What type of insulation is best for a garage door?

Polyurethane insulation is the best type for garage doors, outperforming polystyrene at the same thickness by roughly double the R-value. Polyurethane is injected as a liquid foam that expands to fill every cavity inside the door panel, creating a continuous thermal barrier with no gaps. It also bonds to both steel skins, which adds rigidity and reduces noise. Polystyrene insulation comes as pre-cut foam boards inserted into the panel, which leaves small gaps at the edges and offers about half the thermal performance per inch. Reflective foil bubble-wrap kits sold for retrofits provide minimal R-value, around R-2 to R-4, and are best for non-insulated doors as a budget add-on. For a new door, choose polyurethane and look for a 2-inch panel.