I'm considering building my own insulated windows using two sheets of polycarbonate. Does anyone know if polycarb yellows with age? Or any other bad characteristics of it?

Polycarbonate will not yellow or become brittle if the manufacturer used an appropriate UV inhibitor in the fabrication of the material.  However if the polycarbonate doesn't have a UV inhibitor then it will yellow and become brittle in as little as a year or so depending on UV exposure. 

Polycarbonate will scratch much easier than glass - even polycarbonate with a scratch resistant coating will scratch.

A few other things that you might want to consider before building your own IG units:

The primary advantage of a modern IG (dual or triple pane) window is the ability to include LowE coating and argon gas fill in the unit.  A dual pane IG made with clear glass and air infill has very similar energy performance numbers to a single pane with storm window.  It is the add ons that increase the energy performance.

A sheet of 1/8" polycarbonate is about 10% less energy efficient than a single sheet of 1/8" glass.  A sheet of 1/8" polycarbonate is about half as good a sound blocker as 1/8" glass, if that is an issue at all.  You would need 1/4" polycarbonate to block the same level of sound energy as you would with 1/8" glass.

Additionally, IG units are manufactured with a desiccant between the lites to absorb any excess moisture that may be present in the airspace from the time the unit was manufactured.  These IG's are also manufactured using sealant systems that keep moisture out of the space between the lites.  If moisture is present between the lites, then the units will have condensation issues when the air temperature in the space goes below the dew point.  Adding the desiccant dries up the air in the space significantly lowering the dew point temperature.

I am curious why you are thinking polycarbonate and not glass? 



 

I'm considering polycarb because it's stronger. Perhaps the way to go would be to put a layer of polycarb between two layers of glass that has appropriate coatings. I could even setup a valve for filling with argon if needed. And probably still come out with much lower cost than commercial units.

Not sure about making the middle layer from polycarbonate.

If you're thinking of argon gas filling, that means you'll need to seal the two outer and inner panes of glass together -- this is most precisely called a sealed insulating glass unit. It's commonly know as an 'IG'. If you aren't going to build an IG, there is absolutely no point in trying to fill w/ argon.

In addition to the considerations identified by Oberon i would like add a few more. You can make IG with glass because the moisture in air won't diffuse through the glass. If it could, the moisture would eventually overload the dessicant, creating condensation between the glass - long before the sealant fails. I'm told that polycarbonate is not very resistant to moisture diffusion. So if you'd like to make the middle pane polycarbonate, then it should float in the middle of an otherwise double glazed IG -- just like the decorative beveled glass in a front door.

Another consideration is that polycarbonate expands and contracts significantly more than glass - you need to leave room for it to expand; side to side and upwards (the bottom should rest on rubber setting blocks)

If you're concerned about breakage from the exterior, consider a double glazed glass IG, with an exterior ploycarbonate storm window. This quite common in northern Canadian communities that have no roads to the outside world and where kids throw snowballs and stones.


As an aside, i understand your sentiment about glass. At first glance it seems like a terrible material for windows; relatively heavy, breakable and expensive. But compared to polymer alternatives it is relatively dimensionally stable (doesn't exapnd and contract with changes in temperature), UV stable, scratch resistant, available with a variety of amazing tints & coatings and when all is said and done not that expensive.

If you're on a 'Homesteaders' budget and willing to be flexible on sizes, you can get good deals on 'overstock' from window companies. Overstock is a window company's face saving word for mistakes. Even if you don't count your time, buying overstock is usually less expensive than making your own. Whether it's just the IG or a complete window, most window companies can give you the deal of a lifetime on Overstock. The catch (and there's always at least one catch) is don't expect to find the exact size you need. Also, don't expect to find more than one of any given size.


Stephen Thwaites
Thermotech Fiberglass Fenestration

Any new opinions on this topic? Thanks

Oberon,
I'm curious from where you get your estimate on the relative efficiency of polycarb vs glass. From what I've gathered, poly is 75% less thermally conductive than glass (.22 W/mK vs 1.1 W/mK). Vermonter, any success yet?