Everything You Should Know About Energy Star Windows
You can tell a window is energy-star rated by the sticker that is placed on the window itself. The sticker shows four different components that need to be present in order for the window to be considered energy-star rated and energy-efficient.
U-factor, solar heat gain coefficient, visible transmittance and air leakage are all important parts of an energy-star rated window. Read below to find out what each component is and how it relates to providing you with an energy-efficient window.
The first element of windows that are energy-efficient and energy-star rated is their ability to control heat loss. The window’s capability to resist heat transfer is referred to its U-factor.
Heat will transfer from warmer surfaces to cooler surfaces. An example of this would be heat moving from the inside pane of your windows to the outside in the winter and moving from the outside pane of your windows to the inside of your home in the summer.
There are three different ways heat transfers: conduction, convection and radiation.
Conduction is the transfer of heat between substances that are in direct contact with each other. The better the conductor, the more rapidly heat will be transferred.
Convection is the transfer of heat from one point to another by the movement of an element like water or air. When warm air from inside the home meets a cold window a chilly draft can occur.
Radiation is the movement of heat through infrared rays through the glass panes. The more radiation entering your home, the hotter the space will feel.
U-factor ratings generally fall between 0.25 and 1.25. The lower the U-factor rating, the greater a window’s resistance to heat transfer and the better its insulating properties are.
Solar Heat Gain Coefficient
The second major characteristic of energy-efficient and energy-star rated widows is the ability to control solar heat gain through the window panes. In simpler terms, this controls how much heat is let into your home. This capability is a considerable factor in determining how long it takes your home to be air conditioned or heated to your liking.
Solar heat originates from radiation coming from the sun and sky. It can be directly transmitted through the window pane and straight into your home. It can also be transmitted when radiation is absorbed in between the window panes and then indirectly leaked into the home.
While solar heat gain can provide free heat in the winter, it can also lead to overheating in the summer. To best balance out solar heat gain depends on the climate you live in, where your window is positioned and other factors.
The solar heat gain coefficient is expressed as a number between 0 and 1. The lower a window’s solar heat gain coefficient, the less solar heat it transmits.
Visible transmittance is the amount of light that passes through a window. A higher visible transmittance means there is more daylight coming through your window opening.
Visible transmittance is affected by the number of panes your window has and any additional glass coatings that are added. Visible transmittance of windows typically ranges from above 90% for uncoated clear glass to less than 10% for highly tinted glass.
A typical insulated double pane window has a visible transmittance of around 78%. This value decreases by adding a low-E coating and decreases quite a bit when adding a tint.
Visible transmittance ratings for the whole window are always less than glass ratings since the visible transmittance of the frame is zero. Since the frame does not transmit any light, the overall visible transmittance may be lower than expected.
While visible transmittance varies between 0 and 1, most values among double and triple pane windows are between 0.30 and 0.70. The higher the visible transmittance, the more light is transmitted. A high visible transmittance is desirable since it means your window is allowing the most daylight into your home.
Whenever there is a pressure difference between the inside and outside environment, air will flow through any cracks and gaps between the window and the frame. Air leakage can also occur between the frame and the wall.
Any unwanted air that enters your home can lead to an increased time it takes to air condition or heat your home. Air leakage could also contribute to how long it takes to cool your home in the summer by raising the humidity levels inside of your home.
A properly installed window will prevent any air from leaking in and out of your home. However, tight sealing and weather stripping of windows, sashes and frames is also important to stop any air from leaking.
Air leakage is expressed as the cubic feet of air passing through a square foot of window area per minute. The lower the air leakage, the less air will pass through cracks in the window and its frame. This rating must be .30 or less in order for the window to be considered energy-star rated.
Benefits of Energy Star Rated Windows
While there are many positives to an energy-star rated window, there is one major benefit most people look for when considering to replace their windows with new, energy-efficient ones. This benefit is the overall savings in heating and cooling bills.
The ability or lack of ability of your window to control heat loss, solar heat gain, how much air leaks and how much light is admitted into your home plays a huge part in affecting your energy savings.
Since these four components work together to provide the most energy-efficient window, they will also reduce the amount you spend on your energy bills.
Hopefully with the help of this article, it should now be easier to understand what the energy-star rated sticker means on your windows. In addition to that, you should now also have the knowledge on what these terms mean and how they play in part to make your windows more energy-efficient and overall better for your home.