This is a really nice DIY insulating shutter design from Bruce.
The Astrofoil, which is similar to Reflectex, but somewhat stiffer, has reflective surfaces on both sides that face air spaces for a high overall R value.
I've use Astrofoil stuffed in windows at night for 20 years, and also made interior "thermal shutters" with Astrofoil inserts in maple frames for my earlier solar hot air - rocks under the slab, radiant heated house. It works very well. Astrofoil is a bit more rigid than Reflectex, and if the window piece is cut about 3/4" larger than the window opening, it will stay in place and no support is needed. The window frame holds it a bit off of the glass so there's another trapped air space as well.
Before using Astrofoil in my solar hot air heated home, I could lose 10 degrees overnight in the living room, which had a whole wall of south glass, plus an east facing patio door (glass). With double Astrofoil (and folding Astrofoil/ maple thermal shutters for the patio door) I would lose 3 degrees on a cold winter night. Remember that this loss is really huge as there is 2' of rock storage under the living room floor slab, and 3/4-1" thick Portland cement plaster on galvanized lath wall and ceiling which all results in a lot of thermal mass. The attic had R-70 blown fiberglass.
The thermal shutter for the sliding glass door to patio.
Same shutter in the closed position.
Astrofoil in maple frames insulating window shutter.
Ice on the inside of the window in the morning is common, for the direct Astrofoil inserts and sub zero weather. For some applications, the ice and condensation might cause maintenance problems if the window surround is just latex painted drywall.
The patio door folding insulating panels and shutters don't cause icing and sweating as there is enough leakage and convection loss to keep the glass and aluminum frame temperature warmer. I used inserts in the bedroom windows, as well as the shutters in sub zero weather. Once the shutters were opened for the day, and the inserts removed, the windows de-ice and dry up.
I also do the no-frame Astrofoil in my new house windows in the winter; when the sun goes down, the Astrofoil goes up (just a single layer). It also works well to block heat in the summer from west windows.
Astrofoil thermal shutter without a frame -- just cut a bit oversize and pushed into place.