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Energy Efficiency

Understanding visual transmittance in windows

The amount of light you let into your home is a matter of personal taste. When replacing your windows, understanding the VT will help you get it right.

Blog entry > Understanding visual transmittance in windows

Visible transmittance (VT - expressed as a number from 0 to 1) also known as Visible light transmission (VLT - expressed as a percentage %) is a measurement of the amount of light in the visible portion of the spectrum that passes through glass. The higher the number, the greater the amount of light that is passing through the glass. VT is the typical measurement used for windows.

Examples:

  • VT .5 would be VLT 50% of the light is getting through
  • VT .98 would be VLT 98% of the light is getting through

Although maximizing the amount of light entering through a window is often desired, especially in colder climates, there is often an undesirable solar heat gain.

High-performance windows with low-E coatings can reject the solar heat gain while allowing relatively high amounts of visible light to pass through the glass, however, the more solar gain being filtered out, the less (minimal) the transmittance. Tints can also be added to glass to decrease light transmittance.

Examples:

  • Double-glazed Arg-filled IGU -  VT .80
  • Double-glazed Arg-filled green tinted IGU - VT .68
  • Double-glazed Arg-filled grey tinted IGU - VT .58
  • Double-glazed Arg/Kry-filled IGU - VT .52
  • Double-glazed Arg/Kry-filled bronze tinted IGU - VT .48
  • Triple-glazed Arg/Kry-filled grey tinted IGU -  VT .34
  • A brick wall - VL 0

The amount of solar gain can be adjusted to allow more or less light entering the home. The type of low-E coating that is appropriate for your specific house depends on the orientation of the window area, and shading strategies that are being used.

Examples:

  • A north-facing window in Ottawa gets very little direct light, so you would want to make sure you had a very high VT number for the glass you would be ordering
  • A south-facing window would get direct sunlight all day long, it might be too bright for the eyes and you might want a lower VT

Need help finding understanding visual transmittance, leave the guessing out of it, talk to your local window specialist. They’ll help you find the perfect window, that will maximize your in-home comfort