Here's a quick experiment for you to try. On a cold day move your hand around the edges of your door. I'll wait while you go and do this...
So, did you find a soft cold wind around any edges or corners? If you live in an older home, or perhaps a neglected rental, then the odds are good that you will.
Guess what, that cold wind may be invisible, but it's money! It might not feel like much, but with cold air flowing into your home 24x7, you are wasting a lot of heat energy. Maybe you haven't noticed but the cost of heating oil, or other alternatives, has not been getting any cheaper lately.
Anyway, the attached picture is an example of a recent home project of my own. You might not be able to tell but I'm using a thin plastic foam instead of a clear plastic film. If you look closely you'll see (yeah, sure you will) that I've cut small slices in the foam and used painters tape to hold it in place. This is surprisingly useful.
Tip: Cut a slice in the foam then sneak half a strip of tape through. Half the tape will stick to the object behind the foam while the other half will hold the foam. Be careful not to damage the material under the foam when you cut the slices into it.
Again, looking at the picture, you'll see that the foam covers the entire width of the door which allows it to block air flow around the door. Yes, the door does still open - the foam is flexible! Can you guess what I used?
Nope, it's not that.
It's hardwood floor underlay. You can buy rolls of it at your hardware store at a reasonable price since it isn't sold as a seasonal winterization kit. The painters tape will obviously be there too.
This door was the first project and I've since covered a couple of windows as well. Obviously, the plastic foam is translucent instead of transparent, so don't use it anywhere you'll want to look out of.
Finally, I'd give you another experiment, but you probably don't have any thin foam lying around the house. If you do, put your hand on a cold window and see how much heat it absorbs from your hand. Then, put a thin layer of foam over the window and do it again.
Want to see my other winterization tips?