I made my own storm windows using Optix plexiglass purchased at Home Depot. I'm told that even though Optix has a 10 year non-yellowing warranty, it will probably yellow before then. A better way to go would be Lexan, but it is at least 3 times more expensive. I went with the Optix because I figured I'd be learning how to do this as I went along and I didn't want to waste any expensive Lexan. For me, this turned out to be a good decision. Even though I didn't waste any Optix, I did scratch it several times and made a small crack in one window that I was fortunately able to repair. In any event, it cost me less than $200 to do the job with the Optix and I've kept the receipts and a copy of the warranty in case it does yellow. The warranty says I'll get a refund if it does and I'll be able to then use the windows as templates to make new Lexan windows.
I'd say that making the storm windows was easy yet deceptively time consuming. I have a laser thermometer which is a great tool for lots of things. With it I've compared temps of the storm windows with windows that didn't yet have the storm windows installed. No matter what the outside temperature is, the difference is about 10 degrees (just little less than the difference between the uncovered windows and the walls). So they work very well. Interestingly, I also had one window covered with a cheap/easy plastic film storm window purchased at Walmart. It raised the window temperature by less than 2 degrees. I have no Idea why that is the case. I wouldn't have expected it, but those temporary storm windows are almost useless (though they do reduce condensation fairly well). I've also tested the Optix storm windows on a couple of warmer days we had and they definitely reduce the temperature being transmitted into the coach. I can't remember how much, but it was significant. My intention is to leave these storm windows up all year long and to take them off only for cleaning maybe once a year. I think they look good. People usually don't notice them when they visit. When I point them out, people usually only have good things to say about them. I left a couple of the smaller widows uncovered for ventilation and may end up removing one or two others during the summer. MATERIALS NEEDED: (1) 1/8 inch thick Optix plexiglass (at Home Depot this thickness only comes in a large sheet), (2) a plexiglass cutting tool, (3) #6 stainless steel pan head sheet metal screws 1/2 inches long (buy in quantity at the link below), (4) closed cell foam weather stripping to fit the channel in the window where the new storm window will fit, and (5) high quality clear silicon caulk.
Pictures, slideshows and other helpful links for this project:
Remove window valences, blinds and other coverings.
Cut plexiglass in a rectangle approximately 1 inch longer than the bottom and both sides of the window.
While holding the window securely against the top edge of the window molding channel you will be using, drill two very small holes through the molding and the plexiglass. (Tip: While drilling the plexiglass, use light force and high speed. This will prevent the plexiglass from cracking and, because of the heat produced, actually melt the plastic a little making the hole a little stronger)
Take the plexiglass down and widen those holes with another drill bit so they are a little bigger than the screws you will use to secure the window to the molding.
Place the plexiglass back up against the top of the window and secure it using two screws.
Use a water-based sharp felt-tip pen to trace the inside edge of the molding channel where the window will eventually fit (Note: do this freehand since these windows don't really have straight edges).
Remove the two screws and place the plexiglass on a flat, clean surface.
With the cutting tool, score the inside edge of the lines you made with the felt pen (Note: this will give you a window that is a little forgiving, being just a bit smaller than the space it will fit in).
Keep scoring the plexiglass over the previous scoring until you produce a very deep gouge (Note: the depth of the gouge is especially important on the rounded corners).
Once the gouge is deep enough all around, carefully break off the excess plexiglass (Note: ideally you've gouged it deep enough to use only your fingers, but in some cases you may get better results using a pair of pliers to break off the excess glass).
For a nice touch, you can carefully smooth the edges of the plexiglass using a sanding block. If you do this, be very careful not to scratch the surface area.
The storm window should now fit perfectly into the place where you wanted it to fit. Make sure that it does. If it is too big in spots, now is the time to sand down those spots or otherwise make it fit.
Use high quality clear silicon caulk to seal all seams in the inside channel of the window (i.e., the part that will be between the regular window and the storm window). If the window has vent holes, plug them with silicon. Wait 24 hours before the final mounting of the storm window.
Place the storm window back in place and secure it with the two screws used previously. Make sure the window fits perfectly.
Use the small drill and drill holes like the previous ones all around the window about 3.5 inches apart.
Remove the two screws and take down the storm window. Line the storm window channel with closed cell foam weather stripping being sure not to leave any gaps. Put the storm window back in place and secure it using the two screws making sure you don't squish the weather stripping too much.
Use the larger drill to widen each of the new holes you've created so that they will be just a little larger than the screws you will be using. Make sure you don't drill through the smaller holes in the window frame.
Fill each of these new holes with silicon caulk. Then carefully put screws in each of the holes making sure you don't squish the weather stripping too much and that the amount you squish it is about the same from hole to hole. Use a soft cloth or tissue to carefully remove the excess caulk around each screw head.
Once all the new screws are in place, remove the two original screws, fill the holes with caulk and replace the screws.
Wait at least 24 hours before removing the window again. This will give the caulk time to dry leaving a little bit of rubber between the screw and the plexiglass which will pretty much eliminate the possibility of the storm windows cracking over time due to the bouncing around they will experience as you drive down the road.
Using the smallest drill bit you have, drill a hole near the top of the window in a place that will not be noticed when the coverings are put back on. I don't know whether this hole is actually needed, but I put it there so that the pressure between the windows could equalize while travelling at different elevations.
Replace all blinds, valences and other coverings.
You can now move on to the next window. Believe me, it will go a lot faster than this one.