If you have a way to insulate a window at night, and on overcast days, but leave it free to let in the sun when it is shining, your windows will collect way more heat, than they lose, and won’t threaten to freeze you out on cold nights. If you are going to be there, to put an insulating shutter (usually made of foam insulation) over the glass, you can do it by hand. If you will be away, and need it to operate automatically, you can make a shorter shutter that seals well along the bottom and sides, but has about a 2″ opening across the top. This shorter shutter, would need to be flat black on the window side (or at least a dark color), so that it will absorb sunlight and heat up. It will then warm the air, which will rise into the house. This is the simplest, least efficient way to make window insulation into an automatic Solar collectors, but it is still far more efficient than a plain window, in terms of its twenty four hour per day efficiency.
Note: All foam insulations, just like a lot of other materials in our living spaces, can burn, and give off deadly smoke. The safest foam insulation for making insulating shutters, seems to be isocyanurate, which will not support flame, and also happens to have the greatest insulation value per inch. It burns much like wood, and will not melt and drip like polystyrene. There are a few advantages to taping the edges of isocyanurate with aluminum tape. It increases the durability, strength, fire resistance, and also the length of time that it retains a higher insulation value, than its official rating. Just as with any other material, use at your own risk. There are rigid insulations, which are not plastic foams, but, they don’t have as high insulation values.
You could also use an insulating curtain type material, instead of a rigid foam material. The important factors are that it seal tightly on the sides and bottom, and if you want it to act as a Solar collector, it needs to be black (or dark) on the window side. As with the rigid shutters, insulating cloth window coverings can attach to the window, with Velcro, or magnetic tape. Shutters can also simply hold in place, with a tight fit. If it is made with a cloth, it can be made to seal along all four sides. Then, you can just pull the top lose, when you are not going to be around, to take it down, or put it up. That way, it can do that warm air rising thing.. and when the sun goes down, the cold air at the glass, will just pool behind the insulation, instead of falling into the room. That will only work well, if your window is not leaky.
I have foam shutters on some of my windows. They are simply cut to fit tightly. These are on westerly facing windows. On sunny afternoons, I remove them, to let the sunlight shine in. East and west facing windows don’t get sunlight for as many hours per day as south facing windows, especially in the heart of the winter, but most of the heating season (and the cooling season) they gain a lot of heat. Insulating shutters are actually most effective on windows that have the most loss, compared to their gain. So, the north is the first exposure you would want to insulate, then the west, because the window insulation can also help you control summer overheating. Then the east facing windows, and lastly the south facing ones.
This simple “direct gain” of sunlight just shining through windows, with night insulation, is the most efficient kind of Solar heating we can have, but it requires tending, to change the windows to perform ideally, for the time of day, and year. Windows are extremely effective collectors, but they also have a lot of heat loss. An all glass walled house, would work very well, with insulating shutters, that have one side shiny foil, and one side dark. That would provide a choice of three options, to perform in even more ways, that would do well for all situations, on all exposures, all year. However, it would keep you busy, like sailing a sailboat. With vents, (with back-flow dampers) the shutters , could serve other functions and work automatically, for even better, more hands-off performance.