Nov 15 2007
Windows are important to the efficiency of our homes and often account for a significant portion of our heating and cooling bills. New technologies are beginning to address common issues, such as solar heat gain and insulation, so that we can expect more from our windows in the future.
What is a SHGC?
Windows allow sunlight into our homes. Sunlight is composed of short and long wavelengths. The short wavelengths represent the visible light that makes our homes bright and sunny. The long wavelengths produce the heat we feel from that sunlight. This heat is also known as solar radiation.
The amount of solar radiation transmitted through a window is represented by the Solar Heat Gain Coefficient (SHGC), a number between 0 and 1. A window with a high SHGC allows a lot of solar radiation into your home. The heat provided can be welcome in colder climates, but provides unnecessary heat in warmer climates. A window with a low SHGC blocks solar radiation, which reduces the need for air-conditioning in summer but is not ideal for cold climates.
The amount of insulation a window provides is also important and is represented by a number called the U-value. The higher the U-value, the less insulation the window provides. Conversely, the lower the U-value, the better the window’s insulating properties; typically a U-value of .35 or lower is sufficient.
Thermal energy easily travels through a window with a high U-value, leading to excessive heat gain in the summer and excessive heat loss in the winter. This means that the air-conditioning and heating systems will have to work extra hard, driving up your energy use and utility bills.
The least efficient type of window is the single-paned window. Generally, they are a thing of the past because they offer little insulation or protection from the sun. Their high SHGC means that ample amounts of sun are able to pass through the window and heat the home’s interior. This seems like a positive thing for colder climates, however, the single pane also means a higher U-value (minimal insulation). Therefore, heat generated to warm your home can easily escape through a single-paned window.
Commonly used today are double-paned windows with a special metal coating (low-E, E stands for emissivity). The double-pane increases these windows’ insulating abilities and the low-E coating helps block long-wave infrared radiation (heat) from entering the home while still permitting visible light. The drawback to the low-E coating is that in heating climates, it can prevent some passive (solar) heating in the winter.
There have also been improvements made on insulating windows. Replacing the air in-between the glass panes with gasses, such as Argon and Krypton slows the passing of heat through the window better than air. The performance of these windows is not entirely known. One concern is that leakage can occur, but it is unsure at what rate and how much it affects the efficiency of the window. To find out more about the use of argon and krypton, click here.
According to Popular Mechanics, the future will bring a new kind of window that optimizes its SHGC for all climates. They are called transition metal switchable windows and are good for any climate, especially climates that have four distinct seasons.
A small electrical current travels through the glass triggering a light sensor. As the current passes through the glass, it increases the reflectivity, preventing solar heat from entering the house. When the current is halted, the window becomes more transparent, which allows more light and heat to pass through. Since the current is triggered by light sensors and regulated by controls, it can automatically react to changing seasons and light conditions.
The upshot is that when it is hot and sunny outside, the glass automatically turns reflective to keep heat out of the house. When the temperature or light conditions drop, the glass automatically turns transparent to allow more heat and light into the house. This technology makes the glass optimal for any climate.
The role our windows play in the overall efficiency of our homes is important no matter what climate you live in. I hope this gives you a better understanding of what makes a window efficient and that you are as excited as I am to see the next generation of windows in your new home!
To find more on what makes a window energy efficient visit Energy Star.