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When seeking ways to increase a home's energy efficiency, one suggestion is to replace existing windows with those made of low-E glass. What exactly is low-E glass and how does it improve energy efficiency?
The E in "low-E" stands for emissivity. Low-E glass reflects heat back to the source by way of a thin metallic coating either in or on the glass. The light from the sun is either visible, infrared, or UV. UV is the damaging light that affects skin and fades colors; visible light allows us to see; infrared light is a source of heat.
The glass in low-E windows blocks portions of the UV and infrared light while allowing the visible light to pass through. The infrared light heats up objects, such as floors, furniture, driveways etc. When the items cool off, they emit low powered infrared light. It is this kind of energy that low-E glass reflects.
In warm months, the heat from objects outside remains outside making the house cooler. In the cold months, objects inside the home are heated by the HVAC unit or the sun. This heat is bounced by the low-E glass back into the house.
There is a soft and a hard coated form of low-E glass. Hard coat low-E glass is made by a process of applying tin to molten glass. Soft coat low-E glass most often has a thin layer of silver applied to the surface while it is in a vacuum. Because the silver coating is delicate, soft coat low-E glass is sandwiched between other glass. Argon gas is typically used to prevent oxidation of the soft low-E glass if it is exposed to air, and it also acts as additional insulation.
In cold 0 degree weather, a single glass pane window might have an inside temperature of around 24 degrees. A double pane window might be somewhere around 33. The inside surface of hard coat low-E would be about 47 degrees, and soft low-E would be at around 60 degrees.
Since most houses generally lose around 25% of the home's heat through the windows, it is easy to see that low-E glass is worth the extra cost and it will pay for itself over time from the energy savings it offers. There are varying qualities of low-E glass, so shop wisely and get the best value for your money.