WHAT ARE THESE?
Figure 1 on the left is an example of one of the basic labels used for classifying windows. These communicate the energy efficiency of the window you're planning to buy. WHAT DO THEY MEAN? For most of these values, a low value is a very good thing. A majority of the numbers describe the ability of thermal energy to enter or leave your building; a lower value translates to lower energy costs. Read further below to decipher U-Factor, Solar Heat Gain Coefficient, Visible Transmittance, and Air Leakage ratings.
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U-FACTOR
The U-Factor is a measurement of the amount of energy that will transfer through a surface. You may have seen U-Factors before when either replacing a window or adding insulation. You will want to get the lowest U-Factor while staying in your price range. The figure to the right shows heat dispersion through a window. The red arrows are the thermal energy transferred from the warmer outside into the cooler building. The building's temperature rises due to this energy transfer. |
SOLAR HEAT GAIN COEFFICIENT
Like the U-Factor, a low value is what you want from your Solar Heat Gain Coefficient (SHGC). The SHGC describes how much solar energy will enter through the window. In areas like Houston where the sun is out for a longer time, considering this factor is vital. There are many ways for households to reduce the effects of sunlight--from solar screens to awnings, many outside fixtures can severely reduce the negative effects of windows with high SHGC. Figure 3 shows solar heat gain. The building's temperature rises due to the sunlight transfered into the building, symbolized by the yellow arrows. |
VISIBLE TRANSMITTANCE
Visible Transmittance (VT) rates how well the window acts as a window. A high VT describes a much clearer, more transparent pane of glass whereas a low VT means your window isn't very clear... and may be better as a door. Figure 4 to the right shows this clarity difference. The two different panes contrast VT values. The more opaque pane on the left corresponds to a lower VT value while the transparent pane on the right correspondsto one with a higher VT value. |
AIR LEAKAGE
Air leakage is a serious problem, affecting more than just frames. Windows open up the possibility for air to leak through connections to other panes as well as through the instillation. The government doesn't assign values over 0.1 for air leakage and a score of 0.3 or less is required to obtain an Energy Star rating. As with U-Factors and the SHGC, a lower value for air leakage will improve the energy efficiency of your building. Figure 5 to the left shows air travel through windows. The blue arrows demonstrate how air leaks through gaps between window panes. |
For an explanation of when you should consider replacing your windows, click on the button to the right to go onto the next section, Window Replacement.
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Sources:
[1] "How to Read an NFRC Sticker on Energy-Efficient Replacement Windows," [Online], November 28th 2016, Vytex Windows, website: http://vytexwindows.com/homeowners/care-and-maintenance/how_to_read_a_sticker/
[2] "Measuring Performance: Air Leakage (AL)," [Online], November 28th 2016, Efficient Windows Collaberative, website: http://www.efficientwindows.org/al.php
[1] "How to Read an NFRC Sticker on Energy-Efficient Replacement Windows," [Online], November 28th 2016, Vytex Windows, website: http://vytexwindows.com/homeowners/care-and-maintenance/how_to_read_a_sticker/
[2] "Measuring Performance: Air Leakage (AL)," [Online], November 28th 2016, Efficient Windows Collaberative, website: http://www.efficientwindows.org/al.php