Thermal performance · DASMA TDS #163 and #196
Stop Shopping for R-Value: The Number That Actually Tells You How Warm a Garage Door Is
Every garage door brochure in the northwest suburbs leads with an R-value. The trade association representing the manufacturers who print those brochures says that number should not be used to describe a garage door assembly at all. Here is what to ask for instead.
The number on every brochure
Walk into any showroom between here and Schaumburg and the conversation goes the same way. The salesperson opens a sample section, taps the foam, and tells you it is R-16. The door next to it is R-12, and it costs less. You reason that the bigger number is the warmer door, and you have just made your decision on a figure that the industry’s own technical body says is the wrong figure to compare doors on.
That is not a conspiracy. R-value is a real number. It is simply a number about a slab of material, and you are buying a moving assembly with joints, hinges, end stiles, a header seal and a gap under the bottom section.
What R-value actually measures
The Door & Access Systems Manufacturers Association covers this in Technical Data Sheet #163, and the language is not ambiguous. R-value describes the door section only — the material type and its thickness. DASMA states plainly that section-only R-value “should not be used for garage door assemblies.”
Look at how the figure is built and you can see why. The calculation includes air film values on either face totalling R-0.85. The steel facings are treated as negligible, which they are: thin steel conducts heat so well that it contributes essentially nothing to the resistance. Everything else comes from the k-factor of the core material and how many inches of it there are.
So what is an R-value really telling you?
It is telling you about the foam, and how thick the foam is. That is genuinely worth knowing when you are choosing between a hollow pan door, a single-layer door with a bonded core and a three-layer sandwich section. It is not telling you how the finished door performs once it is hanging in your opening.
You will find articles online printing tidy tables of “typical R-values by construction type.” Neither TDS #163 nor its companion sheet publishes those ranges, so I am not going to invent them. If a manufacturer quotes a range, it belongs to that manufacturer’s spec sheet and should be read as such.
The practical problem follows. Because the printed R-value describes a section rather than a door, comparing R-14 from one brand with R-14 from another compares two cores, not two products. What happens where the sections meet, where hardware bridges the insulation, and around the perimeter of the opening is nowhere in either number. Those are exactly the places heat leaves a garage.
Why you cannot flip one number into the other
Anyone who took high school physics knows that U is the reciprocal of R. Divide one by the other and you are done. For garage doors, that is wrong, and DASMA says so directly: “U-factor and R-value are not reciprocals of each other.”
In DASMA’s words, “these two methods are not comparable and are obtained by entirely different methods reflecting different aspects of thermal performance.” One is a calculation about a piece of section. The other is a measurement of a whole door. Dividing 1 by 16 and printing the result as a U-factor produces a number that means nothing, and I have seen it done.
U-factor and what ANSI/DASMA 105 puts on the rig
U-factor is the rate at which heat passes through the assembly. It is expressed in BTU/(h·ft²·°F), and unlike R-value, lower is better. The arithmetic is refreshingly honest: a door rated 0.20 is twice as efficient as a door rated 0.40. No interpretation required.
It is determined under ANSI/DASMA Standard 105, which the International Energy Conservation Code references, and the test runs on a complete installed door. That phrase is doing a lot of work, so here is what it covers:
- Sections
- The same material R-value describes, but as part of a whole.
- Section joints and their interface
- Every meeting rail between panels, and how two sections seal against each other.
- Assembled hardware
- Hinges, end stiles, roller carriers and struts, including the metal paths bridging the warm face to the cold one.
- Perimeter design
- How the door meets the jambs, the header and the slab.
R-value tests none of that. DASMA’s recommendation follows: U-factor “provides the highest degree of confidence when evaluating and comparing the thermal performance of garage door assemblies.” Read that twice. It is an industry body telling its own members’ customers to stop shopping on the marketing number.
Who checks the number
A tested figure is only as good as the testing. DASMA runs a Thermal Performance Verification Program jointly with ICC-ES, described in TDS #196. Participating manufacturers submit third-party laboratory data, ICC-ES reviews it and maintains the records, and doors that comply carry the program’s badging.
That is the point of it: a U-factor with a name behind it rather than a figure typed onto a sales sheet. If you are specifying a door for a garage you actually heat, ask whether the quoted model is in the program.
What the energy code requires here
Arlington Heights sits in IECC Climate Zone 5A, the cool-humid zone. That designation comes from a secondary source, so cross-check it against the official IECC climate zone map before relying on it for a permit submission. The village’s Building & Life Safety Department will confirm it.
The village adopted the 2024 Illinois Energy Conservation Code, effective 30 November 2025. Now the part that surprises people: under the IECC, a residential garage door U-factor requirement only applies where the garage is heated or air conditioned. An unconditioned attached garage door carries no mandatory U-factor at all. Where the requirement does apply, U-factor must be determined per NFRC 100 or ANSI/DASMA 105 — not converted from an R-value.
The practical reading
If your garage has no heater and no supply register, no inspector is going to hold you to a U-factor. That does not make insulation pointless. It moves the decision out of code compliance and into what the door does for you, which is a better conversation anyway. We cover it on every garage door replacement quote.
What insulation actually buys you
The honest version, from twenty-odd winters of fitting these doors around Cook County. An insulated door on an unheated attached garage does four useful things and one thing it does not.
It makes the garage a milder buffer. The rooms above and beside an attached garage lose heat into it. A garage sitting at 30 °F rather than 15 °F on a January night is a smaller temperature difference across that shared wall and floor, and January here averages 25.2 °F per the NWS Chicago normals. That is the real energy argument, and it is about the rooms.
It cuts condensation. A cold steel skin with humid air against it drips. Put a core behind that skin and the inside face runs warmer, which keeps water off the door, off the hardware and out of the springs. Damp steel is what corrodes cables and coils, which is the subject of what road salt does to garage door hardware.
It stiffens and quietens the door. A bonded-core section is far more rigid than a hollow pan. It flexes less on the lift, rattles less over the rollers, and runs quieter under a bedroom. That is often the real reason people upgrade. If noise is your problem, start with a noisy door diagnosis rather than a new door.
It helps the opener in the cold. A stiffer door with intact seals presents a more predictable load, and openers under 16 CFR 1211 reverse when the load reads like an obstruction. Less flex and less ice bonding means fewer nuisance reversals in February.
What it will not do is heat your garage. Insulation slows heat loss. With no heat source in the space, there is nothing to slow. An insulated door on an unheated garage buys you a few degrees off the outside temperature and a slower swing, not a warm room. Anyone promising otherwise is selling.
One caveat worth knowing before you buy: insulated bonded-core sections bow when the two faces sit at very different temperatures, which is normal rather than defective. We wrote that up separately in why insulated doors bow in a cold snap.
The wall matters more than the door
If the goal is warmer rooms behind the garage, the door is not the first thing to spend money on. The Department of Energy’s Building America program is direct about this in its guide to air-sealing an attached garage: the air barrier between the garage and the house — the common wall, the ceiling under a room above, the band joist, and every penetration through them — is what governs comfort and air quality in those rooms.
Part of the reason has nothing to do with heating bills. An attached garage holds vehicle exhaust, fuel and solvents, and gaps in that shared assembly move that air into the house. None of this argues against a good door. It argues for order of operations: seal the shared wall and ceiling first, then insulate the door for what it does for the garage itself.
What to ask the salesperson
Five questions, and you can ask them over the phone.
“What is the U-factor of the complete door?”
Not the section R-value. If the answer is a conversion from R-value, it is not a U-factor.
“Was it tested to ANSI/DASMA 105?”
That is the standard the IECC references, and the one that puts the whole assembly on the rig.
“Is the model in the DASMA Thermal Performance Verification Program?”
If yes, an independent body has reviewed the lab data.
“What is the core, and how thick is the section?”
This is where R-value belongs. Polystyrene board dropped into a pan and polyurethane foamed in place between two skins are different products.
“What weatherseal comes with it?”
Bottom astragal, jamb seal, header seal. Perimeter design is part of the tested U-factor, and it is the part that degrades first.
A quote that cannot answer question one tells you something. We fit insulated doors across Arlington Heights, Mount Prospect and the surrounding suburbs, and the spec sheet goes on the estimate. For a straight answer on a door you are already considering, send us the model number.
Dale Whitmore
Owner & Lead Technician
Fitting overhead doors across the northwest suburbs since the mid-1990s and running this company since 2011. Our technicians are not paid commission on parts, which is why this article talks you out of things.
Straight answers
Insulation questions homeowners actually ask
Is a higher R-value always a warmer garage door?
Not reliably. R-value describes the door section — material type and thickness — and DASMA states it should not be used for garage door assemblies. Two doors with the same printed R-value can have different joints, hardware and perimeter design, and those are where heat leaves. Ask for the U-factor of the complete door instead.
Can I convert R-value to U-factor by dividing one by the other?
No. DASMA states directly that for garage doors, U-factor and R-value are not reciprocals of each other, and that the two methods are not comparable because they are obtained by entirely different methods reflecting different aspects of thermal performance. A U-factor derived by arithmetic from an R-value is not a U-factor.
What U-factor should I look for?
Lower is better, and the scale is linear: a door at 0.20 BTU/(h·ft²·°F) is twice as efficient as one at 0.40. What you need depends on whether the garage is conditioned and what is above it. For an unheated attached garage in Arlington Heights the energy code sets no mandatory figure, so choose on what the door does for the space and the rooms behind it.
Do I need an insulated door to meet the code in Arlington Heights?
Under the IECC, a residential garage door U-factor requirement only applies where the garage is heated or air conditioned. An unconditioned attached garage door has no mandatory U-factor. The village has been on the 2024 Illinois Energy Conservation Code since 30 November 2025; confirm your specific situation with Building & Life Safety before you assume either way.
Will an insulated door make my garage warm enough to work in?
On its own, no. Insulation slows heat loss; without a heat source in the garage there is little to slow. Expect a milder, slower-swinging space rather than a warm one. If you want a workable garage in January you need a heat source, and then the door’s U-factor starts to matter for real.
What helps the rooms above my garage most?
The air barrier between the garage and the house. The DOE Building America guidance on air-sealing an attached garage puts the common wall, the ceiling below a room above, the band joist and every penetration ahead of the door itself. Seal that first, then treat the door as an upgrade for the garage.
Want the real spec, not the brochure number?
Tell us the opening size and what sits above the garage, and we will quote a door with a tested U-factor on the estimate.