Garage door R-value, explained

R-value is a measure of how well a material resists heat flowing through it. The higher the number, the better it insulates.

How it applies to garage doors

Insulated garage doors are made like a sandwich: two steel skins with insulation between them. The R-value you see in a brochure usually describes that panel section. Our steel insulated garage door, for example, is published at R-18 with a polyurethane core bonded to steel on both sides.

Polyurethane vs. polystyrene

• Polyurethane is injected as a liquid, expands to fill the panel, and bonds to both skins. It typically gives a higher R-value for the same thickness and makes the panel stiffer.
• Polystyrene is a rigid foam board placed inside the panel. It’s lighter and less expensive, and usually rates lower.

What else matters

A door’s real-world performance also depends on things a panel R-value doesn’t capture:

• Weather seals at the bottom, sides and top of the door.
• Windows — glass insulates far less than a foam-filled panel.
• Gaps between sections and around the frame.
• Whether the garage walls and ceiling are insulated too.

Do you need a high R-value?

If the garage is heated or cooled, shares a wall with living space, has a room above it, or is used as a workshop or gym, insulation makes a noticeable difference in comfort and noise. For a detached, unheated storage building it matters much less.

A full-view glass door will always insulate much less than an insulated steel door — choose glass for light and looks, and steel for comfort.