Garage-door section construction — single, double, and triple layer R-values

What this diagram shows

Three garage-door section cuts compared to show why same-size doors have very different R-values and weights — the core material changes, not the thickness. A single-layer steel door is one bare skin with a hollow interior: no useful R-value and the lightest at about 1.5 pounds per square foot, suited to detached or unheated garages but dent-prone. A double-layer door sandwiches polystyrene bead-board between a steel skin and a vinyl backer for R-6 to R-9 at about 2.0 pounds per square foot — the most common mid-market door. A triple-layer door injects dense polyurethane foam between two steel skins for R-12 to R-20 at about 2.4 pounds per square foot, the attached-garage standard: quietest and strongest but heaviest, which is why it needs bigger springs and a stronger opener. Published R-values are face-of-section values and real installed performance runs about 30 to 40 percent lower from perimeter air leakage.