garage door

So I'm checking out the polystyrene sheathing at Lowe's to insulate my double garage door. The door is metal and has a usable space between the frames of 1 1/4" (this allows for the space taken up by the embossing on the door panels).

The sheathing has this chart on it that gives a total R value, considering sheathing thickness, the foil on the sheating, and the air gap.

I was surprised to see that the air gap contributed what it did to the total R value. For example, a 1/2" sheet of this insulation has an R value of 2.0, the foil facing contributes another .17, and an air gap of 3/4" adds 2.8......for a total R value of nearly 5.0. So I'm more than doubling the R value of 1/2" insulation with an air gap....

The 3/4" sheathing had a value of 3.0, with the foil and air gap adding .17 and 2.8, respectively.....to total at nearly 6.0.

With my 1 1/4" space, the 3/4" is a no-go if I want that air gap. But I can just fit the 1/2" with a 3/4" air gap. I'm thinking I can just use some strips of the sheathing to make mounting strips.

Question....by what factor is an R value of 6 better than a 3? Does it literally double the effectiveness?

At about ten bucks a sheet, either one seems reasonable. I'm just trying to figure the value of the air gap in layman's terms.

jtb, You are on the right path. Yes, R-6 is better than R-3 and you will feel it in the garage. If you have the space, go with the air gap. Check out this thread that discussed this topic in depth. It also has great ideas on how to get it to fit in the panels.

I am planning the same install very soon using 3/4" and 3/4" air gap. Since my door panels are only 21" tall, I figure on using the scrap as spacers. I did a pre-install check and had my door at 112 degrees. Can't wait to see and feel the results after its done.

BTW, I did notice my door was filthy inside and had to scrub the dust off to get to a clean surface. This should help the glue hold in the heat.

Good Luck with your project.

Since the AC is up and running, it's on to the garage door. I went with the 3/4" polystyrene......about ten bucks a sheet at Lowes.

To achieve the desired air gap between the door and the insulation, I cut some small strips of the material and glued it inside the panel on the door:


.......nice snug fit between the supports with the desired air gap: