Garage Door Insulation and R-Value: What the Numbers Mean in Winnipeg

R-value is thermal resistance: the higher the number, the more slowly heat passes through. On a garage door the published R-value is a calculation for the centre of one section, done by the industry method in DASMA's technical sheet 163. It adds up the insulation's thickness divided by its conductivity, plus a small allowance for the air films on each face. The steel skins contribute almost nothing.

What the calculation leaves out is everything that makes a door a door: the joints between sections, the end stiles, the hinges, the perimeter seals, and the air that leaks around all of them. DASMA says plainly that the section R-value should not be used to describe a whole door assembly. The number that does describe the whole door is the U-factor, which comes from testing a complete installed door. Lower is better there, and it is not simply one divided by the R-value, however often that gets claimed.

In practice, most residential quotes still lead with an R-value, though DASMA has been pushing the industry toward published U-factors and some manufacturers now list both. It's still useful for comparing doors against each other, as long as you treat it as a comparison of panels rather than a promise about your garage.

It depends on what's on the other side of the door.

A detached, unheated garage that just keeps the car out of the snow doesn't need much. A non-insulated steel door works, and anything up to about R-6 is plenty. Insulation here buys you a quieter, stiffer door more than a warmer one.

An attached garage shares a wall, and often a ceiling, with the house. The manufacturers' usual guideline is at least R-10, and we'd steer most people to R-12 or higher if the garage is where you come and go all winter. A warmer garage means a warmer back hall, less frost on the inside of the door, and a car that starts easier.

A heated garage, a workshop, or a gym is a room you're paying to heat. R-16 or better is what manufacturers suggest for a heated workshop, and it's what we'd put on a heated space. Doors go up to about R-18 to R-20. One honest note: the step from R-12 to R-16 does less than the numbers suggest, because heat flow is the inverse of R-value and the curve flattens quickly. Past R-12, the money is often better spent on sealing.

Polystyrene is rigid foam board cut to fit between the two steel skins. It's cheaper, and it insulates reasonably well, but it leaves small gaps at the edges and the section joints, and it doesn't bond to the steel. Manufacturers put it at roughly half the insulating value of polyurethane at the same thickness.

Polyurethane is injected as a liquid, expands to fill the section completely, and bonds to both skins. That gives a higher R-value in the same thickness and a much stiffer panel, which is why the stiffest and quietest steel doors are polyurethane. Every door we install above about R-12 is polyurethane. The polystyrene doors we stock are for the unheated detached garage, where the extra R-points don't buy anything.

The installation page lists the brands and insulation options we quote.

Because R-value doesn't measure air. DASMA's thermal verification program says in so many words that air infiltration isn't covered by the ratings. A door with a high R-value and a hardened bottom seal, worn side weatherstripping, or no thermal break in the section joints will let in more cold than a lower-rated door that seals properly.

Three things to look for on a quote: a bottom seal rated for cold so it stays flexible at minus 25 or colder, side and top weatherstripping that actually touches the door, and a thermal break, meaning a non-metal separation between the outside and inside skins at the section joints and edges. Without a thermal break, the steel carries cold straight through around the insulation.

If your current door already has decent insulation and the garage is still cold, the weatherstripping guide and the bottom seal guide are the cheaper fixes to try first.

Ask the same four questions of each. What's the insulation type and thickness? What's the published R-value, and is it the DASMA calculated figure? Is there a tested U-factor for the model? And what's the thermal break and seal package? Two doors with the same R-value can behave very differently on the last point.

Then look at the rest of the door: the gauge of the steel, the warranty on the skins and the hardware, and whether the quote includes new perimeter seals and a strut on the top section for the opener. Our installation page covers what's in ours.