R-value

R-value measures thermal resistance: how well a material resists heat flow. Higher is better. The energy code sets minimum R-values for walls, ceilings, floors, and foundations, and the minimums vary by climate zone.

Where the numbers come from

The adopted energy code assigns each part of the country a climate zone, and each zone gets its own table of minimums. A wall assembly that meets code in Florida would be well short of what a Minnesota project needs.

Assembly versus material

The R-value printed on a batt is the material value. What the code cares about is the assembly: the whole wall including framing, sheathing, and any continuous insulation. Framing members conduct heat far better than insulation does, so the effective assembly value is meaningfully lower than the batt's rating. This is why continuous exterior insulation appears in newer codes.

Prescriptive and performance paths

The prescriptive path takes the table values directly and is simple. The performance path allows trading off, where a better window lets a wall fall short, as long as the whole building models out at least as well. The performance path is what an energy calculation usually demonstrates.

At inspection

R-values are verified at the insulation inspection, before anything is covered. What is installed has to match what the energy calculation assumed, and a substitution made for availability during construction is the usual cause of a failure here.