The Magic of Glass- Now Double It

In my last post we described the progress of a sunny day in my addition. But what makes it work? Why is window glass so nifty as a solar collector? And what technology drives the improvement of windows as selective conductors of light, heat and air?

Simple clear glass is selectively transparent to light and heat. The diagram is not terribly technical, just illustrative. The sun's rays are mostly (except for ultraviolet) passed directly through into the room, and at least some of the infrared (radiant) energy is trapped, making clear glass a pretty good collector of solar heat.

When you double the window with two panes of glass, and add an inert gas in the space between panes, you've decreased light transmission very little (app. 10%) and decreased conductive and radiant heat loss by almost 50%. Well done, Andersen! We're getting warmer as we go.

Add selective coatings (called "low E", mostly) to filter radiant energy going in either one or both directions, and you've affected solar heat gain a little (another 10% or so) and reduced radiant losses by another 30% or so. This link may help.

It's hard to refine the glass window as a filter much beyond that, although the engineers, God bless them, will never stop trying (as long as we pay them, and even then, they have garages). As you increase the filtering of radiant energy through the glass, direct light transmission also suffers, and you get eventually to a window that puts one in mind of Paul's phrase "seeing through a glass, darkly." See our previous post on "super windows."

The strategy of using window coverings as nighttime filters for radiant and conductive energy through windows is difficult to escape. I've seen automatic pully arrangements with insulating covers, and the Somfy company gets very geeky with light sensors on motor-driven shades, but if you're an energy player, so to speak, you won't mind operating your shades/drapes/panels at sundown and sunup, turning windows into walls when they aren't working for you as solar collectors. We'll try to get back in the next few days, as work permits, to continue this discussion of windows and sunlight.