Foam insulation is used below concrete slabs to decrease heat loss through floors. This type of insulation is installed before the concrete slab is poured. Prefabricated foam boards are the insulation most commonly used beneath concrete slabs. High-density foam boards are rated based on their insulating capacity and their ability to resist damage from crushing, freezing and moisture.
Rigid foam insulation is installed underneath on-grade concrete slabs before they are installed. Insulating foam boards also are applied to the sides of a slab to prevent heat from escaping through the foundation wall. Installing a vapor barrier beneath the foam boards and concrete slab creates a strong barrier against moisture that prevents groundwater from penetrating through the concrete slab. You also may add foam insulation along the interior of foundation walls beneath a concrete slab. The foam boards are installed between a foundation wall and concrete slab and extend toward the footer to insulate the sides of the slab and the area beneath the slab.
The insulating capacity of high-density foam boards is rated using the R-value system. Boards are rated based on the amount of R-value that 1 inch of insulation provides. Increasing the total R-value of insulation to 6 decreases the amount of heat flow through uninsulated areas 84 percent. Most types of foam board insulation require only 1 or 2 inches of high-density foam insulation to achieve this level of insulation. Foam insulation also is rated based on its ability to withstand compression. Individual types and brands of insulation are rated based on the pounds per square inch (psi) they are designed to withstand. The psi required for your insulation depends on the weight of the concrete slab compared to its overall area.
Installing high-density foam insulation beneath concrete floors keeps them warmer than they would be without the insulation during winter and decreases energy loss through the floors. If a radiant heating system is built into a floor, then installing insulation beneath the floor will improve the radiant heating system's performance. Foam insulation installed beneath a concrete slab does not require fireproofing or protection from termites and other burrowing insects.
If you decide to install insulating high-density foam boards beneath a concrete slab, then ensure the foam boards extend down to the frost line. Doing so will help prevent damage from the ground's movement as it freezes and thaws while providing more effective protection against heat loss. Some areas of concrete slab may put more stress on the insulation beneath it due to uneven loading. Use high-density foam boards with a compressive strength rating that exceeds the stress the concrete slab is expected to create. That measure will prevent localized damage to the insulation that would reduce the floor's energy efficiency.
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