DASMA TDS 163 - U-Factor and R-Value for Residential and Commercial Garage Doors

Most door salespeople quote R-value. Most energy codes and engineers use U-factor. DASMA TDS 163 explains the difference, and why a door with an R-18 sticker on the showroom floor may not outperform an R-12 door once you account for the full assembly.

What this data sheet says

TDS 163 establishes that R-value and U-factor measure different things for a garage door:

• R-value is the thermal resistance of the insulation core only, measured in a simplified slab test. It ignores panels, seals, rails, and the metal skin.
• U-factor is the whole-door thermal transmittance, measured by ANSI/DASMA 105 on a full installed door assembly. U-factor captures heat flow through every path, including the steel or aluminum panels and the gaps at the edges.

"R-value does not account for the total door; it measures only the insulation core. U-factor, measured per ANSI/DASMA 105, reflects the complete door assembly's thermal performance."

The relationship is U = 1/R, but that math only applies to a pure insulation slab. A steel-skinned door with polyurethane foam core rated R-18 may have a measured whole-door U-factor of 0.20 to 0.25 once the steel panels and frame are included. A different door marketed as R-12 with a thicker thermal break at the panel joints and better seals may measure U-0.18 and actually lose less heat.

TDS 163 also notes that DASMA's Thermal Performance Verification Program (further detailed in TDS 196) allows manufacturers to certify their door's U-factor using the ANSI/DASMA 105 test protocol, giving buyers an apples-to-apples comparison.

When it applies

TDS 163 is most relevant when:

• Specifying a door for a conditioned garage (heated or cooled space attached to living area)
• Comparing competing door bids where one salesperson quotes R-value and another quotes U-factor
• Meeting energy code: the 2025 Denver Building Code references the 2024 IRC, which in certain climate zones requires minimum door thermal performance

Denver sits in IECC Climate Zone 5B. For an attached garage door opening into conditioned space, the IRC energy provisions reference U-factor as the performance metric. A lower U-factor door (say U-0.17 versus U-0.27) makes a measurable difference over a Colorado winter.

For example, a 16x7 door (112 sq ft) with U-0.27 loses roughly 30% more heat than the same door with U-0.17 on a 20°F night with 68°F interior. Over a Denver heating season, that gap adds up.

What this means for you

When comparing doors, ask for the U-factor from ANSI/DASMA 105 testing, not just the R-value. If a manufacturer cannot provide a tested U-factor, treat the R-value figure with skepticism.

Higher R-value does not guarantee lower U-factor. The insulation core is one path; the steel skin, panel joints, and bottom seal are others. A door with R-18 foam but thin steel panels and minimal thermal break at the joints can lose more heat than a door with R-12 foam and a well-designed frame.

For attached garages in Denver, a tested U-factor around 0.20 or better is a reasonable target for an insulated steel door. Doors below U-0.15 exist but come at a premium and are usually reserved for highly conditioned attached garages.

G Brothers can provide manufacturer spec sheets with ANSI/DASMA 105 U-factor data for all insulated door lines we carry.

Full text and source

Download DASMA TDS 163 from the official TDS index at https://www.dasma.com/technical-data-sheets/.

This entry covers residential and commercial sectional garage doors. Rolling steel and specialty doors may use different thermal testing methods.

Want to put numbers to this? Use the interactive r-value / u-factor converter below, or open the full r-value / u-factor converter with examples and notes.