Did you know that your windows could be responsible for a whopping 25 percent of your home's heating costs? During the fall and winter months, ordinary glass can allow the stored thermal energy in your home to escape, leading to colder indoor temperatures and more energy wasted on keeping your home warm. Low emissivity or "Low-E" glass is designed to tackle this heat loss, helping your home retain more heat as outdoor temperatures fall.
How Low Emissivity Leads to High Efficiency
Ordinary double and triple pane windows have gas-filled pockets between the windows that act as a barrier against heat loss. However, these windows do little to control the amount of radiant infrared energy that passes through. Uncoated glass is highly emissive of this infrared energy by nature, offering an emissivity as high as 0.95. During the winter, this means that the bulk of the radiant infrared energy in your home escapes to the outdoors while only a small portion is reflected back indoors.
The idea behind Low-E glass involves lowering the emissivity of the glass, allowing it to reflect more of the infrared energy while allowing less to pass through. During the summer, Low-E glass keeps your home cool by reflecting the vast majority of the sun's radiant infrared energy away from your home. During the winter, Low-E glass helps your home retain its heat by reflecting radiant infrared energy that would otherwise escape through your windows.
The key lies in the unique coating that's embedded within the window. This special metallic coating is designed to be microscopically thin enough to let visible light through while reflecting radiant infrared energy. Think of Low-E glass as a thermos for your home -- the coating keeps heat in during the winter and keeps it out during the summer.
Choosing Between Hard Coat and Soft Coat
Low-E glass is typically manufactured with a passive or solar control coat, also known as a "hard" or "soft" coat, respectively. Both coatings are added at different points of the manufacturing process and each one has its own unique set of performance properties:
- A passive Low-E glass coating is created using the pyrolytic process. A tin dioxide coating is applied to the glass ribbon as it moves along the production line. The coating bonds to the hot glass surface as it cools, creating a durable "hard coat" that gives the glass a slight bluish tint.
- A solar control Low-E glass coating is applied after the glass is cut and cooled down. The manufacturer applies several thin layers of silver onto the glass sheet using a Magnetron Sputter Vacuum Deposition (MSVD) coater in a vacuum environment. Afterwards, another glass sheet is placed on top of the coated sheet, trapping the coated layer in between both panes of glass.
Passive Low-E glass coatings tend to be ideal for colder climates, as these coatings allow a small amount of the sun's radiant infrared energy to pass through while keeping the majority of your home's heat inside. These coatings also tend to be more durable than their solar control counterparts. In comparison, solar control Low-E glass coatings are focused on blocking as much radiant infrared energy as possible, making these coatings a better fit for mild to hot climates where cooling is more of a priority.
Low Emissivity Comes at a Higher Cost
As expected, Low-E glass tends to cost a bit more than ordinary glass. For instance, replacing your existing double pane windows with Low-E replacements could cost an average of $40 to $55 per square foot. In addition, it can take 20 to 30 years to fully recoup the purchase and installation costs, making Low-E windows a genuinely long-term investment.
Nevertheless, the high initial investment can be offset each year by a steady reduction in your home's heating and cooling bills. This is especially beneficial if you're in a colder climate and rely on expensive fuel oil or electricity for heat. For more information, see a website such as http://www.newmanroof.com.