The R-value (thermal resistance) of a building material is its resistance to the flow of heat. As explained in the preceding subsection, the R-value is the reciprocal of the k-value or the C-value and represents heat flow resistance expressed as a numerical value. The higher the R-value of a material (or a combination of materials in a wall, ceiling, floor, or roof), the greater its resistance to the flow of heat. The minimum recommended R-values are R-30 for an attic, R-ll for walls, R-19 for ceilings and raised floors, and R-42 for ductwork (Figure 3-1). Energy savings and lower fuel costs will result if insulation with an R-value higher than the recommended minimum is installed. This holds true for both new construction and existing structures.
6" (R-19) in ceilings or attics
Perimeter insulation around slabs or heated crawl spaces Figure 3-1 Minimum levels of effective insulation. (Courtesy Owens-Corning
Returning to the masonry wall illustrated in Table 3-2, you will note that all seven R-values have been added to produce the total resistance of a solid masonry wall (3.47). In other words, the total resistance to heat flow through a solid masonry wall of the type of construction illustrated in Table 3-2 is equal to the sum of the resistances of the wall components:
Rt = Ri + R2 + R3 + ••• + R„
The total resistance (Rt) of a particular type of construction is used to determine its U-value, which represents the total insulating effect of a structural section. The U-value is the reciprocal of the total resistance (Rt). Using the data supplied for the masonry wall illustrated in Table 3-2, its U-value is determined as follows:
U = R = 347 = 029