SAN FRANCISCO - Energy efficient windows can help control our utility costs. They keep the heat inside when it's cold, and outside when it's hot. However, as with anything, there can be a downside.
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Robert Hart, a mechanical engineer and windows researcher at Lawrence Berkeley National Lab, studies energy efficient low-e glass windows. The windows have microscopically thin, transparent coatings, and often multiple panes.
Using heat lamps and radiometers, Robert demonstrated how low-e windows work. "They're made to reflect the solar radiation back out," he said. That reflection makes our homes more comfortable.
One homeowner we spoke to has given those windows a lot of thought. Since her family moved to Sonoma, Tish Lehane said they started having problems right away. "The first summer that we moved here. We moved in October, and then the following summer we put out our furniture and it started melting," said Tish. "It doesn't take any time. It could probably be in a couple hour's time, and then it's gone."
Tish has garbage bags full of lawn furniture cushions, the covers all melted from the sun's reflection over the years. She said the heat and bright sunlight in Sonoma, along with the low-e windows, is causing damage to her furniture and to her backyard. She pointed out plants that will have to be cut back soon before reflection from the house's windows burns them.
"Everything out here is just kind of for show," she said. "We don't get to enjoy the backyard as much as we'd like to because it feels a little dangerous." Tish and her family said they only spend time outside when under the cover of a patio umbrella.
Reflected sunlight can damage home siding, too. I didn't find that in the Bay Area, but Robert noted several examples published in a Lawrence Berkeley National Lab report he authored. In that study, Robert found clear glass will reflect about 13 percent of the sun's energy. "A typical low-e window reflects about 1.5 to 3 times that much energy," he said.
Robert says damage is pretty rare because, "there's a lot of things that have to be just right for this to occur, which is why you don't hear about it very often."
What type of things? Window curvature, time of day and distance between the window and the object among them.
Robert says he has measured temperatures up to 170 degrees, which is enough to warp plastics and harm plants.
If this is happening at your home, there is some good news. Usually, a common screen used to keep insects out of the home is enough to diffuse the light and energy.
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