I have a corner room in our house, in the summer it traps heat and in the Winter (like right now) it lets it all out. In the summer my room is regularly between 3 to 10 Celsius hotter than the rest of the house, and in the Winter it is usually the reverse, hovering around a 5 degree difference. I have three windows, one single one and two that serve to make a large double window. They are those idiotic non sensicle windows that open outwards and have to be hand cranked. All of them are broken, they cannot open, unless forced open (and I mean forced) and I hate having to get a ladder and climbing up two stories to forcefully close them, so they never open, and I have given up on any airflow and just use an air-purifier year round with just makes the room feel colder in the winter. I have currently put up plastic sheets around my Windows, and since the large window is facing the street and everyone can see into my room very well, especially at night, I just covered it with a large sheet (I tore down the stupid blinds that came with the house in a fit of rage over how idiotic and stupid and half broken they were). I can't fix the rest of this house (most studs range from 14" to 17" apart because the guy who built this place was an absolute fool) so I was wondering, since I have a bunch of this left over fiber glass insulation form when I worked on our basement, and I block my windows anyways, would putting it between the plastic which if it comes off, would only be to refresh the tape, and the Window itself, somehow help insulate it further? And even if it does help, is it safe to do in the place I spend probably around 90% of my time in, even with an air filter.
Edit: I live about 30 minutes south of Boston, MA. In the summer it gets fairly hot, average of about 26c (up to 35c, that is around the highest I've personally experienced during a heat wave) and is very humid (upwards of 90%) and in the Winter it gets very cold, below freezing usually and extremely dry.
Best Answer
Insulating the inside of your windows will have at least one direct consequence: the effective R-value of the overall window will be increased. That is, less heat will radiate through the window. This will have a warming effect during the heating season (ie, when it's cold outside) and a cooling effect during the cooling season (ie, when it's warm outside). The magnitude of this effect can only be roughly estimated so you probably won't really know how much of an improvement this is without trying it.
This will result in a secondary consequence. The window itself (mainly the glass) will be closer to the outside temperature than the inside temperature.
During the heating season, this means it will be cold (how cold depends on your climate, of course; it shouldn't get colder than it is outside). If you live anywhere but a dry climate, this probably means it will be below the condensation point and water from the house will condense on the glass (then run down to the sill). Chances are, over a long enough period of time, this will result in water damage to the window and nearby wall. If it is cold enough where you live, some of this water may freeze which could result in more immediate damage from the expansion-contraction cycle.
Conversely, you will also be constructing a miniature greenhouse. When the sun shines on the window from the outside, it will heat up the air space between the inside glass and the insulation. There's no extra energy coming from the sun in this case but you're concentrating it into a smaller mass (the air in the gap). If ambient temperatures in your climate are high enough or the window is situated to get a lot of full sun, and depending on the construction materials of the window, temperature in this air space could get high enough to damage the window. This doesn't seem extremely likely but it might be possible given just the right circumstances.
Now, set thoughts of R-value aside. A common problem around windows is air sealing. That is, they make a room cold when it's cold outside not because of heat radiating through them but because of cold air blowing in through holes (or warm air blowing out). These effects can be just as important as those due to low R-value windows. Fiberglass insulation will do essentially nothing to improve this situation. Air blows right through fiberglass. The plastic sheets you put up over the window may already have dealt with this part of the problem, depending on how well you've managed to seal around the window with those.
If you really don't plan to use the window as a window, your best bet is probably to rip it out and replace it with an insulated wall. Or perhaps with some higher-quality windows and some drapes that you can be happy with.