Homeowners are increasingly interested in building materials that are energy-efficient and reduce heating costs over the long-term. All sorts of things are going green nowadays – from renewable resource-based bamboo wood floors to low-flow toilets and smart heating systems, it’s easy to increase your home’s efficiency and save money as well as the environment. There are many ways in which you can modify your house for this purpose, and Low-E windows are an extremely popular type of replacement for standard glass panes. How do these work?
The “E” in Low-E windows stands for “Emissivity.” Emissivity refers to the ability of a material to absorb incoming energy and emit it via radiation. All materials will either absorb and emit or directly reflect energy. A mirror, for instance, reflects most light energy and only absorbs a little, giving a shiny reflective surface we can look into. Concrete, on the other hand, will absorb most of the thermal energy that hits it, and emit it as heat; this is why it will feel warm to the touch on a summer day.
Sunlight contains a mixture of visible light, ultraviolet rays, and infrared light in the form of heat; window glass is naturally highly emissive, and it lets all three of these wavelengths through into your home. While it’s great to let visible light into your home, the other two are not as helpful; they will interact with surfaces in your home like fabric, wood, tile, and furniture, many of which will absorb and emit the rays. Ultraviolet light causes fabrics to fade and skin to burn; infrared heat will be emitted by your surfaces and increase the heat in your house – like when the sun-drenched tile floor of your kitchen becomes painfully hot to walk on in bare feet.
Low-E windows are coated with a metal that reflects UV and infrared light, while still allowing visible light to pass through. There are two types of Low-e glass: hard coat and soft coat. This refers to the type of material used in coating the glass. In hard coat, a layer of tin is applied directly to the molten glass during production, and it is very tough to scratch it off. Soft coat windows are coated with silver or zinc, applied in a vacuum and sandwiched between another pane of glass to protect the coating. It is also sometimes infused with argon gas to prevent the metals from oxidizing if exposed to air.
While Low-E windows may cost more, they are effective in reducing energy costs for your home; by reflecting ultraviolet and infrared rays away, they prevent overheating in the summer (reducing air conditioning use) and they also help the house retain its heat in the winter by preventing it from escaping through the windows. It’s estimated that they can filter out 40-70% of the heat normally transmitted through window glass, and reduce energy loss by about 40%. However, there are some strange disadvantages to Low-E windows, which we’ll discuss next week.