ayawi,
There are no codes or laws that prohibit you from using either polycarbonate (Lexan) or acrylic (Plexiglas) or other plastics in your windows. Many people have done so in the past and many will do so in the future.
There are some inherent problems with using plastics in windows, however; including significant loss of visibility due to scratching, lowered energy performance when compared with glass, and significantly increased sound propagation thru the material when compared with glass about twice as loud as glass at the same thickness. If the plastics were not been manufactured using a UV inhibitor (some are and some aren't), then they will eventually turn yellow and become brittle in just a few years.
Many people don't realize that the dual pane window was originally developed in order to match the energy performance of a single pane window / storm combination in a single, rather than two-piece, construction - i.e. to achieve the same level of performance in the window unit alone without needing the storm window as well.
So to say that you can match the energy performance of a dual pane window by restoring the original and then adding a storm window is really backwards. It is really more correct to say that a basic dual pane window should be able to match the performance of a well constructed and maintained single pane window / storm window combination.
Ultimately, I would suggest that the real comparison should be the performance of the original window before and after the restoration - how much does the restoration improve window energy performance? Adding plastic in place of glass is not a good trade-off from an energy performance standpoint.
And as was pointed out in a previous post, the plastics will not match glass in clarity, color, or tint“ you will notice the difference. Obviously, if the glass is missing and it was replaced by cardboard, then the windows probably are a very significant factor in energy usage and any change would be an improvement.
Your desire, as stated, is to attempt to restore your original windows to optimum performance condition. Any home (or any other structure), is ultimately a complete working system that performs best when all components of the system work in harmony with one another. One could easily argue that harmony with the home environment is an aesthetic consideration as well as an energy performance one, and one could also argue that keeping as much of the original system intact as possible is in harmony with the what the home is.
Heck, if someone wants to achieve the maximum possible energy efficiency, and damn the aesthetics, then close off every window in the home with layers of insulation and that will achieve maximum energy performance - and it will be much less expensive than either replacement or restoration - and it is much simpler too.
A very extreme approach of course, but one that will work if a person is willing to box themselves in, and I strongly suspect that just about everyone reading this has seen examples of the "less windows / smaller windows" idea in some new homes (especially those built during the 1970's energy performance era) and in older homes that were modernized at some time in the past - certainly less extreme than my example, but they out there none-the-less.
A couple of considerations:
First, when the "restoring versus replacing" argument comes around to the energy performance differences between the restoration window and the replacement window, they will eventually be lost in the rhetoric of the aesthetic reasons for restoring versus replacement. And these are often very valid reasons by themselves.
Second, because no matter what the numbers show about individual unit performance, ultimately the individual window is still part of the overall structure of the home and its performance as part of that overall structure is what is important - not necessarily how well does it perform in the lab environment.
I happen to really like classic older windows. I believe that they are often so much a part of the charm of an older home that it is worth every possible effort to try to save them. And while I very much understand the appeal to some folks of replacing older windows for the potential improvements that replacements will bring in energy consumption, comfort, and the potential ease of maintenance; I also understand the desire of many people to try to do everything possible to save the original windows in a classic home.
There are a lot of misconceptions about old windows versus new windows and many opinions often based on feelings and not on facts that come up in discussions like this one.
I would offer a few for consideration:
FACT – if a homeowner was to opt for top-quality custom replacement windows, he or she could get them made in any style that they wanted in wood, fiberglass, aluminum, steel, and yep, even that old villain – vinyl. There are available styles that would fit their home and that would be virtually indistinguishable from the original windows in the home – and, these windows could last every bit as long as the home. But note that I said "top-quality custom" - potentially spelled $$$$$$$$.
FACT – a low end replacement window doesn't come close to meeting the same sort of standards as does a top-quality product – from the materials used, to the glass used, to the hardware used – it could be like comparing a Yugo to a Ferrari (and whatever happened to Yugo BTW?) – and from a performance standpoint a good restoration will almost always be a much better investment of time, trouble, and money than changing to a junk replacement.
The guy in the Sunday paper who promises to replace every window in a home at "$99.95" with his "super-dooper-highest-quality" vinyl or aluminum window likely has neither the highest quality window nor does he have the homeowner's long term interest at heart. He needs to get in and to get out – and what happens in five or 10 years? That's no longer his problem.
A very common comment from folks who favor restoration over replacement is that restoring a single-pane window and storm window combination will result in energy performance numbers that are comparable to any new window on the market regardless of any glass coatings or gas used with the new window.
Fact“ the newest dual or triple pane windows made with LowE2 or LowE3 coated glass and argon or krypton fill are substantially more energy efficient than old windows single or double pane. A triple pane window with LowE2 coating on surfaces 2 and 5 combined with krypton gas infill and a low conductance spacer system can achieve energy performance numbers as much as 10 times better than an original single pane window and as much as six times better than a single pane with storm (or a low-end dual pane window as well).
R-values aside, since they measure only conductive thermal transfer, and any discussion of windows has to include radiant performance (60% plus of total window performance), the primary advantage of multi-pane windows over dual panes is the ability of a multi-pane window to include LowE coatings. The coatings improve both conductive and radiant performance of the window over clear glass.
If one wants to discuss R-values in window systems, then the current leader in the field has a measured R-value of 20. A good triple pane window with LowE2 coatings on glass surfaces 2 and 5 and inert gas infill can achieve an R-10. A dual pane with a single LowE on surface 2 and gas infill can achieve an R-6. But, again, that is only a measure of conductive performance and it is not the full story.
Comment – old windows may have lasted 50, 75, 100, or more years and new windows will always fail in short order only lasting 5 or 10 years.
Fact – quality products will outlast non-quality products no matter when produced. For all of the older windows that are on homes today and that deserve the chance to be around even longer, there are tens-of-millions of windows that were made in the last few centuries that didn't make it into today's world for a wide variety of reasons – including simple window failure – often as a result of poor or non-existent maintenance. The ones that have made it thru are often the best that were produced and they likely had a good bit of maintenance at some point. These are all very good things that make the idea of keeping them around even longer worth serious consideration.
Ultimately, older windows have lasted as long as they have because they are of simple, uncomplicated construction that has had the distinct advantage of using a material (old growth lumber) that is no longer widely available. The pyramids will easily outlast a modern skyscraper – that doesn't make a pyramid inherently superior than the skyscraper - it is simply different. Old growth lumber can be worth preserving if only for itself.
The better, newer, dual and triple pane window systems now on the market and installed all over Europe and North America should easily reach 50 years life expectancy - many will go well beyond that time. There are vinyl windows in Europe that were installed over 50 years ago that are performing flawlessly today.
Fact is many old houses have their original windows and some of these windows (if they have had adequate maintenance for their lifetime) are in very good shape. And some are also in very poor shape, most are somewhere in between. But, they have all made it this far and if they are worth restoring, then by all means that should be considered as a very definite option. Again, most (but not all) older windows are made with old growth lumber which is better than almost any lumber readily available today.