[08/09/2021]

Visible Transmittance of the Window, or Tvw (and sometimes referred to as VLT) is an optical property and measurement that relates to how well your windows transmit light energy into your home. 

In a nutshell, Tvw/VLT measures how light your windows can make your home.

Why is Tvw Important to Understand? 

Tvw/VLT ratings have complex, but big, implications for the aesthetics and the energy efficiency of your home. Specifically, Tvw/VLT has a complex relationship with Solar Heat Gain Coefficient (SGHC), a measurement that determines how much heat is produced from the sun’s rays.

This relationship can be briefly summed up this way:

Low Tvw/VLT rating

Cons = Darker Room
Pros = Lower Solar Heat Gain

High Tvw/VLT rating

Cons = Higher Solar Heat Gain
Pros = Light Room

As we examined in our recent article on Solar Heat Gain, a higher SHGC can be a problem if you live in a warmer climate, and no one wants to sit around in the dark for the sake of a cooler home.

Do I Have To Reduce Light To Make My Home Energy-Efficient?

Modern-day Low-E glass coatings and Double-Glazed Systems typically allow for the best of both worlds, meaning you can have a high Tvw/VLT rating, but keep SHGC at an effective level for your home.

This allows you to select windows that are appropriate for your location, window orientation, window area and shading without compromise.