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What is a Window's U-Factor?

The U-factor (U-value) is the difference in temperature between either side of a window's glass. The lower the U-factor of a window is, the better it will be as an insulator. 

How the U-factor plays a part in home insulation

Fixed windows, for instance, are sometimes located in solar strategizing areas inside the home to transfer incoming solar heat to be stored in the walls, floors, and ceilings. Windows such as these act like heat banks because they have a low U-factor, and during the colder parts of the day, the quantities of heat stored will begin to dissipate and fill the home.

Let's look at how the U-factor of home windows are mitigated in terms of the choice of glass, and how that affects the needed heat transfer for comfortable indoor temperatures.

Four important determinants for the desired U-factor

Insulated glazing. 
An extremely effective method manufacturers use to reduce heat gain is glazing which creates a finished product referred to as insulated glass units (IG Units). IG Units are windows that use two or three glass panes with some space left in between, usually filled with air or a low heat conductivity gas. Air can be used but gases such as argon and krypton have lower-conductivity properties that limit the transfer of heat from the outermost glass pane to the innermost one.

Low-E glass.
Short for low-emissivity, low-E glass is coated with a material that is visibly unnoticeable which reflects infrared and, in turn, lowers the U-factor (better insulation) and the solar heat gain. Low-E glass is very useful not only because it lowers the U-factor but because it enhances the performance of the glazing by blocking a lot of the heat before it reaches the glazed layers, allowing for a better overall insulation performance. 

Glass surface.
Manufacturers typically apply the low-E coating in two different ways that eventually give you two differently performing glass panes. Number 2 surfaces are used to block heat gain while number 3 surfaces are used to trap it. Window manufacturers produce #3 surfaces by applying the low-E coating on the outside of the inner glass pane to make the window more exposed to heat gain in colder climates. To produce #2 surfaces, on the other hand, they coat the inner side of the outer pane to make it block as much heat gain as possible.

Some modern window variations allow you to alternate the panes of glass to better accommodate for the changing seasons.

Glass thickness.
Another important element that alters the U-factor is the thickness of the glass panes themselves. For homeowners that find argon or krypton filled windows to be expensive, thick glass can be a more affordable alternative. The thickness of the glass will also play a major role in how impact resistant your windows will be.

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