Just a sheet of steel — zero insulation. Acts as a direct thermal bridge between the garage interior and outside air. Steel is an excellent conductor (R-value of ~0.003 per inch). Cheap, light, and common on budget doors. Your garage temperature essentially matches outdoor temperature within a couple hours. Common on builder-grade homes.
Rigid foam board (EPS or XPS) glued between two steel skins. The foam does the insulating work, but the steel-to-steel edges at each panel create thermal bridges that bypass the insulation. The glue bond can fail over time, creating air gaps that reduce effective R-value. Decent mid-range option. R-value depends on foam thickness (typically 1.5" to 2").
Liquid polyurethane is injected between the steel skins and expands to fill every void — no air gaps, no thermal bridges at panel edges. The foam bonds chemically to both steel surfaces, adding structural rigidity (the door is stiffer and quieter). R-value per inch is ~6.5 vs ~4 for polystyrene. This is the premium option and the best thermal performance available in standard residential doors.
Q = U × A × ΔT, where Q is heat loss (BTU/hr), U is the U-factor (1/R-value), A is door area (ft²), and ΔT is the temperature difference. A typical 16×7 door is 112 ft². At R-0 with a 38°F ΔT, that's 112 × 1.0 × 38 = 4,256 BTU/hr escaping through the door alone. At R-18, it drops to 112 × 0.056 × 38 = 237 BTU/hr — a 94% reduction.
The rubber or vinyl strip along the bottom edge. This is the #1 leak point — it must seal against an often-uneven concrete floor. Types include T-style (slots into a retainer), bulb seal (compression), and threshold seals (a raised bar the door presses against). Replace when cracked, brittle, or daylight is visible under the door. Check annually.
PVC or vinyl strips nailed or screwed to the door jamb. They compress against the door face when closed. The top seal bridges the gap between the top panel and the header. These wear out faster on the hinge side because the door flexes during operation. Replace when gaps appear or the material loses its spring-back.
Each horizontal joint between panels is a potential leak path. Insulated doors typically have tongue-and-groove or vinyl compression joints between sections. Single-steel doors often have no inter-panel seal at all — just overlapping steel. Adding foam tape to panel joints on an uninsulated door can reduce infiltration by 15-20%.
Wind creates positive pressure on the windward side and negative pressure (suction) on the leeward side. A 20 mph wind creates ~1.6 psf of pressure. On a 16×7 door, that's 180 lbs pushing air through every gap. Infiltration rates can triple in windy conditions. This is why sealed garages feel drafty during storms even with the door closed.
An insulated door upgrade (R-0 → R-18) typically costs $800-1,200 more than a basic door. At estimated annual savings, the insulation pays for itself in approximately 3-4 years — then saves money every year after. In extreme climates (very hot or very cold), payback can be under 2 years.
If your garage shares a wall with heated/cooled living space (attached garage), the garage temperature directly impacts your HVAC load through that shared wall. An uninsulated garage door in winter can drop garage temps to near-outdoor levels, making the shared wall a massive heat sink. The door-to-house connection is often the single largest uninsulated surface in the building envelope.