I used fiberglass batt and reflectix to insulate the non-window panels of my garage doors, and the process was relatively quick and easy.I decided to use two layers of 3/4″ PolyIso insulation boards with aluminum facing to insulate the window opening panels. The process of cutting out two panels to fit around each window opening was labor intensive.
First, I made cardboard templates sized to fit around the windows and cut out my first layer of insulation. On the first layer, I placed the reflective layer “outwards” facing the door, as there is a small air gap between the panel and most of the steel door material due to a decorative “indent”. This first layer goes around the window frame. I would mark each piece of foam with a template, then use a carpenters square to cut straight (and mostly perpendicular) lines.
In the case of the 2nd layer, I had to freehand trace the curves with my razer knife. To keep the reflective side looking nice on the 2nd layer, I would mark and cut from the back, and then finish off the corners of the cuts from the reflective side.
The second layer fits over the window frame, so the openings are cut smaller, allowing light to enter, but covering up any opaque portions of the window or frame.This comes into play more on the front of my garage, which has decorative coverings over part of the square windows.
I faced the reflective side inward to reduce radiant heat transfer to the interior, and to match the reflectix aesthetically.
Two layers of this PolyIso 3/4″ insulation has an R value of 10, although there are some gaps between the layers, along with large openings cut out for the windows, but I prefer the natural light over thermal benefits of covering the windows entirely.
The take-away message here is if you have the choice, always pay extra to buy the insulated garage doors from the factory, as retro-fitting insulation takes time and money.