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XPS Foam Insulation for Foundation Applications

A versatile, moisture resistant closed-cell foam insulation to insulate foundations and control moisture

View the ToolBase TechSpecs- Extruded Polystyrene (XPS) Rigid Foam Insulation for an overview of this technology.

Extruded polystyrene (XPS) foam insulation is a rigid, closed-cell foam product that can be used in various applications for insulating building foundations. Foam plastics are composed of millions of tiny cells or bubbles. When the cells are completely intact with no holes or spaces between the cells - it is called closed cell foam. When the cells have holes connecting one cell to another - it is called open cell foam. All other factors being equal, closed-cell foam also tends to be stronger (in bending and compressive strength) than open-cell foam. In addition, XPS tends to maintain its R-value over time due to its moisture resistant properties.

XPS properties allow it to be used for several foundation configurations. It is a preferred insulating material for constructing frost protected shallow foundations (FPSFs). FPSF construction provides certain advantages, such as a reduction in materials and cost (see additional resources on FPSF construction). Placing insulation board under the soil and concrete exposes it to high compressive loads and significant moisture. XPS is well suited for the task, making it ideal for insulating under slab (see related document section).

The moisture resistance and high R-value to thickness ratio also make XPS ideal for insulating the exterior and interior of basements and crawlspaces. Because moisture can cause significant problems to a structure, XPS can offer builders a product that impedes moisture while insulating at the same time, as opposed to having to deploy multiple strategies to accomplish this task. For finished interior applications, the high insulating value (per inch of thickness) can be valuable for obtaining good insulation levels and fully utilizing basement and crawlspace areas of a home.

XPS insulation comes in a variety of dimensions to suit many applications. Sheets can range from ½ inch to 2 inches in thickness, 2 feet to 4 feet in width and 8 feet to 10 feet in length. It can be easily cut to fit in certain areas, and can be handled easily by one person. Its higher cost compared to some other forms of insulation is offset by the variety of applications for its use. It is important to note that there are different classifications of XPS, based on density. These different types provide varying compressive strength. When selecting XPS that must support loads, it is important to obtain the correct type (see resources for FPSF).

While XPS has many advantages, it is important to follow proper installation guidelines. Like other types of foam insulation, XPS can be damaged mechanically, which can diminish its insulating and moisture properties. XPS can also be damaged by prolonged exposure to UV rays (sunlight). Given these considerations, it is important to follow installation instructions and protect the sheathing when exposed on the exterior, above grade. For interior applications, building codes require XPS to be covered by a thermal and ignition barrier, such as gypsum wall board.


XPS incorporated into foundation frost protection provides an affordable alternative to deep footings in cold climate zones. Where inground basements are included in the house design, rigid foam and a gypsum thermal barrier applied to the interior of a basement wall to meet code requirements provide the added value of a finished wall surface.

Energy Efficiency

The closed-cell structure of extruded polystyrene insulation board enables it to maintain its stable, long-term R-value and enhances its resistance to moisture intrusion. In addition extruded polystyrene insulation serves as an air barrier material that can significantly reduce air movement through walls, thus improving the thermal efficiency of the wall assembly. The material is recognized under the EPA Insulation Program, which encourages the use of thermal insulation as a means to reduce energy usage.

Quality and Durability

Extruded polystyrene insulation board is ideal for below-grade perimeter foundation walls; since it is highly resistant to moisture it can effectively control moisture migration through concrete foundation walls and slabs. In addition, the closed-cell structure enables the insulation to minimize the potential for condensation to occur within a perimeter foundation wall. This high resistance to moisture can enhance the durability of perimeter foundation wall systems. Extruded polystyrene insulation board is not biodegradable, so it will provide long-lasting thermal and moisture resistant properties throughout the life of the structure.


Implementation can be easy depending on the application. When installing XPS on interior or exterior walls, installation is straightforward. Contractors are generally familiar with installation methods and equipped with the necessary tools. Some extra finishing details will be required when installing on exterior applications, and if used on the interior, it may be wise to do so in conjunction with a finished basement. For FPSFs, slightly more instruction may be required for contractors unfamiliar with how the foam is installed. Additionally, when using XPS that must support compressive loads, it is important to pick the correct type. XPS comes in various densities that have slightly different strengths, so choosing the correct product is essential.

XPS insulating sheathing is comparable in cost to other insulation material options for foundations, especially when compared on an equivalent R-value (thermal resistance) basis. Using R-10 as a basis for comparison, XPS has a cost* range of about $0.54 to $1.12 per square foot. When combined with FPSF construction, the labor and material savings from using less concrete more than offsets any higher cost from XPS.

* Cost my vary widely based on the region of the country, the supplier, and other factors.

Because XPS maintains its R-value over time and in the presence of moisture, there is less of a chance that it may require replacement over other types of insulation.

XPS insulation is recognized and accepted by model building codes. XPS insulation must meet requirements set forth in ASTM C 578.

K. Hovnanian Inc, Freedom Township, NJ, January 2002
Utilized XPS in a Frost Protected Shallow Foundation.

Oakwood Homes, Denver, CO, April 2002
Utilized XPS in a Frost Protected Shallow Foundation

Depending on the application and the material the foam is attached to, specific fasteners may be required. XPS can be applied to masonry and concrete. Manufacturers' instructions usually specify fastener type and spacing, sometimes labeling the spacing on the facer of the product. To optimize the moisture barrier properties of XPS, taping the seams is recommended. Some fasteners can be applied with automated tools, but it may require a change to the tool's impact setting.

Warranties for XPS products will vary depending on application and manufacturer.

XPS' high R-value per inch, resistance to moisture, high compressive strength and ability to maintain its R-value makes it an excellent product for foundation applications. It can be used on interior and exterior foundation walls, and for slab-on-grade or frost protected shallow foundations. This offers flexibility when designing a home. However, some measure must be taken to protect the XPS from physical or UV damage, and there may be some higher cost and training required for installing XPS insulation in foundations.

Disclaimer: The information on the system, product or material presented herein is provided for informational purposes only. The technical descriptions, details, requirements, and limitations expressed do not constitute an endorsement, approval, or acceptance of the subject matter by the NAHB Research Center. There are no warranties, either expressed or implied, regarding the accuracy or completeness of this information. Full reproduction, without modification, is permissible.