In building science, the effectiveness of an insulating material is quantified by its R-value, which stands for 'thermal resistance.' Specifically, R-value measures a material's capacity to resist the conductive flow of heat. The fundamental rule is straightforward: the higher the R-value, the greater the insulating power and the slower heat transfers through the barrier.
In practical terms, during the winter, a high R-value slows the escape of heated air from inside the home to the cold outdoors. During the summer, it prevents solar heat absorbed by the roof and walls from penetrating the living space. R-value is determined by the material's thickness, density, and specific chemical or structural composition.
Comparing materials by R-value per inch
Different insulation materials offer vastly different thermal resistance per inch of thickness. Standard fiberglass batts—the most common residential insulation—typically provide an R-value of 3.1 to 3.4 per inch. To achieve a typical wall rating of R-13, you need about 3.5 inches of fiberglass, which conveniently fits perfectly within standard 2x4 framing.
Closed-cell spray polyurethane foam, on the other hand, is a high-performance material that provides an impressive R-value of 6.0 to 7.0 per inch. This allows builders to achieve an R-21 rating within that same 2x4 wall cavity. While spray foam is significantly more expensive, it also acts as a robust air barrier, sealing microscopic drafts that fiberglass cannot stop.
Diminishing returns and climate zones
The U.S. Department of Energy divides the country into distinct climate zones, recommending specific total R-values for different parts of a house. For example, unvented attics in cold northern climates (Zone 6) require R-49 to R-60, whereas southern climates (Zone 2) may only necessitate R-30 to R-38 in the attic.
However, it is crucial to understand that R-value operates on a curve of diminishing returns. Upgrading an uninsulated attic from R-0 to R-20 halts approximately 95% of heat transfer, resulting in massive energy savings. Upgrading that same attic from R-20 to R-40 requires doubling the material cost but only halts an additional 2.5% of heat flow. Therefore, strategic air sealing often yields better results than blindly piling up extreme R-values.