R-value

R-value measures a material's resistance to heat flow, and it is the standard number used to compare insulation products, including garage door insulation kits and wall or ceiling insulation. A higher R-value means the material does a better job of slowing heat from moving through it, whether that heat is trying to escape in winter or get in during summer.

Garage doors are one of the biggest sources of temperature swing in an attached garage, since a large uninsulated metal door has very little R-value on its own and transfers outside temperature almost directly into the space. Adding an insulation kit rated for the door raises that R-value and can noticeably reduce how quickly the garage heats up or cools down compared to the rest of the house.

R-value is additive when materials are layered, but only if there are no gaps, since even small air leaks around panels, gaps at the door track, or an uninsulated service door undermine the benefit of high R-value insulation elsewhere. A common mistake is insulating the door but leaving obvious drafts around the frame or an adjacent entry door unaddressed.