An R-value is the measure of a material’s ability to resist conductive heat transfer. The higher the R-value, the more resistant that material is to transferring heat. R-values are stated on a variety of insulation products, from fiberglass batts to loose-fill cellulose. Full values are given for products of a uniform thickness, and per-inch values are given for materials that are blown or pumped into cavities.
The R-value of any building assembly, such as the floor, walls, or ceiling, equals the R-value of individual components added together. A typical stud-framed wall with 5.5 inches of fiberglass insulation (about R-3 per inch) has an overall R-value of approximately 20. Likewise, an earthen wall (such as adobe or rammed earth) insulated on the exterior with 4 inches of extruded polystyrene (R-4 to R-5 per inch), has a similar R-value. Even though the R-values of these two different wall structures is the same, the earthen wall will typically have less air leakage and much more thermal mass than the wood-framed one.
The R-value of a studded cross section is less than a studless section because every stud is a thermal break that conducts heat faster than the insulated sections. Eliminating unnecessary framing has a direct effect on a wall’s overall thermal performance, in addition to reducing the amount of wood used. When framing walls on 16-inch centers, as is most common, 15% to 25% of the wall’s total surface area is wood (not counting windows and doors). When framing on 24-inch centers, this number shrinks to 10% to 20%. Of course, all of this depends on how the wall is built—for example, how many window jacks, extra studs in corners, trimmers, and jack studs are installed. How well the insulation is installed (no compression and no voids, and touching all six sides of the cavity), what type of insulation is used, and air leakage will also influence whole-wall R-values.
To determine the actual R-value of the walls in your home design, use the calculator at www.ornl.gov/sci/roofs+walls/AWT/home.htm. This program allows you to specify the wall type, the cavity insulation, thickness of any foam sheathing, and exterior finish type.