R-value
A measure of a garage door’s resistance to heat flow — the higher the number, the better the door insulates.
- Outer steel skin, facing the street
- Insulating foam core — this is what R-value measures
- Inner steel skin, facing the garage
- Aluminium retainer holding the seal
- Bottom seal, compressed against the floor
An uninsulated single-skin steel door has an R-value around 0 to 2. A polystyrene-insulated door reaches roughly R-6 to R-9, and a polyurethane-injected door reaches R-12 to R-18 or higher in the same thickness, because the foam is denser and bonds to both skins.
R-value matters most where the garage is attached, heated, used as a workshop, or shares a wall or ceiling with a living space. On a detached, unheated garage it changes very little except door weight and rigidity.
A higher R-value door is heavier, which means the springs must be sized for it. Fitting a heavier insulated door onto the old springs is a common shortcut that leaves the door out of balance from day one.
R-value is worth paying for in three situations and largely wasted outside them: the garage is attached and shares a wall or ceiling with a room you heat, you use it as a workshop or gym, or it houses something that suffers in temperature swings. On a detached, unheated garage the practical gains are small. What does change in every case is weight, because an insulated door is significantly heavier than a single-skin one and the springs must be sized for the new door. Reusing the old springs on a heavier replacement is a common shortcut that leaves the door out of balance from the first day.
Be careful comparing published figures, because they are not always measured the same way. Some manufacturers quote a calculated R-value derived from the insulation core alone, which ignores the steel skins and the frame and produces a flattering number for the same physical door. A whole-assembly figure is the honest one. You may also see U-factor quoted instead, which measures heat transfer rather than resistance — it is simply the inverse, so a lower U-factor is better where a higher R-value is better. When comparing two quotes, check the two doors are being described on the same basis before concluding one insulates better. DASMA, the industry association for door manufacturers, publishes guidance on how the figure should be stated, and reputable manufacturers follow it.