How Stuff works has breakdown of the R value of adding bublewrap
Build It Solar also has a breakdown of the cost savings. Plus many other wonderful tips that are worth trying.
I bought a small amount of bubblewrap and used up what I had on hand from packages I have received. This project cost me about $16 total. I'm sure if you ask around you can do it for zero dollars just by finding stores willing to give you left over bubble wrap.
All you need are bubble wrap, some scissors, a ruler and a spray bottle of water.
*important note - water - NOT FREBREEZE. I didn't have an empty spray bottle and I figured, why not, the Frebreeze is right here. I had to redo that window because it didnt' stick as well. Water is cheaper anyway, but I'm impatient and didn't want to wait. The kid had an empty spray bottle so I was back in business in a couple of hours. Didn't have to use extra wrap, that was still fine - it just came loose as the frebreeze dried.
Measure the window and then cut the bubble wrap to size.
Spray the window with water
press the bubble wrap to the glass, bubble side down.
easy peasy - you're done.
On my North facing windows I added a second layer. Same method, just put it on top of the first layer.
Lots of great looking photos on line - but I'll use the ones I took of my actual windows. Got a nice blurred effect from the diffused light coming through the wrap.
In the second photo I played around with the expose just to show the bubbles in the bubblewrap.
This was about a million times easier than putting up plastic.
One drawback is that I can't look out and see the birds, but I'll trade that for lower bills this winter.
A HUGE plus is my dog. One of my dogs is terrified of bubblewrap. He's also the one most likely to jump up on the door. Now the can't stay far enough away. My door frame might stay clean this winter!
So there you have it - an easy, fast way to insulate the windows. One of those things from Pinterest that actually does work the way people say it does. (Sometimes I think people on Pinterest just make things up so I'll look stupid when I try them)