Research study
The R-value on a garage door is not the R-value of the garage door
It is worked out for one slice of one panel. The industry’s own standards body says in writing not to use it for the door.
Written by HyreGarage Research Desk Primary-source research and data analysis
Audited by HyreGarage Research Desk Citation, computation and retrieval-date audit
The finding
A garage door’s advertised R-value describes one panel section, not the installed door.
DASMA Technical Data Sheet #163 says this calculated method “should not be used for garage door assemblies” and that U-factor and R-value “are not reciprocals.” Ask instead for the tested U-factor from an ANSI/DASMA 105 test.
In DASMA’s own example, 5.2% of the R-value is air, not door.
What did we find?
You may be comparing two doors on a number that describes neither. The trade group that publishes the method says so. DASMA Technical Data Sheet #163 states that “[f]or purposes of garage door thermal properties, U-factor and R-value are not reciprocals of each other.”
The R-value is a calculation on one section. The U-factor is a measurement of the whole door. ANSI/DASMA 105 tests a complete door in a chamber. The calculated method adds up a single section. DASMA says it “should not be used for garage door assemblies.”
What gets left out is not a small detail. An assembled door “may include features that greatly impact the thermal performance, such as section joints, stiles, thermal breaks, air spaces, weather seals, etc.” Some of these “provide a ‘thermal shortcut’ around the insulation”. The calculated R-value sees none of it.
HyreGarage math on DASMA’s own worked example: of the R-16.47 it reaches, 0.85 is air film. That is 5.2 percent coming from the air next to the door, not the door. Steel facings are “considered negligible”.
What this page claims, and what it does not
The claim is narrow. It is made almost entirely in the words of the industry’s own standards body.
A calculated section R-value is a real, defined and correctly computed number. DASMA publishes the method. The problem is what buyers think it describes.
In DASMA’s words, the method covers nothing beyond the section and “should not be used for garage door assemblies”. That limits what it covers. It does not make it inaccurate.
The worked example, the air film value and the R-16.47 total are all DASMA’s. Splitting out the air film share and showing why the inversion fails are HyreGarage calculations, labeled as such.
Both are sold commercially. The 0.68 and 0.17 air film figures, the ASTM C177/C518 k-factor basis and the ANSI/DASMA 105 test description are quoted as TDS #163 states them. We name that go-between each time.
This page compares methods, not doors. It names no manufacturer’s figure and rates no product.
Whether an insulated door saves you anything depends on whether the garage is inside the heated part of the house. This page raises that question but does not settle it. Our garage door insulation study looks at it.
Why do R-value and U-factor look like the same number?
Because most people learn R-value from attic insulation, where bigger is better and it works. A garage door looks like a panel with insulation in it, so the same rule gets applied to it all the time.
The trouble is what a garage door really is. DASMA puts it plainly: “In actual use, a garage door consists of multiple sections coupled together, plus track, hardware, and accessories. The assembled door may include features that greatly impact the thermal performance, such as section joints, stiles, thermal breaks, air spaces, weather seals, etc.”
Attic insulation is one continuous blanket. A garage door is a stack of separate panels, hinged together, running in a metal track and sealed at the edges by a rubber strip. Its steel skins also carry load.
Every joint between panels is a path for heat, and every piece of steel that crosses from outside to inside is a bridge. DASMA names it: “Some elements of door construction can provide a ‘thermal shortcut’ around the insulation in a door section, thereby increasing heat flow.”
So the industry has two ways to express insulation. One tests the whole door (U-factor). The other calculates one section (R-value). TDS #163 explains they differ: “Please note, these two methods are not comparable and are obtained by entirely different methods reflecting different aspects of thermal performance.”
HyreGarage analysis: only one of the two is expensive to produce. A U-factor needs a physical test of a complete door in a chamber. An R-value needs a spreadsheet. Both are legitimate. The cheaper one is the one that ends up on the showroom card.
How do the two rating methods differ?
One tests the whole door; the other calculates one panel section. The table draws on DASMA Technical Data Sheet #163. The last column decides how much the number is worth to you.
| Method | Reports | Units | What is measured | What it captures |
|---|---|---|---|---|
| Tested Door Assembly | U-factor | Btu/(h·ft²·°F) — lower is better | The entire, installed garage door assembly | Section joints, stiles, thermal breaks, air spaces, weather seals — including the “thermal shortcut” paths around the insulation. |
| Calculated Door Section | R-value | (h·ft²·°F)/Btu — higher is better | One door section only | Nothing beyond the section. DASMA: it “estimates the insulating value of a door section without accounting for the complexities of door section joints, stile attachments, etc.” |
DASMA TDS #163 (10/10/02, Rev. 10/15/08, Rev. 7/6/22), retrieved 2026-09-06. The sheet notes that “[s]imilar test methods are used for many building products for which the performance of a complete product is relevant, such as windows and entry doors.” Nobody sells a window on the R-value of a slice of its frame.
What is DASMA’s R-16.47 example made of?
The sheet publishes a full calculation, so we can show exactly what the number is made of. The example is a 22-inch-tall section. It has polyurethane foam 1⅜ inch thick over 16 inches of its height and 1⅞ inch over the other 6, with a k-factor of 0.12.
| Component | Calculation as DASMA publishes it | R | Share of total |
|---|---|---|---|
| Air films | Rair films = .68 + .17 = .85 | 0.85 | 5.2% |
| Insulation, 1⅜″ over 16″ of the section | R = (1.375) × (16/22) / .12 = 11.36 | 11.36 | 69.0% |
| Insulation, 1⅞″ over 6″ of the section | R = (1.875) × (6/22) / .12 = 4.26 | 4.26 | 25.9% |
| Steel and other metal facings | R-values “considered negligible” | 0 | 0% |
| Section total | Rsection = .85 + 11.36 + 4.26 | 16.47 | 100% |
| Joints, stiles, thermal breaks, seals, track | Not in the calculation at all | — | — |
Example, formula and values: DASMA TDS #163; share column: ours. DASMA credits the air films to the 2017 ASHRAE Handbook — Fundamentals, p. 26.21, Table 10: “Still Air, Vertical Surface, Non-reflective shows R = 0.68. Moving Air, 15 MPH wind, Non-reflective shows R = 0.17.” We did not retrieve the Handbook (sold commercially).
Can you turn an R-value into a U-factor?
No. It is a very reasonable mistake, so here is how it happens during a purchase.
A buyer sees a door rated R-16.47. They know that codes and energy calculations use U-factors. They also know that for many building materials, U and R are reciprocals. So they divide: 1 ÷ 16.47 = 0.061, and use that figure to compare doors.
DASMA says plainly that this is invalid: “For purposes of garage door thermal properties, U-factor and R-value are not reciprocals of each other.”
HyreGarage analysis: the reason is what was measured. The R-value comes from a section with no joints, stiles, edge seal or track, so heat has no shortcuts. The U-factor is measured on a whole door that has all of them. Finding those shortcuts is the point of a thermal test.
Flipping the first number cannot give you the second, because they never described the same object. The two numbers do not disagree. They are about different things.
The same calculation has a second, quieter problem. The 0.85 air film is a fixed value from the setting: still air on one side and a 15 mph wind on the other. It is added to every door’s section R-value equally.
On a heavily insulated door that is a rounding error. On a lightly insulated door it is a big share of a small number. In proportion, it flatters cheap doors more than expensive ones.
And the steel adds nothing. DASMA: “R-values of steel and other metal facings are considered negligible.” The skins give the door its strength, look and much of its cost, yet add nothing to the figure. In the real door, steel crossing the section is exactly the shortcut the calculation cannot see.
What should you ask for instead?
Ask for the tested U-factor first; that is DASMA’s own advice. Here are six questions, in the order they help.
Ask for the tested U-factor
DASMA: “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.”
Ask whether the figure comes from an ANSI/DASMA 105 test.
Ask whether the number is tested or calculated
This one question separates the two methods. A tested figure is a U-factor from a chamber. A calculated figure is an R-value from a spreadsheet. Both may be quoted honestly, but only one describes the door you are buying.
Ask about the edge seal and the joints
These are the parts the calculation cannot see and the test can. A door with a good U-factor usually has well-designed joints and seals. Those also keep out water and drafts.
Ask whether your garage is inside the heated part of the house
HyreGarage analysis: this decides whether any of it matters. An unheated garage behind an insulated, air-sealed shared wall sits outside the house’s heated shell. There, the door mainly affects the garage. A heated garage, or one under a bedroom, is a different case.
Ask what problem you are solving
A comfortable workshop, a warm room above, unfrozen pipes and lower house energy use are four goals with four different answers. Only one of them is really about the door. If you are pricing a new door anyway, our garage door replacement cost study shows where cost figures come from.
Do not compare an R-value with a U-factor
Not by flipping either one. If two quotes give you different kinds of number, ask both companies for the same one. Comparing them as they stand is the mistake this page is about. Our garage door cost calculator helps you price the options line by line.
What do the insulation terms mean?
- U-factor
- DASMA: “an expression of thermal transmittance, reported in Btu/(h·ft²·°F); the lower the U-factor, the better the insulation.” For garage doors it comes from testing a complete door.
- R-value
- DASMA: “an expression of thermal resistance, based on material type and thickness, and is reported in units of (h·ft²·°F)/Btu; the greater the R-value, the better the insulation.” For garage doors it is calculated for a single section.
- ANSI/DASMA 105
- The test standard DASMA calls “nationally recognized”. A complete door is installed in a chamber. The inside face is held at a constant temperature and the outside face is cooled. The energy needed to hold the temperature gives the door’s U-factor. We did not read the standard itself.
- Air film
- The insulating effect of the air next to a surface. DASMA assumes “air will be stagnant on the inside of a closed door and the exterior will have air moving against it at 15 MPH”, for R = 0.85 from ASHRAE values. It belongs to the setting, not the door.
- k-factor
- How well a material conducts heat. DASMA: “The k-factor for a particular material is traced to testing in accordance with ASTM C177 or C518, using an average material temperature of 40°F.” The insulation R is thickness divided by k.
- Thermal shortcut
- DASMA’s phrase for parts that carry heat around the insulation: stiles, joints, fasteners and steel crossing the section. Every real door has them. No calculated section R-value includes them.
What could we not find?
ANSI/DASMA 105 and the ASHRAE Handbook. Both are sold commercially, and we bought neither. Every value credited to them here is quoted as TDS #163 states and cites it. That covers the 0.68 and 0.17 air film figures, the ASTM C177/C518 k-factor basis and the test chamber description.
Any manufacturer’s tested U-factor. We publish no product figures. The natural next study would set tested U-factors against advertised R-values across many doors. That would show how big the gap is, not just that it exists. It needs manufacturer test reports, which are not published in any systematic way.
How big the gap is. This is the limit of the page. We can show that the calculated R-value leaves out joints, stiles and seals, because DASMA says so. We cannot tell you how much heat those paths carry, because that is what an ANSI/DASMA 105 test measures, and we have no test data.
What the building code says. DASMA states that “recognized building codes and standards deal with garage door thermal performance in terms of U-factor”, and we quote it for that. We could not retrieve the IECC text to confirm how it treats doors on unheated garages. So we raise that question and do not answer it.
Whether an insulated door saves money. We did not try to answer this, and we would be skeptical of anyone who does in general terms. It depends on how the garage connects to the heated house, the climate, the door’s tested performance and how long the door stays open. Nobody models that last one.
Questions
What does the R-value on a garage door actually mean?
Is R-value or U-factor better for comparing garage doors?
Can I convert a garage door R-value to a U-factor?
Why does a high R-value door still let heat through?
How much of a garage door R-value comes from the door itself?
How is a garage door U-factor tested?
Is a higher R-value garage door worth the money?
Do insulated garage doors have benefits besides heat?
How do I calculate a garage door section R-value myself?
Which DASMA document says all this?
Written and audited by
HyreGarage Research Desk
Primary-source research, data analysis and fact checking
We are not a garage door company. We read the agency file, the code record, the standards document or the public register ourselves, compute the figure from it, and publish it with the source and the date we retrieved it.
Where a number cannot be traced to a primary source, we publish the shorter page and say what we could not verify. Our own company records cover ten states; nothing national is ever derived from them.
- 10
- states our own company records cover — and the limit of any claim made from them
- 3,901
- garage door companies in the store
- 457
- license records verified against a state board
- 0
- national claims made from a ten-state store
How this desk works
- Primary sources only. Injury counts come from CPSC. Housing counts come from the Census file, not from a summary of it. Code history comes from the building commission that adopted the code. We do not cite an article that cites a source; we retrieve the source and do the arithmetic ourselves.
- Every figure carries its retrieval date. Registers change, datasets are revised and codes are amended. A number without the date it was read cannot be checked, so every study states one.
- Fact, calculation and analysis are labeled apart. A quote is a quote, a HyreGarage computation says so, and an interpretation says “HyreGarage analysis”. Presenting our reading of a dataset as something the agency stated would be the easiest way to lose the only thing this desk is for.
- Limitations go above the fold. If a figure is an upper bound, a bracket, or an association rather than a cause, that is said before the figure is quoted rather than in a footnote underneath it.
- No DIY instructions for spring, cable or track work. Those components hold enough stored energy to cause serious injury, and CPSC records the consequences. We describe what has failed and what a competent repair involves; we do not tell you how to do it.
Data as of DASMA Technical Data Sheet #163, retrieved 2026-09-06. Authorship on this site is organizational: the analysis belongs to the desk rather than to a named individual, and we do not publish credentials we do not hold. Our editorial policy sets out how we source, date and correct what we publish.
Sources & retrieval dates
Comparing insulated doors?
Ask every company for the same number in writing: the tested U-factor from an ANSI/DASMA 105 test. Quotes with different kinds of number cannot be compared. Matching is still being built, so nothing goes to a company unless you agree.
HyreGarage is not a garage door company and does not perform, supervise or warrant garage door work. This page compares measurement methods. It publishes no thermal figure for any product and rates no manufacturer.