Garage Door System GuideA garage door labeled R-16 does not perform like R-16 across the whole door. DASMA, the trade association that writes the industry test standards, explains that R-value is a calculation for a single door section, while U-factor is a measurement of the complete, installed door. The two are obtained by different methods, are not reciprocals, and are not comparable. This guide walks through what each number means, how they are derived, and why DASMA and building codes favor U-factor.
R-value and U-factor measure different things
R-value is an expression of thermal resistance, based on material type and thickness, reported in units of (h.ft2.F)/Btu, where a greater number means better insulation. U-factor is an expression of thermal transmittance, reported in Btu/(h.ft2.F), where a lower number means better insulation. DASMA states both definitions in its technical data sheet on garage door thermal values.
The critical point DASMA makes is that these two methods are not comparable and are obtained by entirely different methods reflecting different aspects of thermal performance. For garage door thermal properties, U-factor and R-value are not reciprocals of each other. A homeowner comparing an R-value on one door to a U-factor on another is comparing two things that do not translate.
R-value is calculated for a single door section
The R-value method involves the door section only and results in an R-value. It estimates the insulating value of a door section without accounting for the complexities of door section joints, stile attachments, and similar features, and DASMA states it should not be used for garage door assemblies.
The calculation adds the R-values of the air films, the outside and inside surfaces, and the insulating core. DASMA assumes stagnant air inside a closed door and 15 MPH air moving outside, and based on ASHRAE data these air films total an R-value of 0.85. R-values of steel and other metal facings are considered negligible. In DASMA's worked example, a section with polyurethane foam calculates to R-16.47, a section-only figure that does not represent the whole door.
U-factor is tested on the whole installed door
The testing method involves the entire, installed garage door assembly and results in a U-factor. An assembled door adds sections coupled together, plus track, hardware, and accessories, and includes features such as section joints, stiles, thermal breaks, air spaces, and weather seals that affect thermal conductance.
DASMA notes that some elements of door construction provide a thermal shortcut around the insulation in a door section, thereby increasing heat flow, while thermal breaks and weather seals resist it. To capture this, DASMA developed the nationally recognized test standard ANSI/DASMA 105. A complete door is installed in a chamber where the inside face is held at a constant temperature and the outside face is cooled, and the energy required to hold that temperature measures the rate of heat flow, or U-factor, through the whole door.
Why building codes and DASMA favor U-factor
DASMA concludes that U-factor provides the highest degree of confidence when evaluating and comparing the thermal performance of garage door assemblies. The accuracy of a tested door assembly U-factor exceeds other types of thermal performance ratings, and recognized building codes and standards deal with garage door thermal performance in terms of U-factor.
DASMA's Thermal Performance Rating program exists to make U-factor the industry standard for representing thermal performance of sectional garage doors. The program explains that R-value is a calculated measurement that reflects the insulating material only and represents the thermal property of only an individual door section, whereas building codes and knowledgeable professionals recognize that assembly value is more important than component value.
How insulation materials compare
When you do compare the insulating cores themselves, the numbers are more modest than a marketing R-16 suggests. A 1.5-inch-thick fiberglass batt offers a thermal resistance value of R-4.7 to R-6.45 depending on density, according to Bob Vila.
Rigid foam boards vary by chemistry. Expanded polystyrene (EPS) has an R-value of about 4.6 per inch of thickness, extruded polystyrene (XPS) offers about R-5 per inch, and polyisocyanurate offers a higher R-5.8 per inch. These per-inch figures explain why thicker foam cores earn higher section R-values, but they still describe the core material, not the finished door that a U-factor would capture.
Frequently asked questions
Is a higher R-value garage door always better insulated?
Not necessarily as a whole door. DASMA explains that R-value is a calculation for a single door section only, while U-factor is measured on the complete installed door, and the two are not comparable and not reciprocals of each other.
What is U-factor on a garage door?
U-factor is thermal transmittance measured on the entire installed door assembly, including section joints, thermal breaks, and weather seals, under DASMA's ANSI/DASMA 105 test. A lower U-factor means better insulation.
Why do building codes prefer U-factor over R-value?
DASMA states that a tested door assembly U-factor provides the highest degree of confidence when comparing thermal performance, and recognized building codes deal with garage door thermal performance in terms of U-factor rather than section-only R-values.
How do the insulation materials themselves compare?
Per the guide's figures, a 1.5-inch fiberglass batt runs R-4.7 to R-6.45 depending on density, EPS foam about R-4.6 per inch, XPS about R-5 per inch, and polyisocyanurate about R-5.8 per inch. These describe the core material, not the finished door.