What R-value actually measures
R-value is resistance to heat flow. A single-layer steel door is effectively zero. A polystyrene-core door lands in the low teens. A polyurethane-core door, where the foam is injected and bonds the steel skins together, reaches the high teens and beyond.
It matters most on attached garages
If your garage shares a wall or a ceiling with living space, or if a bedroom sits above it, an insulated door changes how hard your heating system works. That is where the payback is.
Detached garages and workshops
If you use the space, insulation is the difference between a workshop you can stand to be in during February and one you cannot. If it only stores a lawnmower, the case is weaker.
The quiet benefit
Polyurethane-core doors are noticeably quieter because the foam bonds the panel skins and kills the drum effect. People often notice this before they notice the temperature.
Durability
A bonded foam core makes the section far more rigid, so the door resists denting and flexing better than a hollow single-layer door of the same gauge.
What to check before you buy
Ask for the R-value of the whole door, not just the panel center. Ask about the weather seals along the sides and top, because a well-insulated door with poor perimeter seals leaks heat anyway. And make sure the springs are re-sized for the heavier door.
When to hand it over
If any of this points at springs, cables or a door that has gone out of balance, that is the point to stop. Supreme Garage Doors Repair Services covers garage door installation and every other garage door service across New Hampshire, with same-day slots most days and a line answered 24 hours at (888) 904-6236.