Has anyone ever heard of or used a product/system like this?
Bi-Glass Systems

I am a window restoration specialist of 22 years experience. BiGlass once approached me with a franchise offer, around 1995. Don't know about their current technology but at that point they were routing out a portion of the glazing rabbet into the interior moulding. Then they hollowed out the jamb edge of the sash to receive the mechanical balance. The structural integrity of the sash was so degraded that the insulated glass was glued in (with adhesive caulk) to make the sash usable. Also the IG (insulated glass) units used were so thin as to be ineffective.
First off, even the best, factory made, IG units must be replaced at least every 50 years. If your windows are ninety years old and had been originally made with this wonderful technology you would be getting ready for your second retread. Since your custom shaped glass is not being made in a quality assembly line situation you can likely count on about 15 years.
Next, the sash weight balance system relies on gravity, not springs and strings. Tape balances can be more durable but they cost around $100 a window. The best way to make your weight pockets not leak is to caulk all the air intrusion points, around casings and at jamb seams.
The reason more windows aren't restored is that few tradesmen have the experience and wherewithal to attend to all the details. Biglass is a system of gizmos that allow vinyl to take up the slack that the installers don't have time or skills to deal with.
One of the first rules of restoration is to first do no harm; Make no irreversible alterations. Once Biglass has had their way with your windows you can pretty much commit them to the trash bin 20 to 50 years down the road.
Check out interior or exterior storms and try and find someone who can make your windows work as they were meant to.

Hello there. I restore windows as well and after looking at the BiGlass site, all I can think of is how much work for such a short-lived result.

Why not simply fix the windows to work as they were meant to? If you really, really need better glass (for sound or to insulate against the giant disco ball next door) replace the existing glass with laminated.

I spent a summer restoring a handful of windows, just back to regular glazing. I found that any shortcut I tried ended up taking more time to fix the side effects. I think one reason that restorations aren't done much is not just skill, but time which equals money. However, I also think that there are a lot of false assumptions in the decision making.

Old wood sashes were made with a much higher quality wood than available today. I've heard that modern, fast-growth wood sashes are only expected to last about 25 years. Also, people mistakenly think that the problems with old windows being cold is the single glazing, and that double glazed windows will just beat anything you can do with an old sash. The leading problem with old windows is probably infiltration, and new windows fix that in the process. However, you can fix old window infiltration by reglazing to fix leaks around the glass, weatherstripping around the sashes, such as with spring metal for durability, and sealing the weight pockets. I've chosen tape springs so I can completely seal the pockets with insulation, but you can also use PVC pipe channels for the weights, with chains instead of rope to avoid wear and breakage, then seal around the pipes. By the way, fiberglass insulation does nothing for infiltration without an intact air barrier.

After solving infiltration, you can then get to the double glazing with storms. An outside storm has the benefit of protecting wood sashes from exterior weather. They also allow a much bigger air space, which is really where the insulation is, not the second layer of glass, which is just there to keep the warmed, sandwiched air from leaking away. Thin double glazed panes don't have enough air to be much of an insulator. The extra air space can make up for leakage compared to a sealed pane. I think I read a old triple-track in good condition matches a double pane with a 1/2" air gap. If you already have an exterior storm window, you can paint the frame to make a huge upgrade in look. And if you chose a contrasting color, it makes a nice accent that seems to indicate you intended it to be there and part of the building. If you are buying new storms, you can get low-E coatings for even better performance (heat reflection and UV).

Finally, restoring is much better environmentally, rather than trashing the old stuff to put in manufactured new stuff.

---mike...
p.s. for the window restorers: Have you tried steaming for old paint removal? I was able cut through all dozen or so layers of original paint in one pass with no chemicals or destruction of the sash. The steam keeps the temperature low enough to avoid fumes or fires, and it softens glazing compound, too. I used a $35 Conair steamer from Target on a small project. I made some comments and a short video at:

http://forums.finehomebuilding.com/brea ... aint-steam

I've done a lot of my own windows and also help teach a window restoration class once a year locally. I've tried pretty much every technique except for the steam method. I've wanted to build a steam cabinet similar to the one on Olde Window Restorer http://www.oldewindowrestorer.com/steamcabinet.html but haven't gotten around to it. Maybe this summer when I tackle a couple more windows.

Diane