Thermal bridging — why R-21 batts give you about an R-16 wall

What this diagram shows

A plan section looking down a stud wall, warm inside at the top and cold outside at the bottom. The bays between the studs are filled with R-21 batt, but the wood studs — spaced 16 inches on-center and making up about 25 percent of the wall — are only about R-6.9, so heat takes the path of least resistance and bridges straight through the studs while it is slowed by the batt in the cavities. Because the studs conduct heat far faster than the insulation, the area-weighted parallel-path R-value of the whole wall drops to roughly R-16, below the cavity rating. Adding continuous rigid foam across the outside of the studs covers the framing and breaks the thermal bridge.