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How Energy-Efficient Are Your Windows?

Even though most new windows include labels of their energy ratings, it is almost impossible to know what the energy performance of a current window is. This article will give you pointers on identifying the energy efficiency of existing windows, what components and materials are involved, and how they are rated in terms of your home's regional climate.

Contact Your Window Company

If the energy rating sticker is not visually apparent on your current window, it is usually hidden in between the sash and sill. You will be able to find either the energy ratings of the window or the model number. If you can only find the model number you can contact your manufacturer, give them the number, and they will walk you through how energy-efficient your current window is.

Qualities of Energy Efficiency 

  • Double/triple glazing: Window glazing involves having more than one layer of glass separated by a spacer. The space in between is filled with gasses (i.e. argon or krypton) with densities lower than that of air to trap and deter a certain amount of heat gain. Double-glazed windows insulate better than standard windows by up to 50% while triple-glazed windows improve insulation by roughly 75%. Improving home insulation reduces your energy bills since there would be a lesser need to use fans and heaters.
  • Polymer interlayer: Impact-resistant windows use an interlayer of a plastic polymer sandwiched in between two layers of glass to withstand the impact of a 2-by-4 traveling at fifty feet per second. This polymer can block up to 100% of UV radiation and up the energy efficiency of your home.
  • Low-E coating: Low-E stands for low-emissivity which is an invisible coating that acts as a heat barrier.
  • Airtight fit: A good airtight window is sealed well in the interface between the wall and window frame, uses foam or caulk to seal any gaps, and uses weatherstripping in the area where two sashes interlock.
  • The window frame material: Wood is the best natural insulator when compared to vinyl, fiberglass, clad-wood, and aluminum. Each can be improved with the techniques mentioned earlier but, as far as standard windows go, listing them in descending order of energy-efficiency would have wood in first place, clad-wood in second, fiberglass in third, vinyl in fourth, and aluminum in last place.

Quantities of Energy Efficiency

Windows are rated with several quantities, the most important of which are the U-factor and the solar heat gain coefficient (SHGC) which gauge how well your windows insulate within region-specific climates.

  • The U-factor is used to measure the amount of heat loss in the winter. The lower the U-factor, the more heat is trapped by your windows in the winter, which is why windows sold in the northern U.S. regions have lower U-factors than the southern regions.
  • The SHGC measures the amount of heat from the sun is allowed to transfer through the glass. The higher this number, the more heat gain is transferred into your home. The southern U.S. regions will have windows with low SHGC ratings to protect the home against excessive heat.

The table below shows close U-factor and SHGC approximations of wood, wood-clad, vinyl, and aluminum window frames. The values depend primarily on the material's heat retention properties and the number of the window's glass panes.

 Frame Type Value Single Glass Pane Double-Glazed Triple-Glazed
 Aluminum U-factor 1.20 0.80 0.60
SHGC 0.80 0.70 0.60
 Wood, wood-clad, and vinyl U-factor 0.95 0.55 0.35
SHGC 0.80 0.70 0.60

The NFRC and Energy Star Ratings

The National Fenestration Rating Council (NFRC) is a non-profit organization that regulates standards of energy performance architectural products which include home windows. Energy Star-rated windows meet those standards and indicate the regions for which the final window product is built to perform best with an Energy Star sticker. 

Below you can see how Energy Star-rated windows differ in their U-factor and SHGC quantities and how they suit different regional climates.

 
 Climate Zone U-factor SHGC
 Northern regions ≤0.30-0.32 0.35-0.40≥
 North-Central regions ≤0.32 ≤0.40
 South-Central regions ≤0.35 ≤0.30
 Southern regions ≤0.60 ≤0.27

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