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Figure 4-4. Potential Locations for Slab on Grade Insulation

Insulation is included in slab-on-grade construction for two purposes:

  1. Insulation prevents heat loss in winter, and heat gain in summer. This effect is most pronounced at the slab perimeter, where the slab edge otherwise comes in direct contact with outdoor air.
  2. Even in climates and locations on the slab (perimeter vs. middle) where slab insulation may not confer large energy benefits, thermal isolation of the slab can prevent cool slab temperatures that can otherwise cause condensation inside the house. This can lead to mold and other moisture-related problems, especially if the slab is carpeted.

A wide variety of techniques can be employed to insulate slab-on-grade foundations (Figures 4-4 and 4-5).  Good construction practice demands elevating the slab above grade by no less than 8 inches to isolate the wood framing from rain splash, soil dampness, and termites, and to keep the subslab drainage layer above the surrounding ground. The most intense heat transfer is through this small area of foundation wall above grade, so it requires special care in detailing and installation. Heat is also transferred between the slab and the soil, through which it migrates to the exterior ground surface and the air. Heat transfer with the soil is greatest at the edge, and diminishes rapidly with distance from it.  In hot climates, direct coupling of the soil to the slab may moderate cooling loads, though at the risk of condensing moisture from the indoor air. 

Both components of the slab heat transfer — at the edge and through the soil — must be considered in designing the insulation system. Insulation can be placed vertically outside the foundation wall or grade beam. This approach effectively insulates the exposed slab edge above grade and extends down to reduce heat flow from the floor slab to the ground surface outside the building. Vertical exterior insulation (Figure 4-5a) is the only method of reducing heat loss at the edge of an integral grade beam and slab foundation. For stemwall foundations, the major advantage of exterior insulation is that the interior joint between the slab and foundation may not need to be insulated, which simplifies construction.  One drawback is that rigid insulation must be covered above grade with a protective board, coating, or flashing material. Another limitation is that the depth of the exterior insulation is controlled by the footing depth. However additional exterior insulation can be provided by extending insulation horizontally from the foundation wall. Since this approach can control frost penetration near the footing, it can be used to reduce footing depth requirements under certain circumstances (Figure 4-5a).  This method is known as a “frost protected shallow foundation” (FPSF).  A variation for unheated buildings is shown in Figure 4-5b.  See NAHB (2004) for more information on this technique, which can substantially reduce the initial foundation construction cost.

Exterior insulation must be approved for below-grade use. Typically, three products are used below grade: extruded polystyrene, expanded polystyrene, and rigid mineral fiber panels. (Baechler et al. 2005). Extruded polystyrene (nominal R-5 per inch) is a common choice. Expanded polystyrene (nominal R-4 per inch) is less expensive, but it has a lower insulating value. Below-grade foams can be at risk for moisture accumulation under certain conditions.  Experimental data indicate that this moisture accumulation may reduce the effective R-value as much as 35%-44%.  Research conducted at Oak Ridge National Laboratories studied the moisture content and thermal resistance of foam insulation exposed below grade for fifteen years; moisture may continue to accumulate and degrade thermal performance beyond the fifteen-year timeframe of the study.  This potential reduction should be accounted for when selecting the amount and type of insulation to be used (Kehrer, et al., 2012, Crandell 2010).

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Figure 4-5. Potential Locations for Slab on Grade Insulation

Insulation also can be placed vertically on the interior of a stemwall or horizontally under the slab.  In both cases, heat loss from the floor is reduced and the difficulty of placing and protecting exterior insulation is avoided. Interior vertical insulation is limited to the depth of the footing but underslab insulation is not limited in this respect. Usually the outer 2 to 4 feet of the slab perimeter is insulated but the entire floor may be insulated if desired. Remember that condensation control is an important consideration, along with heating energy use.  It is essential to insulate the joint between the slab and the foundation wall whenever insulation is placed inside the foundation wall or under the slab. Otherwise, a significant amount of heat transfer occurs through the thermal bridge at the slab edge.  The insulation is generally limited to no more than 1 inch in thickness at this point.  Figure 4-4d shows insulation under the slab and at the slab edge to control the temperature of the slab, with exterior insulation placed vertically and horizontally to prevent frost penetration to the footing.

Another option for insulating a slab-on-­grade foundation is to place insulation above the floor slab (Figure 4-5c). This may be the only option for retrofit applications. It can be appropriate for new construction as well, especially when wood is the desired floor finish. These techniques have critical details that must be followed to avoid moisture problems; full descriptions can be found in Lstiburek (2006).

Other specialty systems can be used for slab-on-grade stemwalls. These include insulated concrete forms (ICFs), post-tensioned slabs, and systems that place foam insulation between two layers of cast in place concrete.

For more information visit Minimum Thermal Bridging and Insulating Foundations within the Building America Solution Center.