What insulation actually does on a garage door
An insulated door reduces heat transfer through the panels, but on most garages the bigger win is air sealing. A warped single-skin door with a hardened seal loses far more heat through gaps than through the steel itself.
Three construction types
Doors fall into three broad groups, and the difference is structural as well as thermal.
- Single-skin: one layer of steel, no insulation, lightest and loudest, easiest to dent
- Double-layer: steel with polystyrene backing, moderate insulation, quieter
- Sandwich (triple-layer): steel-insulation-steel, the highest R-values, stiffest and quietest
When insulation is clearly worth it
Attached garages, heated garages, garages with living space above, and any garage used as a workshop. In those cases insulation pays back in comfort, noise and usability.
When it matters less
A detached, unheated garage used only for parking. Here insulation mainly buys quieter operation and better dent resistance, which may still be worth it, but not for heating reasons.
Don't forget the weight
Insulated doors are heavier. They need correctly sized springs and, on wide doors, an opener with enough capacity. Fitting a heavier door on the old springs is a false economy that shows up as an early spring or opener failure.