A sheet of steel with no insulation. Steel conducts heat well, so it passes heat straight through between the garage and the outside. Cheap and light.
Rigid foam board glued between two steel skins. The foam does the insulating work, but heat can bypass it where steel meets steel at the panel edges.
Liquid polyurethane is injected between the steel skins and expands to fill the space. Because it bonds to both skins, it leaves fewer gaps than glued board.
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. Illustrative arithmetic, with our assumptions: a 16×7 door is 112 ft², a 38°F difference, and single steel treated as R-1. That gives 112 × 1.0 × 38 = 4,256 BTU/hr. At R-18 it is 112 × (1/18) × 38 = about 236 BTU/hr.
The rubber or vinyl strip along the bottom edge. It has to seal against an often-uneven floor. Types include T-style (slots into a retainer), bulb seal (compression), and threshold seals (a raised bar the door presses against). Clopay says most seals need replacing every two to three years, depending on climate and use; replace one when it is cracked, brittle, or you can see daylight under the door.
PVC or vinyl strips fixed 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. Replace them when gaps appear or the material loses its spring-back.
Each horizontal joint between panels is a potential leak path. Insulated doors usually have a tongue-and-groove or vinyl compression joint between sections; a single-steel door may have none.
Wind pushes air toward the windward side and pulls it from the leeward side, so air moves through every gap. This is why a leaky garage can feel drafty in a storm even with the door closed.
HomeAdvisor puts an insulation upgrade at $600 to $3,000 depending on the insulation type and door size. Compare your own quote with the modelled savings above.
If your garage shares a wall with heated or cooled living space, the garage temperature can affect your HVAC load through that wall. A garage that follows the outdoor temperature makes that wall work harder.