The big problem with insulating stud walls is…the studs. Wood only insulates to R-1 per inch of thickness, while most insulation used in wood stud walls is more like R-3.5 per inch. So every 24 inches we have a stud that lets out 3.5 times as much heat as the insulation next to it. These passages for heat to escape are called thermal bridges. They reduce a 2x6 at 24” stud wall with R-21 insulation from a potential R-23 wall (including all the material layers) to an effective R-20.5 wall, an 11% reduction. If the studs are spaced closer at 16” it is even worse, an effective R-19.4 wall, a 16% reduction.
In order to reduce thermal bridges in the walls of the Eastside Harvest House, we decided to wrap them in rigid foam insulation. At 2 inches thick, the foam adds another R-9 to the wall, for a potential R-32. Significantly, the wrapped wall is an effective R-30, only a 7% reduction and one and a half times as good as the effective R-20.5 of the unwrapped wall.
Rigid foam is plastic resin foamed with a gas that can contribute to global warming. The worst offender in this regard is XPS (extruded polystyrene, often colored pink or blue or green) which is foamed with a greenhouse gas over 2000 times worse than CO2, the major climate change gas. We used EPS (expanded polystyrene, usually white) instead which is foamed with pentane, only 3 times worse than CO2.