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Replacement Window Terms

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Many clients have communicated to us over the years that they were surprised at how much there is to know in deciding on which replacement window and door to choose. Many customers feel overwhelmed and are shocked when they start this process on what windows cost, what details and specifications to choose from, what kind of replacement window to buy, and on and on….

Ultra Windows is here to help! We want to help you navigate all the terms and guide you to the best value in replacement windows. Use the below glossary to understand some of the common terms used in this process and contact us if you have any questions!

  1. Solar Heat Gain Coefficient – The Solar Heat Gain Coefficient (SHGC) is a number assigned to a window that tells you how much heat that window lets pass into your home from the sun. SHGC numbers range from 0 to 1, and the lower the number, the less heat will enter your home. Thus, in hot climates a low SHGC is desirable, while in cold climates a higher SHGC is desirable. The National Fenestration Rating Council (NFRC) offers reliable SHGC ratings for replacement windows that they have certified.
  2. U-value The U-factor is a rating given to a replacement window based on how much heat loss it allows. U-factors generally range from 0.2 (very little heat loss) to 1.2 (high heat loss). The U-factor is the inverse of the R-value of a replacement window, which measures a window’s insulating value. Thus, a high R-value is the same as a low U-factor, and means that a window does not allow much heat to escape. A poorly-made replacement window cannot get a low U-factor. Single-pane windows are about 1.0 and double-panes are about 0.4. If you live in a colder climate, or find that you are always heating your home, buying windows with a low U-factor is a good way to save energy and money. The National Fenestration Rating Council (NFRC) offers reliable U-factor ratings for replacement windows that they have certified.
  3. Visible Transmittance – Visible Transmittance (VT) is a measure of how much light passes through a replacement window. VTs range from 0 (no light) to 1 (all light). VT is an important quality to consider when purchasing a replacement window because sunlight can fade furniture and carpets or damage precious art. Sunlight can also affect the ambience of a room in your home. A special coating on windows called low-E can provide even better protection against UV rays along with greater insulation. The National Fenestration Rating Council (NFRC) offers reliable VT ratings for replacement windows that they have certified.
  4. Low-E – A low-E (low-emissivity) coat is a microscopically thin glazing for replacement windows that changes the amount of heat that can pass through them. Low-E coats save money and energy by maintaining a more stable temperature inside buildings, and thus reducing the need for heating or cooling. They can also block UV rays. A high solar gain low-E coat allows more heat to pass through a replacement window, and is useful for colder climates. A low solar gain coat is appropriate for hot climates because it allows less heat to pass through a replacement window. A hard coat is longer lasting than a soft coat.
  5. Air Leakage (AL) – Heat loss and gain occur by infiltration through cracks in the replacement window assembly. It is indicated by an air leakage rating (AL) expressed as the equivalent cubic feet of air passing through a square foot of replacement window area. The lower the AL, the less air will pass through cracks in the window assembly. At this time, the AL is optional among NFRC ratings. For code compliance purposes, however, air infiltration is often tested in accordance with the North American Fenestration Standard (NAFS), which produces similar results to the NFRC air leakage rating. Select replacement windows with an AL of 0.30 or less (units are cfm/sq ft).
  6. Design Pressure Rating (DP) – The “DP” measures the amount of pressure a door or replacement window will withstand when closed and locked. Each DP rating also establishes other performance factors such as water penetration, air infiltration, structural pressure, forced entry and operational force. The higher the DP numbers, the better the performance.
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