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Choosing the Right Glass

The type of glass you choose is probably the most important decision you’ll make when buying new windows— after all, about 90% of a window is its glass unit. Your choice of glass can dramatically affect your home’s comfort and energy efficiency.


Remodeling Magazine coverHow Much Energy is Lost Through My Windows?
According to the U.S. Energy Information Administration, 1/3rd of a typical home’s heating is lost through its windows and doors. Most homeowners could easily cut that loss in 1/2 by replacing old windows and doors with current technology.

In fact, the savings can be so great that Remodeling Magazine consistently ranks window and door replacement as one of the top five investments you can make in your home.

Std_Double_Pane2Modern Day Insulated Glass
Modern day insulated glass is essentially a “sandwich” of two or three pieces of glass separated by a spacer bar, and sealed on all four sides to create a dead airspace between the two panes. Designed to function much like a Thermos® Bottle, this dead airspace slows the heat transfer between the outside and inside of the home. With multiple high tech coatings and gas injection, today’s insulated glass can be more than twice as energy saving as windows built just 20 years ago.

Interesting Fact: The patent for insulated glass was issued over 150 years ago, to Mr. Thomas Stetson in 1865. (Sure looks like people have been worrying about high fuel costs for a very long time.)

Available Glass Systems & Energy Savings

Glass SystemTypical U-ValueEnergy SavingsWeatherMaster Model
Standard Double Pane0.49-N/A
Double Pane w/ Argon Gas Filling0.466%Earthwise 3800
Triple Pane Glass0.3333%N/A
Double Pane w/ Low E and Argon Gas Filling0.3039%N/A
Double Pane w/ SB70 Low E, DuraSeal Warm Edge Spacer and Argon Gas Filling0.2745%Earthwise 4700e
Triple Pane w/ SB70 Low E, Intercept Warm Edge Spacer and Argon Gas Filling0.2647%Earthwise 4700t
Double Pane w/ Ultraflect Low E, Premium DuraLite Warm Edge Spacer and Argon Gas Filling0.2255%Insulator 8710i
Triple Pane w/ two SB70 Low E coatings, Intercept Warm Edge Spacer and Krypton Gas Filling0.1667%Insulator 8270
Double Pane w/ two Heat Mirror Films, Proprietary Warm Edge Spacer and Krypton Gas Filling0.1569%Alpen HPP 925 Series

Glass Terminology


R-Value is a measure of the resistance to heat flow through a material. Higher numbers mean more insulation. A typical double-pane window achieves an R-2.0.

Unlike U-Values, it is usually a measure only of conduction, not air leakage or radiation heat transfer. As a result, U-Values are preferred when making comparisons.


U-Value is a measure of heat transfer through a material. Lower numbers indicate less heat transfer (more insulation) with a typical double-pane window achieving U = 0.5. It has a relationship to the R-Value where U-Value = 1/R (approx).

Solar Heat Gain Coefficient (SHGC)

SHGC expresses how much heat from the sun is blocked by the glass as a number between 0 and 1. The lower the SHGC, the more a window is blocking solar heat gain.

Controlling solar heat gain is particularly important during the summer cooling season. By contrast, people in Northern climates may desire solar heat gain to reduce the cost of heating their homes. Learn more at:

Visible Transmittance (VT)

Measures how much light comes through a window. VT is expressed as a number between 0 and 1. The higher the VT, the higher the potential for natural daylighting.

Warm Edge Spacer

Warm edge spacer is a glass spacer system specially designed to minimize heat transfer. Ordinary spacer bars are made of aluminum or other metals that readily conduct heat or cold from the glass surfaces, while warm edge spacer bars provide an insulating barrier. As a result, windows with warm edge spacers are less prone to condensation during cold winter months.

Oftentimes these specially engineered spacer systems incorporate other advantages that can drastically improve the longevity of the glass unit. Some brand names are Intercept, Swiggle, DuraSeal, DuraLite, and Super Spacer. By using a warm edge spacer, manufacturers can increase the energy efficiency of a window by approximately 5%.

Argon Gas

Argon gas is an inert gas that is often injected and sealed between the two panes of glass to improve energy efficiency. Since it is a naturally occurring component of the atmosphere, it is 100% safe. Argon gas-filled windows are approximately 6% more energy efficient than windows without argon gas.

Krypton Gas

Krypton gas is an inert gas that is often mixed with argon and injected into the dead airspace between the glass panes. It is more prevalent in triple-pane windows since it is more effective when the glass panes are close together.

Low E Glass

Low E glass contains a highly engineered surface designed to reflect heat before it passes through the window. While early versions were often mirror-like or heavily tinted, today’s Low E glass is designed to allow most visible light to pass through it, just like ordinary window glass. Additionally, Low E coatings often are designed to reflect UV light that can damage draperies and interior furnishings.

By incorporating Low E glass, manufacturers can decrease energy loss through double-pane glass by up to 50%. NOTE: Not all Low E glass is equal. Be sure to check both winter and summer U-Value ratings, and the solar heat gain coefficient, SGHC.