Welcome to the forum Jennifer.
Well now i have several questions. My intial inquiry was just wondering if it was possible to use a 2” board on a door bit meant for 1 3/4? But now with your other information im wondering how a basically plywood sandwich is fastened as well as how the decorative pieces would be fastened? Also wondering how the ends would be dealt with in order to keep the elements outHi Jennifer, You could most certainly do that, but the door is likely to be pretty heavy by the time you're done. So either you'll have to have a strong motorized lift or carefully set up garage door springs. Another option is to use 1x lumber for all the inside the door rather than 2x4. Use the 2x4 for the outside frame. If you are using a plywood back, that will add rigidity and strength to the door made with 1x lumber. You might have to rip the 1 by to the same width as the 2x4. Not a big deal to do.
I have a steel door that I insulated with foam, but I also put a layer of radiant barrier, which looks a little like bubble wrap between two layers of aluminum foil. It adds about R4 insulation factor, which may not seem like much. However, I put this under the roofing in my garage and we found that it cut 35 degrees of transmitted heat.
On my garage door, there's a layer of barrier, then a 2 inch layer of foam board, then a second layer of radiant barrier on the inside. Total R value is not all that high, probably R16 or so, but the radient barrier not only reflects heat away from the outside, it also reflects in back into the garage. Our dryer is out there and it produces enough incidental heat so it's never icy in there, even on the really cold days.
With a wood door, you'll get less heat transfer than my steel doors. You can find radiant barrier in 25 foot rolls at Home Depot or even Lowes. I used the 4 foot wide that just fit the steel door segments, and the 20 inch for the roof. Last hint, I used aluminized duct tape to seal the insulation into the door. Even a small gap will let heat out and cold in.
One last thing, You need to use insulation strips to close all the gaps beside, under and above the door. Without sealing those gaps, the insulation won't do you much good.
Hope this helps. Here's a picture of the radiant barrier...
View attachment 400443
I missed the Carriage Door item. That would make it much easier to seal. I assume you're garage has a concrete floor, You can purchase door seals to go under the door. They wear eventually, but are fairly easy to replace. If you mortise the hinges in, you'll get a pretty tight fit, but you can also find rubber seals that would fit inside the framing of the door. I use the kind with a solid bar and a flexible lip about an inch wide that presses against the door. If you make the doors a close fit, you can probably just use flexible insulating strips on the door's edge where they come together.Well now i have several questions. My intial inquiry was just wondering if it was possible to use a 2” board on a door bit meant for 1 3/4? But now with your other information im wondering how a basically plywood sandwich is fastened as well as how the decorative pieces would be fastened? Also wondering how the ends would be dealt with in order to keep the elements out