# Garage Door R-Value vs U-Factor: Why the Big Number Misleads > A garage door advertised at R-18 is not an R-18 wall. R-value is calculated on a single section; U-factor is measured on the whole installed door. DASMA states the two are not comparable and not reciprocals. - Canonical HTML: https://www.garagedoorman.ca/guides/insulated-garage-door-r-value/ - Section: Home > Guides > R-value on a garage door, and why the number is misleading - Business: Garage Doorman, 6 Chestnut Dr, Grimsby, ON L3M 0C4, Canada - Phone: (905) 536-5369 | Email: info@garagedoorman.ca - Page updated: 2026-08-31 --- A door advertised at R-18 is not an R-18 wall. The R-value on a garage door brochure is calculated on a single section in isolation. The U-factor is measured on the whole installed door. The industry body that defines both says plainly that they are not comparable, and are not even reciprocals of each other. By Garage Doorman, Grimsby, Ontario · Updated 31 Aug 2026 **R-value**: Calculated. One door section only. Ignores joints, stiles, track and seals. **U-factor**: Tested. The complete installed door assembly, to ANSI/DASMA 105. **Not comparable**: DASMA's own conclusion. Lower U-factor is better; higher R-value is better; they are different measurements. ## What the two numbers actually measure DASMA defines them like this. U-factor is thermal transmittance, reported in Btu per hour per square foot per degree Fahrenheit, and the lower it is the better the insulation. R-value is thermal resistance, based on material type and thickness, reported in the inverse units, and the greater it is the better the insulation. The important part is how each is arrived at. DASMA states there are two methods for determining thermal properties of a garage door: > "1. Testing — This involves the entire, installed garage door assembly, and results in a U-factor. 2. Calculation — This involves the door section only and results in an R-value." > > DASMA Technical Data Sheet #163 And then, in the same document, the sentence that most garage door marketing quietly ignores: > "Please note, these two methods are not comparable and are obtained by entirely different methods reflecting different aspects of thermal performance. For purposes of garage door thermal properties, U-factor and R-value are not reciprocals of each other." > > DASMA Technical Data Sheet #163 Source: DASMA Technical Data Sheet [#163, U-factor and R-value for Residential and Commercial Garage Doors](https://www.dasma.com/wp-content/uploads/2022/09/TDS163..pdf) (PDF). Checked 31 August 2026. ## Why the section number flatters the door A real garage door is not a single slab of insulation. It is several sections coupled together, plus track, hardware and accessories, and DASMA notes that the 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 those elements provide what DASMA calls a "thermal shortcut" around the insulation, increasing heat flow. Others, such as thermal breaks and weather seals, resist it. The whole point of the tested U-factor is to capture that. ANSI/DASMA 105 is the recognised test standard: a complete door system is installed in a chamber with the inside face held at a constant temperature and the outside face cooled, and the energy needed to hold that temperature gives the rate of heat flow through the whole system. It is the same approach used for windows and entry doors, for the same reason. The calculated R-value skips all of it. In DASMA's words, the calculation method "estimates the insulating value of a door section without accounting for the complexities of door section joints, stile attachments, etc., and therefore should not be used for garage door assemblies." ## How a section R-value is built up If you want to see why the number gets large, here is the arithmetic DASMA sets out. A door section's R-value is the sum of the air films, the outside surface, the insulation and the inside surface: Rsection = Rair films \+ Routside surface \+ Rinsulation \+ Rinside surface The air films are assumed to be stagnant air inside and air moving at 15 mph outside, and based on ASHRAE data they total an R-value of 0.85. Steel and other metal facings are treated as negligible. So in practice, almost the entire number is the foam: insulation thickness divided by the material's k-factor, which is traced to testing under ASTM C177 or C518 at an average material temperature of 40°F. That is a real calculation and it is not dishonest. It is just a calculation about a piece of foam, not a measurement of the thing bolted to your house. ## So what should you actually decide on? Three things, in this order, if the garage is attached to the house: 1. **Is there a room above the garage?** If yes, insulation in the door is worth real money in comfort, because the garage ceiling is that room's floor. This is the single best reason to pay for it. 2. **Is the garage heated, or do you work in it?** If yes, ask for a tested U-factor rather than a section R-value, and compare doors on that. 3. **Is the garage unheated and are the walls uninsulated?** Then a high R-value door is buying you a quieter, stiffer, more dent-resistant door and very little thermal benefit, because the heat is leaving through the walls and the ceiling. That is still a legitimate reason to buy it. It is a different reason from the one on the sticker. ## Three side effects of a well-insulated door ### It is heavier An insulated steel door weighs materially more than the uninsulated one it replaces, and the springs have to be sized for the new weight. DASMA lists applying a spring to a door heavier than the manufacturer's specification, including weight added after installation, as a cause of premature failure and a manual-operation safety issue. Source: DASMA Technical Data Sheet [#190, Factors Affecting Spring Cycle Life](https://www.dasma.com/wp-content/uploads/2021/12/TDS190.pdf) (PDF). Checked 31 August 2026. ### It bows more in the sun Insulated bonded-core panels bow when there is a temperature difference between the two skins, because the warmer skin grows relative to the cooler one. DASMA calls this thermal bowing, states it "is not considered a product defect", and lists the influences: the size of the temperature difference, door size, a darker colour, a thermally broken design, the skin material, and sun exposure. Without reinforcement, DASMA notes the thermal bow on a 20-foot-wide door "could exceed several inches". Source: DASMA Technical Data Sheet [#185, Thermal Bowing of Garage Doors with Bonded Core Sections](https://www.dasma.com/wp-content/uploads/2021/12/TDS185.pdf) (PDF). Checked 31 August 2026. ### It can make condensation more visible A better-sealed, better-insulated garage holds humidity, and humidity meets a cold surface. DASMA describes garage door condensation as characteristic of "a warm, humid garage and a cooler garage door surface due to a colder exterior temperature", names a wet or snow-covered car and an adjacent laundry area as sources, and notes that "even double or triple pane glazing units may experience condensation in humid garages". Source: DASMA Technical Data Sheet [#195, Garage Doors and Condensation](https://www.dasma.com/wp-content/uploads/2022/03/TDS195.pdf) (PDF). Checked 31 August 2026. ## The Canadian bit: what the code says about the foam The insulation inside a door is a code matter and the Canadian rule differs from the American one. The National Building Code of Canada requires the foam plastic itself to be rated for flame spread and smoke development by testing to CAN/ULC-S102, with a flame-spread rating not exceeding 200 and a smoke-developed classification not exceeding 300. Unlike the US model codes, the NBCC has no prescriptive requirement on the material or thickness of the covering over the foam. Source: DASMA Technical Data Sheet [#159, Garage Doors and Foam Plastics: Fire Protection — Canadian Applications](https://www.dasma.com/wp-content/uploads/2024/03/TDS159-.pdf) (PDF), citing 2020 NBCC 3.1.13.2.(2) and 9.10.17.1.(2). Checked 31 August 2026. The same sheet carries a note worth repeating: sectional garage doors are not intended for installation in a fire-rated wall. ## How to compare two quotes honestly - Ask whether the number quoted is a **calculated section R-value** or a **tested assembly U-factor**: . If nobody can tell you, it is the R-value. - Compare like with like. Two R-values from two manufacturers are comparable to each other. An R-value and a U-factor are not. - Ask what the door **weighs**: , and confirm the quote includes springs rated for that weight. - If the door faces south and you want it dark, ask what reinforcement is included. ## Sources 1. DASMA Technical Data Sheet #163, *U-factor and R-value for Residential and Commercial Garage Doors*. [dasma.com](https://www.dasma.com/wp-content/uploads/2022/09/TDS163..pdf) (PDF). Checked 31 August 2026. 2. DASMA Technical Data Sheet #185, *Thermal Bowing of Garage Doors with Bonded Core Sections*. [dasma.com](https://www.dasma.com/wp-content/uploads/2021/12/TDS185.pdf) (PDF). Checked 31 August 2026. 3. DASMA Technical Data Sheet #195, *Garage Doors and Condensation*. [dasma.com](https://www.dasma.com/wp-content/uploads/2022/03/TDS195.pdf) (PDF). Checked 31 August 2026. 4. DASMA Technical Data Sheet #159, *Garage Doors and Foam Plastics: Fire Protection — Canadian Applications*. [dasma.com](https://www.dasma.com/wp-content/uploads/2024/03/TDS159-.pdf) (PDF). Checked 31 August 2026. 5. DASMA Technical Data Sheet #190, *Factors Affecting Spring Cycle Life*. [dasma.com](https://www.dasma.com/wp-content/uploads/2021/12/TDS190.pdf) (PDF). Checked 31 August 2026. ### Getting quotes on a new door? Ask each company whether their number is a calculated R-value or a tested U-factor. It is a fair question and it tells you a lot. [Call (905) 536-5369](tel:+19055365369) ### On this page - What each number measures - Why the section number flatters - What to decide on instead - Weight, bowing, condensation - The Canadian foam rule [New door installation](https://www.garagedoorman.ca/services/new-garage-door-installation/) ## Related - [New door installation](https://www.garagedoorman.ca/services/new-garage-door-installation/) - [Ontario winter failures](https://www.garagedoorman.ca/guides/garage-door-winter-problems-ontario/) - [Weather seals](https://www.garagedoorman.ca/services/garage-door-weather-seal/) - [Spring cycle life](https://www.garagedoorman.ca/guides/how-long-do-garage-door-springs-last/) - [All guides](https://www.garagedoorman.ca/guides/) --- Garage Doorman repairs and installs residential garage doors from Grimsby, Ontario. (905) 536-5369. Full site: https://www.garagedoorman.ca/