Going green is one of the hottest trends in America and having energy efficient windows in your home is quickly becoming a must-have for many people. How can understanding the role your windows play help with energy efficiency, green living, lower heating costs, and airsealing? What are the energy ratings that factor into window replacement?
Not only is the “green” movement near the top of many political agendas, its presence is often seen throughout the mainstream and minor media. The bottom line is this: green sells and going green is cool.
I feel the trend is positive. With energy costs reaching new heights, we need to be proactive and intentional. We should not only reduce the amount of energy we use, but also conserve the energy we don’t. And since nearly 15-20% of all home energy is lost via windows and doors, having energy efficient windows in your home should be a priority.
I’ll help explain (without getting too technical) the current energy ratings scheduled to today’s energy efficicient windows. These ratings are crucial in making informed decisions regarding the purchase of energy efficient windows for new home construction or replacement.
With the green trend gaining momentum, an informed decision concerning one of the most critical areas of energy loss (or conservation) in your home is paramount.
I’ll begin by explaining a few terms needing definition.
INSULATED GLASS: (IG)
Two or more individual panes of glass separated by a spacer and then sealed to be air and water tight. The “captured” airspace between the panes of glass forms the insulating barrier. The majority of modern energy efficient window systems utilize some type of insulated glass.
Emissivity is the capability of a surface to emit heat radiation. A black surface is often used as a constant in measuring other surfaces against it. With energy efficient window systems, lower emissivities are generally desirable.
U-VALUE: (AND ITS RELATION TO R-VALUE)
U-Value is the measure of a window’s ability to reduce heat loss; such as during the winter months in moderating climates. Lower U-values translate into less heat lost from the interior of the home resulting in lower heating costs.
U-value is the inverse of R-value (a more common term used in the insulation business). To find a correlating R-value from a given U-value, simply divide the number 1 by the U-value. Lower U-values correlate to higher R-values. For example: 1 divided by a .50 U-value gives us an R-value of 2.00.
This may sound like a low insulating value but even the finest energy efficient windows today carry U-values hovering in the .22 through .30 ranges; thus correlating to R-values of 4.55 through 3.33.
And given the fact most exterior wall cavities are a minimum R-13, the most energy efficient windows are 3 to 4 times less efficient than the wall they’re installed in.
SOLAR HEAT GAIN COEFFICIENT: (SHGC)
Solar Heat Gain Coefficient (SHGC) is a measure of a window’s ability to reduce heat gain from direct heat (sun). A lower SHGC translates into less direct heat being pulled into the home resulting in lower cooling costs. SHGC and U-value are closely linked.
LOW EMISSIVITY: (LO-E)
Low-E refers to the ability of insulated glass to suppress direct heat radiation and absorb indirect heat radiation. By placing a Lo-E coating, which usually consists of a microscopically thin layer of metal (primarily silver), on a glass surface. The heat remains on the side of glass where it originated.
In a nutshell, Lo-E coatings reflect direct heat radiation and absorb indirect heat radiation.
Due to the sun’s differing angles at various times of the year, Lo-E coatings work well in all seasons. In summer, when the angle is more direct, or “a high sky” they reflect heat. In winter, when the sun’s angle is less direct, “a lower sky” they absorb the indirect heat.
Hopefully, this helps clarify what those silly acronyms and numbers signify on today’s energy efficient windows. No matter what climate you live in it’s always a smart thing to lower your energy costs. And becoming knowledgeable concerning energy efficient window ratings is a great start.