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Understanding Window Efficiency Terminology

What can we help you find today?

  1. Low-E
  2. Argon filled
  3. Insulated Glazing (IG) Insulated Glazed Unit (IGU)
  4. U-Factor
  5. Solar Heat Gain Coefficient (SHGC)
  6. R-Value
  7. Visible Transmittance (VT)
  8. Air Leakage

A part of being a window installation service company is to inform homeowners of the latest window trends. Today we’ll be focusing on popular window efficiency terms that you may encounter while looking for the windows right for your home. 

Understanding how efficient a window will be is essential to making a great replacement window choice. The best way to understand window efficiency is to start with the terminology.

Here are a few common terms you might come across when researching window efficiency.


Low-E is short for low emissivity. Emissivity is the measurement of whether radiant heat is absorbed or reflected. In home windows, high heat reflection, or low emissivity is desired. It means that when your house is warm in the winter, the heat stays in your house. Conversely, when it is hot outside and cooler in your house, more of the heat stays outside.

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Argon filled

When a window is Argon filled, it is at least a double pane window. Double pane or Triple pane windows normally have a layer of air in between them. When a window is Argon filled, the air in between these windows is replaced with much heavier and better insulating Argon gas. Argon gas also has the added benefit of preserving the inner pane of the window by removing oxygen from the inside of the window, meaning that the inner glass will not oxidize, and it will last longer.

Insulated Glazing (IG) Insulated Glazed Unit (IGU)

Glazing is the installation of glass into windows, can be done by window contractor. Insulated glazing is the practice of installing more than 1 pane of glass in order to insulate. An Insulated Glazed Unit (IGU) is another way to describe double-pane or triple-pane windows.

High-quality, energy-efficient windows in your home

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How well a window keeps heat IN. Lower numbers are better ratings.

Solar Heat Gain Coefficient (SHGC)

How well a window keeps heat OUT. Lower numbers are better ratings.


Is the overall resistance to heat transfer. The higher the number, the better the insulation.

Visible Transmittance (VT)

The amount of light a window lets through. Higher VT means more visible light is transmitted.

Air Leakage

How well a window blocks air flow. The lower the rating, the better.

Understanding window efficiency terminology - Infographic by Opal Enterprises