High Density Insulation


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.

High Density Foam Insulation In Stock

Under Concrete and Foundations

 In thicknesses of 1½” and  2″

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15-Year In-Situ Research Shows EPS Outperforms XPS in R-Value Retention

Studies show that as much as 25% of energy loss from a structure can be attributed to a lack of insulation on below-grade foundations, crawl spaces and under slabs. Insulation R-value is directly correlated to maximum energy efficiency in a building envelope; higher R-values translate into increased savings. In below grade applications, foam insulation is exposed to moisture and could lose R-value over time if this moisture is absorbed.

As shown in an independent, third-party test program expanded polystyrene (EPS) maintains its R-value even after long-term exposure in northern climates. A competing insulation material, extruded polystyrene (XPS), was shown to have lost R-value over time. The results of this test program demonstrate that EPS insulation is a perfect choice to reduce energy loss.

In August 2008, independent testing evaluated the field performance of EPS and XPS insulation in a side-by-side, below grade application following a continuous 15-year installation period. EPS Type I and XPS Type X test samples were excavated from the exterior of a commercial building in St. Paul, MN at a depth of approximately 6 feet below grade.

Specimens were tested for thermal resistance using ASTM C518 “Standard Test Method for Steady-State Thermal Transmission Properties by Means of the Heat Flow Apparatus” immediately after excavation. Moisture content was determined by measuring the sample weight at the time of removal and again after being oven dried.

Excavation Site St. Paul, MN
Side-by-Side Installation

In-Situ R-Value Retention & Water Absorbtion

The results demonstrate that EPS Type I outperforms XPS Type X in both R-value retention and decreased water absorption. Further, whereas the in-service R-value of the XPS insulation is reduced by half, expanded polystyrene still delivers 94% of its specified R- value of 3.6 per inch after 15 years . These long term performance advantages make EPS insulation a preferred choice when compared its competition.

This testing further confirms that water absorption results determined using ASTM C272 “Standard Test Method for Water Absorption of Core Materials for Structural Sandwich Materials” cannot be correlated to the in-service performance of foam insulation. The main reason is that the laboratory test procedures call for partial or full submersion conditions which are not encountered in field applications. In fact, laboratory test methods were not developed for predicting actual performance, but were intended for use in specifications as a means of comparing relative physical properties of different cellular plastics and for product evaluations and quality control.

© 2008 EPS Industry Alliance | The EPS Industry Alliance publishes technical bulletins to help inform building professionals on the performance characteristics of expanded polystyrene (EPS) building products. The information contained herein is provided without any express or implied warranty as to its truthfulness or accuracy. Original article here: http://www.epsindustry.org/sites/default/files/Below_Grade_103_33116.pdf