R-Value

R-value is the standard measure of thermal resistance used across the construction industry. For garage doors, it expresses how effectively the door's foam insulation, steel skins, and any airgap within the section resist the flow of heat. The formula is R = ΔT / Q, where ΔT is the temperature difference across the material and Q is the heat flow per unit area. In practice, a higher R-value means a more thermally resistant door.

Most residential insulated garage doors fall between R-6 and R-18. Polyurethane-foam doors—where the foam is injected and bonded to both steel skins—achieve higher R-values than polystyrene-foam (bead board) doors of the same thickness because polyurethane has a higher thermal resistance per inch and eliminates the air gaps that form when a bead-board insert is not perfectly bonded to the skins. A two-inch polyurethane door can reach R-18, while a two-inch polystyrene insert door may measure R-9 or less.

The thermal break at the door section's edges matters significantly for the measured R-value. Without a thermal break, heat conducts rapidly through the steel frame around the foam core, making the section's actual heat loss much higher than the foam's rated R-value would suggest. This is why the same foam thickness can show different effective R-values on different door constructions.

For an attached garage that shares a wall with living space, a higher R-value door reduces the energy load on the home's heating and cooling system. For a detached or unheated garage, the benefit is primarily comfort—a high R-value door keeps the garage warmer in winter and cooler in summer without necessarily reducing the home's energy bill.