What Are Insulation R-Values?
Insulation's primary function is to reduce heat transfer, and its effectiveness at doing so is referred to as its R-value. An R-value is a measurement of insulation's thermal resistance, or its ability to resist the flow of heat. R-values for home insulation, such as in an attic or wall, range from R-1 to R-60. The higher the R-value, the more effective the insulation is at resisting heat flow.
Factors that Determine R-Value
All insulating materials, and all building materials for that matter, have a corresponding R-value that is based on the material, its thickness and its density. Four-inch-thick cellulose will have a different R-value than 4 inches of fiberglass. There are also high-density options for most types of insulation, each with a slightly higher R-value per inch than a low-density option of the same thickness.
Common Insulation Types and Their R-Values
Fiberglass is by far the most popular type of insulation used in the United States. It comes in two forms: batts and loose fill (blown). Batt insulation typically has an R-value of between 2.9 and 3.8 per inch of thickness,. Higher-density fiberglass can have an R-value of between 3.7 and 4.3 per inch. Loose-fill fiberglass is typically rated between 2.2 and 2.7 per inch. Cellulose insulation, which is made from recycled newsprint, is another popular type of loose-fill insulation. Its R-value ranges from 3.6 to 3.8 per inch.
Rated R-Value vs. Effective R-Value
All insulation materials have an R-value assigned by their manufacturer, much like miles per gallon for an automobile. That value is contingent on a proper installation. Small gaps in insulation, much like a hole in the bottom of a boat, will severely degrade performance. For example, a 4 percent void in fiberglass batts (gaps between batts or batts not adjacent to joists) can degrade the overall R-value by up to 50 percent. Loose-fill insulation tends to settle over time, which also can reduce its R-value.
R-Value and U-Value
Even windows, bricks and drywall have R-values. But with windows, for example, manufacturers advertise their U-value, sometimes referred to as the U-factor. The
U-value is the inverse of the R-value. So in the case of windows, the lower the U-value, the better it performs as an insulator.