What Type of Insulation Do You Need to Go Under a Home?

Insulation below your home is vitally important.
Insulation below your home is vitally important. (Image: Jupiterimages/Photos.com/Getty Images)

Approximately 15 percent of your home's heat is lost through the floor. For this reason it is important to insulate under your home. Whether it's an unheated or heated crawlspace or a heated basement with a concrete slab floor, insulation in this area is one of the most effective ways of making your home comfortable. Choosing the right insulation for this application is important because of its possible exposure to moisture.

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Fiberglass Batts

Fiberglass batts are one of the least expensive ways to insulate an heated or unheated crawlspace. The batts are designed to fit in between the floor joists, but should also be secured with chicken wire or some other type of mechanical fastener to ensure it stays in place. Fiberglass has a R-value per inch of approximately 3.2.

Mineral Wool Batts

Mineral wool is similar to fiberglass but is made with slag or rock spun into wool. It is water resistant, non-combustible, rot resistant and inexpensive. It fits in between the floor joists and is secured in the same manner as fiberglass. It has a R-value per inch of 3.3.

Rigid Fiberglass

Rigid fiberglass is a board-type product that can be installed on the exterior of foundations walls. This provides a thermal barrier from the cold soil and slows the transfer of heat. There are two types of fiberglass boards and only one is designed for below grade use. This type will be a solid board without a house-wrap bonded to its exterior. Rigid fiberglass has an R-value per inch of 4.3.

Polystyrene Boards

Extruded polystyrene is a close-cell insulation that has balls of air surrounded by foam. It is a rigid board typically purchased in 2-by-4-foot sections and comes in thicknesses ranging from 1/4 to 4 inches.This type of insulation is also sold with water drainage grooves cut into its surface to move water from the foundation wall down to the footing drainage system. It can also be used below concrete slab floors to provide a thermal brake that will make your basement feel warmer to your feet. Polystyrene has an R-value per inch of 5.0.

What Not to Use

Do not use cellulose or some types of mineral wool and expanded polystyrene. These are not designed for use below grade and will quickly degrade when exposed to the elements. The main problem is that these insulations absorb water readily and can rot or compress. The compression is typically permanent and reduces their R-value.


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