Products are available with U-factors ranging from 0.57 to 0.36 (R= 1.75 to 2.75). Steel stiffened, polystyrene and polyiso doors were included in the tests. Ratings are significantly better than those obtained from tests performed on wood and aluminum doors.
Thermal performance tests measure a door's thermal conductivity known as "U-Factor" and its inverse which is termed thermal resistance or R-Value. U-factors have units in the Inch-Pound system of BTUs per hour per square foot per degree (Fahrenheit) or Btu/hr-ft2-F. R-values have the rather unintuitive units of: hr-ft2-F/Btu. U-factor is based on measured heat flow through a sample at the temperature difference of the air on the indoor and outdoor sides. New test criteria provide realistic values representative of actual room conditions and are based on the full surface of an operable door rather than just the center portion of the door surface as with old test procedures.