Your metal garage door is an enemy of your budget. In the summer, the sun beats down and heats it up, adding heat to your garage. In the winter the metal gets cold, which is a sure sign the heat inside the garage is being lost. In short, insulating your metal garage door is a really good idea. I'd tell you how much you can save by doing so, but I really have no idea. I just know that it is sufficiently non-trivial to justify the manufacture of any number of garage door insulation kits, as any search on Amazon or Google will quickly reveal. There are several instructables that address installation of these kits, which generally run fifty bucks and up.

https://www.instructables.com/id/Garage-Door-Insul...

and

https://www.instructables.com/id/Make-your-garage-...

Other Instructables discuss using construction materials like hardboard isulation to "roll your own"

https://www.instructables.com/id/3-Steps-Most-EFFE...

and

https://www.instructables.com/id/Insulating-A-Gara...

These Instructables are all great solutions, particularly if your garage doubles as your shop, like mine does. However, what if I told you you could insulate your garage door AND divert material from the landfill AND it wouldn't cost you a dime? Win-win-win, right?

Supplies

All you need are sheet styrofoam, cardboard., and a way to cut them.

Step 1: Gathering Materials

I got the idea for this through several related events.

First, I did a little idle on-line shopping shopping for an insulation kit. I could see many of them were just styrofoam and cardboard, and I knew that mountains of styrofoam and cardboard were going into the dump every day. Styrofoam is notoriously hard to recycle, and many recyclers won't even take it.

At the same time, Covid-19 hit, and the wife and I were both suddenly working from home, sitting around the same dining table to work. I apparently fidget too much (among other things), so we suddenly needed to make her a dedicated work space away from me. There was unfortunately not a year or so available for me to make her a desk, so I bought a modular unit from a big box office supply. Guess what I found in the package, besides a do-it-yourself desk? Styrofoam and cardboard! Not nearly enough, though.

That sent me to Craig's List, to see if anyone was offering free styrofoam and cardboard. That takes us to the real hero of this story: James. James works at a place assembling and delivering furniture, and like me he thought that surely there was a better use for all the carboard and foam they had to dispose of every week--so much so that he posted a Craig's List add asking if anyone needed any foam or cardboard for art projects and things like that. I got in touch, and he hooked me up.

Incidentally, furniture-related styrofoam is far better for this than electronics-related styrofoam, which is molded to accommodate the weird shapes of the electronics. In the furniture line, styrofoam packing is often just rectangular sheets, which is perfect for our purposes. You can make electronics packaging work, but plan on doing lots more work as you cut it and fill all those gaps

Yay, James! You rock!

Step 2: The Process...

Once you have the materials, the process is pretty simple. Each panel of a metal garage door is like an elongate boxy "C" shape in profile, with a low (about 1 inch) lip at the top and and the bottom. I think the upper one is slightly longer on my door, but I'm not sure that's always true. In any case, the internal cavity is a couple of inches longer than the opening. With a properly-sized piece of foam, you can slide it up inside the upper overhang, then flex it and press it in so it settles into the lower cavity, and it will stay in place as the door is raised and lowered. See the middle diagram.

I suppose I could have stopped there, but styrofoam is pretty fragile stuff and I am a certified klutz--it was only a matter of time before I ran something into it and gouged out a huge hunk of foam. For this reason i covered over the styrofoam with a sheet of cardboard cut to fit the opening of each panel. Installation worked the same way, except that with the foam I could line many pieces up side by side, while I tried to make each panel cover a single piece. I almost succeeded there, and packing tape handled the rest.

Step 3: Precision Counts...

The trick to this is cutting your materials with enough precision that they can slip in the opening with a little flexing, but are still wide enough to stay securely in place. In pursuit of this goal, I made a simple foam cutter like the one described in this instructable:

https://www.instructables.com/id/How-to-Make-Elect...

...except using a 9-volt battery and a guitar E-string (the high E). It worked, but the wire would cool and the rate of cutting would slow as i made long cuts through the foam (I assume the foam was absorbing the heat from the wire, but that's a guess). More ominously, the battery got pretty hot, and I discovered that I could work faster with a simple utility knife.

The cardboard was a different story. I could have used the knife for that, too, but the blade tended to wander away from the straight-edge as I uncomfortably leaned over long (40-inch plus) cuts (klutz, remember?). I discovered that I could use my tablesaw to slice through corrugated cardboard like butter, and I could even free-hand it because the danger of being injured by kickback is almost nonexistent (DISCLAIMER: I AM NOT ENDORSING THIS. TRY IT AT YOUR OWN RISK. HECK THE SAME GOES FOR THE UTILITY KNIFE AND PRETTY MUCH THE WHOLE INSTRUCTABLE.)

My one final caution is that you should measure each opening, as I found mine were not uniform: the aperture in the lower two panels was 20 in high, while the upper were 18 inches. Similarly the width of the outer panels was 46.25 in, while the inner ones were 43.5 inches. As a result, I needed panels with four different dimensions to finish the door.

I worked at this a few hours at a time, and had to go back to James for a little more foam and cardboard, but in about 3 pretty easy days it was done. My door cost nothing, but if the pockets on yours are configured differently, I would suggest investing in a roll of aluminum duct tape to make sure the panels stay put.