Thursday, March 25, 2010

What are the Odds?

Being that I work for a company owned by Andersen Corporation, maker of Andersen Windows, I've heard through the grapevine about windows concentrating and reflecting sunlight onto neighboring houses and in many cases causing damage. (Sorry, I don't have any inside knowledge or access to damaging "internal memos" or any other smoking guns that a class-action attorney would sell out his best hunting dog for. If any lawyers are reading this, go subpoena sumebody else.) Plastics News had a article on it recently, but this has been going on for a few years.

What is thought to occur is that with a sealed, double-paned window, pressure differences between the sealed interior and the exterior can give the window glass a concave geometry of sorts, which can then serve to concentrate sunlight. Low-E glass is particularly troublesome in this regards. The reflected and concentrated sunlight can then heat and soften vinyl siding on neighboring houses.

I'd love to run a Monte Carlo simulation on this sometime to see how it plays out. Or maybe a derivation of the Drake equation would be better. Think about what has to work out just right for this to occur. First the right pressure differential has to be there, not an easy set up to control. External atmospheric pressure is constantly changing as is the pressure inside the window due to changes in temperature. Then once you have the concavity, you have to have sunlight. And then the 3-D geometry of the sun, window and neighboring house has to be just right. And then the focal length of the lens has to match (not exactly, but close enough) the distance to the siding. My gut feeling is that this would be a "one-in-a-million" type problem, comparable to winning the lottery, but apparently not.

1 comment:

Phil Ball said...

My neighboor has a house with Low E glass, one of his windows (a small dormer window in the roof) reflects light onto his own garage wall. His vinyl siding was warped and damaged within a couple of years of the house being made. So yeah - this happens. The house next door has a window which reflects onto my lawn in the evenings. If I walk through the beam while mowing the grass I feel the heat instantly like stepping into a death ray. Luckily that beam is only there in the evenings and moves very quickly.

You are right about the cause, slight curvature + very reflective glass (low E coatings reflect the infra red heat energy).

This is probably common in the south where glass is used to stop solar gain. In the north the coatings are used to keep heat in and might not work as effectively