A. John Siegenthaler, a consulting engineer who specializes in hydronic-heating-system design in Holland Patent, N.Y., responds: Downward heat loss from a radiant slab should not exceed 10 percent of upward heat output, a ratio derived from European installation standards for floor heating systems. It’s possible to calculate the R-value needed to meet this goal while accounting for such factors as floor coverings, soil temperature, and required upward heat flux. But lately, I’ve begun simply specifying a minimum of 2-inch extruded polystyrene insulation under all heated slabs, even those in basements with no floor coverings. Here’s why:

In my area, the cost difference between 1-inch-thick and 2-inch-thick extruded polystyrene insulation board is currently about 48 cents per square foot. Using 2-inch rather than 1-inch extruded polystyrene adds about $720 to the cost of insulating under a 1,500-square-foot slab.

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