Once again, this post is Sharon's fault. She posted a bit a couple days ago about facing a future with highly erratic fuel costs- making your choice of how to heat your home pretty significant. The subject came up because apparently, worry warts are already eagerly worrying about the unpredictable price of next winter's fuel.
In the comments, the idea of putting bubble wrap on your windows as a way to decrease heat loss came up- and it turns out Sharon's family already does this.
I may have a better trick; at any rate it's certainly a different one, useful in other situations.
We cover our windows- very tightly- with "space blankets" during extreme weather- either hot (blanket outside) or cold (blanket inside). (Our definition of extreme- colder than -10°F/-23°C, or hotter than 100°F/38°C.)
We use the very inexpensive ones inside; the ones that are just a piece of mylar with aluminum vapor deposited on one side- those can run as cheap as $2.50 for a 4 foot x 8 foot piece. The ones with a more complex construction and tough fiber reinforcement run around $12, but if it's going to be subject to wind, you'll need it.
The outdoor ones come with grommets installed- you can position them over any window that is letting sunlight into the house (shiny side out!), and drastically cut your heat gain.
The indoor ones are fragile, and tear easily- handling them is a job for an adult, or you'll run through them quickly. Well handled though, one blanket can easily last 5 years. A torn piece can be pretty easily mended with any kind of tape, even Scotch.
What we do is put 1" long pieces of self- adhesive velcro tape on the inside of the window frame. Clean the frame well beforehand; finishing with isopropyl alcohol; this is permanent. We usually put the hook part on the window frame, and the pile on the mylar (less abrasive in storage.)
Then, put the mating pieces to the velcro on top of those in place on the window frames- and expose the adhesive. You need to determine which side of the blanket has the aluminum on it- for two reasons. That's the side you want inside, for best reflection; and - the velcro tape will not stick to the aluminized side- it'll just strip the aluminum off, and you'll have to do it over. (I would know.)
Have a good idea of how your piece is going to fit. (If it needs to be cut, we usually try to cut it after this next step; more efficient; leaving bigger pieces for other windows.) Start in any convenient corner, and press a corner of the blanket onto the sticky tape; leaving a good margin of blanket to cover the window thoroughly, including cracks.
Press it in hard; hold your thumb on it for 30 seconds or so to heat it a little. Then; gently stretch the blanket to the next corner, and repeat. You want the blanket to be stretched quite taught when you are done. The velcro is somewhat forgiving- you don't have to match pile and hooks exactly, so you can increase the stretch when you are re-mounting the blanket on the window. Don't pull it too tight during the initial fitting, or you can pull the bottom velcro off the window frame.
Leave the blanket in place for 24 hours if possible, to allow the adhesive plenty of time to really set up. When removing the blanket, you need to work your fingers in between each pair of velcro tabs- just trying to pull on the blanket to get a tab to let loose is guaranteed to tear the blanket eventually. Find the junction between the hook and the pile pieces with your finger nail, then wiggle your finger between- no stress on any of the components.
This REALLY cuts the heat loss from windows- when we put them up in winter, it feels like standing in front of a stove when it goes up. When the blanket is reasonably taught, it will act to cut convection and conduction losses through the glass, acting as another storm window. It doesn't have to be perfect to make a big difference. If you have a very drafty window, you can increase the effectiveness of the blanket by adding velcro tabs in between the corners, holding it down tighter.
And, of course, it cuts radiative loss by around 80%. You can easily make that 90% if you want to- by using a double layer of mylar.
Another huge benefit- when the space blankets go up- your winter house becomes much brighter inside. It also makes the house feel smaller- which can be cozy in a cold snap. I should point out- here in the Little House, our windows do not have curtains or blinds. We want to see the world- and there's no one to peek in. Doing this on windows with accoutrements is liable to be a little more tricky; but still doable.
We take the blankets down when it's sunny- to let the sun and heat in; and to help prevent cabin fever. It's easy to release the velcro; fold the blanket and put it into a 1 gallon ziplock bag; and keep it next to its window.
For summer- label each bag, and store away with the winter hats and gloves.
I think this is very likely to be more effective than the bubble wrap, which cuts no radiative loss to speak of, and varying amounts of convective and conductive.
Of course- if you use BOTH - you could probably cut your fuel needs drastically!
The other problem with bubble wrap of course, is that 13,341 children were murdered by their parents last year for chronic bubble wrap popping. They just can't help it, it seems. The little kids love to jump on it, and the big kids love to pop it between their fingers. Parents sometimes just snap.