Radiant Slabs: Rigid Foam vs. Spray Foam

Most radiant-heated slabs are insulated with rigid foam board. It’s the industry standard for good reason: it performs well, it’s reasonably cost-effective, and crews know how to install it.

On a recent project, though, we tried something different. Instead of rigid foam, we used closed-cell spray foam beneath the slab – and the results were excellent.

Why make the switch?

Spray foam gives a tighter seal. It conforms to irregular subgrade and footings, eliminating the seams and gaps that can creep in with board insulation. It also delivers a higher R-value per inch, which can be critical when slab thickness or elevation transitions are tight.

That doesn’t mean it’s the right choice for every job. Spray foam is more expensive, and it requires coordination with a specialized crew. But for this project, a custom studio and workshop where the client prioritized long-term comfort and efficiency, it was the best fit.

As builders, we’re constantly weighing tradeoffs like this. The goal isn’t just to know what works, but to know what works for this project. That means understanding the options, recognizing when they’re worth the investment, and being clear about where the real value lies.

If you’re planning a slab-on-grade build or remodel with radiant heat, don’t overlook what’s under the concrete. That insulation is what keeps the heat moving upward into your living space instead of bleeding into the ground. Get it wrong, and you’ll pay for it in higher energy bills and uneven comfort for decades. Get it right, and you’ll have a slab that performs beautifully, season after season. And if you want a team that’s thoughtful about these choices, we’d be glad to talk.