Garage Door System GuideInsulation is the single biggest variable in a garage door's energy performance and its price. Two foams dominate the market, polystyrene and polyurethane, and they behave differently. Just as important, the R-value a manufacturer prints can mean two different things depending on how it was measured. The facts below come from the DASMA technical data sheet and manufacturer literature.
How door R-value is measured
DASMA, the Door and Access Systems Manufacturers Association, defines two ways to state a garage door's thermal performance. A calculated door section R-value looks only at the mean thickness through the center of a section, while a tested value uses the full installed door assembly in a lab chamber.
The calculated method sums the air films, the surface layers, and the insulation. DASMA assigns the air films a combined R-value of 0.85 and treats the steel skins as negligible, so nearly all of the calculated R-value comes from the foam core. Because section joints, stiles, and thermal breaks create shortcuts for heat, DASMA developed the ANSI/DASMA 105 test standard to measure the U-factor of a complete installed door, and warns the two numbers are not comparable to each other.
Polystyrene vs polyurethane foam
The core material sets the ceiling on how much insulation fits in a given thickness. DASMA lists typical thermal conductivity, the k factor, at 0.235 to 0.265 for expanded polystyrene foam and a much lower 0.13 to 0.18 for polyurethane foam. Because R-value is thickness divided by k, a lower k means more R-value per inch.
That difference shows up in the field. Wayne Dalton states that polyurethane insulation delivers twice the thermal performance of polystyrene of the same thickness. Polystyrene is a rigid panel that slots into the door, while polyurethane is foamed in place and expands to fill the cavity, which is why it also stiffens the door.
Worked example from the standard
DASMA publishes a worked calculation so the numbers are not a black box. For a steel skin sandwich door with a uniform 1.75 inch polyurethane core rated at R-7.0 per inch, the insulation contributes 12.25 and the air films add 0.85, giving a calculated section R-value of 13.1.
Manufacturer specs land in this range. Wayne Dalton's Model 8300 posts a calculated R-value of 12.12 with a tested U-factor of 0.15, and its Model 8500 reaches R-16.22 at a U-factor of 0.12. Overhead Door's Thermacore line runs from R-9.31 on the one inch Model 5720 up to R-17.5 on the two inch Model 5760.