Below Grade

From below the ground . . . up. . . . XPS is very suitable for use.

XPS is often used in foundations because of its high, stable, long-term R-value of 5 per inch and its excellent water resistant properties. It is also unaffected by common soil acids, mildew and fungus growth, decay or corrosion — properties that make it ideal for long-term durability.

Extruded polystyrene foam (XPS) insulation is most commonly-used below-grade in poured concrete, or concrete block foundation systems. Foundations play an important role in keeping a building comfortable, moisture-resistant, and structurally sound.

Learn how XPS can help.

Insulation that comes in contact with the soil is subjected to severe conditions, including long-term exposure to water, high soil humidity and freeze thaw action. These environmental factors can diminish an insulation’s effectiveness. Therefore, insulation in contact with the ground should be inert to the action of soil and water such that its insulating properties are not diminished due to this exposure. XPS insulation is ideal for below-grade applications due to its high moisture resistance and long-term insulating performance. In “slab-on-grade” applications, shown to the left here, XPS boards are applied vertically to the inside of the foundation wall. Additionally, XPS can be used to insulate beneath the floor slab.

Freezing has little or no harmful effect on insulation that remains dry, such as XPS. Similarly, insulation that does not absorb moisture from the surroundings will benefit. Conversely, insulation that gets wet does not perform as well. This is an important consideration when specifying an insulating material where freeze-thaw cycles are common. Weather data demonstrate that many locations in North America will experience several hundred freeze thaw cycles over a 10 to 20 year period. Independent studies demonstrate that only XPS insulation is suitable for use in wet, below grade environments where freeze-thaw cycles are typical.

There are several advantages to insulating the exterior surface of foundation walls. The block or concrete wall insulated on the exterior surface is not subjected to large temperature differences and so will not act as a thermal bridge. Block walls insulated on the exterior undergo less air convection in block cavities. At nearly room temperature, the basement walls act as a heat reservoir, buffering interior temperature fluctuations. In some instances, adfreezing forces are prevented from acting directly on the basement wall.

In “slab-on-grade” applications, XPS boards are applied vertically to the inside of the foundation wall. Additionally, XPS can be used to insulate beneath the floor slab.

XPS insulation is available with varying degrees of compressive resistance from 25 psi to 100 psi. The higher compressive strength insulation should be considered in applications subjected to high loading, i.e., mechanical equipment, etc. XPS is durable and versatile, thus it stands up to the rigors of job-site handling and is available in a variety of sizes. XPS insulation for below grade applications is typically provided in standard 2’ or 4’ widths by 8’ lengths and in thicknesses ranging from 1” to 4”. XPS insulation is easily cut on the job-site to needed sizes by scoring with any common knife and snapping.

Case Study

Silver Valley Condominium Complex, Munroe Falls, Ohio

Insulation for both above- and below-grade applications saved installation time and money for the builder of Silver Valley Condominium Complex in Munroe Falls, Ohio. Set among 50 acres of land with an 11-acre lake, the complex consists of 200 two-story condominiums. “One of the main advantages of using extruded polystyrene insulation (XPS) was its versatility,” said Mr. Jim Fox, superintendent of Botnick Building Company, builders of the Silver Valley Condominiums. “Once it is on the job, we can use it virtually everywhere.”

XPS TYPE X as listed in ASTM C 578 2002 is being used at Silver Valley in the foundations, the vaulted ceilings, and behind the entire envelope of the construction. XPS is also being applied in non-living quarters, such as around the garage area and the gable end walls of the attic.

In the foundation, XPS was installed vertically against the concrete block foundation and was held in place by the backfill. A plastic membrane was then placed onto the dirt floor, followed by XPS, and then a 4” slab of poured concrete. The insulation provides the benefits of a thermal break between the soil temperature and the inside temperature. In the condos that have a full basement, they installed the XPS TYPE X as listed in ASTM C 578 2002 on the outside of the concrete block, followed by backfill.

XPS is often used in foundations because of its high R-value of 5 per inch of material and its excellent water resistant properties. It is also unaffected by common soil acids, mildew and fungus growth, decay or corrosion — properties that make it ideal for long term durability.

In addition, XPS TYPE X as listed in ASTM C 578 2002 was installed in the walls of the Silver Valley project. “We were able to erect the walls without pre-insulating,” explained Mr. Fox. “We used tongue and groove XPS, starting at the slab and completely encompassing all aspects of the entire exterior. By running it in this manner, it takes less product. We found approximately 10% less waste compared to other 4’ x 8’ insulation products, and the tongue and groove edges fit tightly and did not break at the stud. There is less waste on the windows and doors,” he continued.

“XPS was also installed above ground, in the garage and attic area, particularly in places where stucco walls were used. Other insulations, like mineral impregnated fiber-board, will not allow the stucco to adhere as well. The fiber-board draws the water out of the stucco, causing it to dry and crack. Because of XPS’s excellent water resistance, no protection is required underneath the stucco application. We also eliminated the necessity for a paper liner that is needed when using other insulations. We simply apply the stucco after the lath, this saves time and material,” stated Mr. Fox.