R-Value
insulation
A measure of thermal resistance — how well a material resists heat flow. Higher is better, and the values of layers in an assembly add together.
R-value is quoted per inch for loose materials and as a total for manufactured products like batts. The figure printed on a batt describes the cavity, not the wall: wood framing conducts about 2.5 times better than fibreglass and occupies 15 to 25 percent of a typical wall, so a nominal R-13 assembly performs closer to R-11 once thermal bridging is accounted for. This is why advanced framing at 24 inches on centre gives a better whole-wall value than 16 inches with identical batts, and why continuous exterior insulation has become common — it is the only layer the studs do not interrupt.
Formula
Nominal R = R per inch × depth · R-value is the inverse of U-factor
Example
Fibreglass at R-3.2/inch in a 3.5-inch 2×4 cavity gives a nominal R-11.2, or about R-9.9 whole-wall at 16 in OC.
Common mistake
Compressing a batt to fit. Squeezing an R-19 batt into a 3.5-inch cavity delivers about R-13 — you pay for performance the wall never receives.
Use the Insulation Calculator
Work out the R-value of a cavity by material and depth, and the whole-wall R-value once thermal bridging through the framing is accounted for.
Open Insulation Calculator →Related Terms
- On Center (OC) — The spacing between framing members measured from the centre of one to the centr…
- Coverage Rate — How much area one unit of a material covers — square feet per gallon of paint, p…