Pour a slab over the wrong foam and the mistake stays buried forever. The U.S. Department of Energy’s Building Foundations Handbook names extruded polystyrene (XPS), expanded polystyrene (EPS), and rigid mineral-fiber panels as the typical products for below-grade use, because a slab forces insulation to survive permanent moisture, soil contact, and the weight of concrete above it.

Two details separate a slab that performs from one that quietly bleeds heat: the material’s compressive rating, and where the panels actually sit in the assembly. The sections below cover both, plus the code values that decide thickness.

Which Rigid Insulation Materials Are Approved For Slab Use?

XPS, EPS, and specifically rated rigid mineral fiber are the three materials DOE guidance identifies for below-grade and ground-contact applications. Each one trades cost against thermal performance.

XPS carries a nominal thermal resistance of roughly R-5 per inch and is the most common choice under slabs. EPS runs about R-4 per inch, costs less, and needs more thickness to hit the same value. Rigid mineral fiber works in some slab-edge applications, but only when the manufacturer rates that specific product for ground contact and the loads it will carry.

The non-negotiable property is compressive strength. A slab transfers every load — walls, furniture, vehicles — straight into the foam beneath it. Ordinary interior wall foam has no business under a slab unless the manufacturer explicitly approves that use in writing.

Moisture is the other hidden cost. A material rated R-5 per inch on the spec sheet does not deliver R-5 forever when it sits wet against soil.

Where Does The Insulation Actually Go?

Insulation can cover the entire underside of a slab, but many designs insulate only the outer 2 to 4 feet of the slab perimeter plus the slab edge. Full under-slab coverage earns its cost where a project needs tighter thermal or condensation control.

The logic is simple: heat escapes fastest at the slab’s exposed edges and at the joint where the slab meets the foundation. DOE guidance states that the slab-to-foundation joint must be insulated whenever insulation sits inside the foundation wall or under the slab. Panels in the slab’s center with a bare perimeter is a common — and wasted — installation.

On a slab that runs independent of the foundation wall, DOE details show insulation at the slab edge and beneath the slab perimeter, thermally separating the slab from the foundation. With a monolithic slab and grade beam, panels go outside the slab edge or grade beam and continue vertically down to the bottom of the grade beam.

Matching the right board to your project takes more than picking a material name, which is why a shortlist of products tested for under-slab use saves you from comparing compressive ratings and thicknesses one spec sheet at a time.

How Much Insulation Does Code Require?

The exact number depends on climate zone, whether the slab is heated, and the code your local building department actually adopted.

Model-code examples show the range.

Those are examples, not a national rule. DOE’s Building Science Education program is explicit that states and municipalities may amend or replace the IECC, so verify the adopted code and local requirements before ordering material. The slab edge insulation code guidance from DOE walks through how the tables apply.

Two construction-phase details matter as much as thickness. Vapor retarders go in continuously with sealed joints and penetrations, and insulation needs protection from construction traffic, impact, UV exposure, and damage during the pour.

Insulation with insufficient compressive strength under a structural slab, a product not approved for ground contact, and an insulated slab center with an exposed edge are the three failures worth checking twice before the concrete truck arrives.