The Recommended R-Values for Basement Walls

The U.S. Department of Energy has issued insulation performance guidelines for buildings in various regions of the United States. The units of insulation performance, called R-values, measure the insulation thermal resistance, or resistance to heat flow. Different regions with different climates require different R-values. The Department of Energy recommends that homeowners first weigh the advantages and disadvantages of insulating the basement walls, especially if the basement is damp.

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Value of R-Value

R-value is measured in numbers; the higher the number, the greater the effectiveness of the insulation. R-value is determined by the material composition, thickness, density and proper installation. Improperly installed insulation, such as compressed fiberglass batts stuffed into a cavity, reduces the effectiveness of the insulation and thus its R-value.

Benefits of Basement Wall Insulation

Properly installed, wall insulation conditions the basement from the colder underground and makes the space more comfortable, even if the basement is left unheated. Insulation seals air leaks and provides consistent thermal resistance from cold walls. Basement wall insulation also provides greater efficiency than insulating basement ceilings.

Disadvantages of Basement Wall Insulation

Compared to insulating the basement ceiling or providing no insulation at all, basement wall insulation can be costly. If the basement is damp, the homeowner must install a perimeter drain, or the moisture will ruin the insulation and encourage mold growth. Additionally, insulation materials, particularly exterior insulation, may become a breeding ground for insects. If hazardous radon gas is present in the basement, the homeowner must also install a mitigation system.

R-Value Insulation Guidelines

Cold, humid climates require insulation with greater R-values than warm, dry climates. The Department of Energy provides a service to determine the exact R-value required based on ZIP code and type of heating fuel. Generally, zones 1 and 2 cover the southern states, which suggest no necessary values to R-10 for basement walls. Zones 3 and 4 include the Midwest and Northern regions, requiring R-10 to R-19 values. Extremely northern regions and high elevations are zone 5, requiring R-15 to R-19 values. The Department of Energy strongly advises homeowners to properly install basement wall insulation to achieve the maximum R-value.


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