The R-value of certain insulation materials can vary considerably with density. This has important implications in the use of blown-in insulation, the installed density of which is under the direct control of the contractor. For example, blown-in fiberglass is usually listed as having an R-value of about R-2.2 per inch. But this is measured at its “settled” density of about 0.7 lb/ft3; if that same material is forced into walls at a density of 2.0 lb/ft3, the R-value jumps to almost R-4.0 per inch. The same effect does not hold for cellulose, which decreases in R-value as its density increases.4

Blanket insulation is also affected by density. Stuffing a thick batt (or roll) into a narrow stud cavity will result in a more densely installed material. R-13 batt insulation is designed for proper place­ment into 2×4 wood frame wall. However, if an R-19 fiberglass batt, which is designed for a 2 X 6 wood frame wall, is stuffed into a stud cavity that is only 3.5", the total R-value of the batt will be less than R-19. This is simply due to the fact that the fiberglass batt insulation R-value relies on air as part of the resistance equation. Compressing the batt reduces the airspace between the fibers, which in turn reduces the R-value per inch.

This explanation of density is not to be confused with insulations that have different design densities. Typically specified in units of pounds per cubic foot, different products can be optimized for certain locations and higher R-values when manufactured with different densities.

Technically, any air-based insulation material such as fiberglass batt cannot exceed a theoretical maximum R-value of R-5.5 per inch because 5.5 is the R-value of still air. Plastic foams such as urethane and polystyrene sometimes exceed this value by using a fluorocar­bon gas instead of air within the insulation cells. These factors will be discussed in greater detail later in the book. Other exceptions to the preceding maximum are experimental air-based insulation materials that contain very fine powders. These materials increase R-value by virtue of extremely small powder particles that interfere with conduction through air. Although they are not available com­mercially at present, they may appear on the market some time in the future.

As mentioned earlier, U-value is the customary unit used by the fen­estration industry to quantify conducted heat gain or loss. With other building materials such as insulation, roofing, and flooring materials, the R-value is used frequently for conducted heat gain or loss. There is a simple relationship between u – and R-values, namely,

U = 1/R or R = 1/U

For example, a U-value of 0.25 equals У25, or an R-value of 4. Conversely, in order to establish the R-value from the U-value, divide 1 by the U-value, that is,

R = 1/U or U = 1/R.