Is it possible to construct “home-made” thermal windows (double pan) so condensation can be avoided? I work in stained glass and want to make an energy efficient window. — JAA, York, PA
Yes, you should be able to make your own thermal windows. The value of having two vertical panes of glass that are separated by a narrow gap is that heat has trouble flowing across gap. While air is a poor conductor of heat, it carries heat reasonably well via convection. But with only a narrow gap of air between two vertical glass panes, convection doesn’t work well. Air heated by its contact with the warmer pane tends to flow directly upward, rather than toward the cooler pane. Similarly, air cooled by its contact with the cooler pane tends to flow directly downward, rather than toward the warmer pane.
But as you’ve anticipated, you may have trouble with condensation on the inside surface of the cooler pan. Your best bet at avoiding this problem is to completely seal the space between the two panes and to fill it with very dry air or even bottled nitrogen gas—which can be obtained cheaply from a local gas supply company. You’d have to blow the dry air or nitrogen in through one hole and allow the trapped air to flow out through another hole. After the trapped air has been replaced several times with dry gas and you’re sure there is little moisture left between the panes, you can stop replacing the air and seal both holes. But with stained glass, you have many potential gaps through which moisture can enter the trapped air, so achieving a seal could be very difficult. In that case, you might just put a desiccant at one edge of the window. Drierite is an inexpensive material that resembles little white pebbles and that can absorb quite a bit of moisture. If you put some Drierite between the two panes before you did your best to seal the space between them, I would expect the Drierite to remove enough moisture from the trapped air to avoid condensation problems. After a few years, enough moisture may have leaked in through cracks to cause trouble, in which case you would simply replace the Drierite. One useful type of Drierite is blue when fresh and turns pink when it has absorbed its fill of moisture.