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Everything you should know about U-Value and R-Value for Glass

In this section, we will explore the difference between two of the most significant glass

performance measures; U-Value and R-Value. Join as we dive deeper to understand the differences and why this is important for both commercial and residential properties.

Quick Summary:

At a quick glance - the U-value is the measure of how well a material resists heat flow. The lower the number, the better it is at resisting heat flow. The R-value measures how well materials resist conductive and convective losses of heat.

What is the U-Value?

The U-value measures the insulating qualities of the glass, or how much heat flow or heat loss happens through the glass as a result of the temperature differential between interior and exterior temperatures.

U-values indicate how efficiently an insulated glass unit (IGU) keeps hot or cooled air within. The lower the number, the more effective the insulation. U-values typically vary from 0.1 (very little heat loss) to 1.0 (very significant heat loss) (high heat loss). The number of BTUs that will travel through each square foot of space per degree of temperature variation from one side of the window to the other is the U-value of a window.

How to Calculate R-Value?

Divide 1 by the U-value figure to get the R-value. A U-value of 0.20, for example, equals an R-value of 20. (1 divided by 0.20). Divide 1 by the R-value to get the U-value—a 4.50 R-value yields a U-value of 0.22.

What is the key difference?

R-value is used to test the performance of most other components of the building envelope, such as walls, floors, and roofs, whereas U-value is used to measure the performance of performance assemblies such as IGUs.

Another way of looking at it is: the U-value of a window is the measure of heat transfer by conduction, while the R-value measures the efficiency of insulation.

How to measure both energy performances?

U-value and R-value are two different ways of measuring the energy performance of a window or door.

The u-factor is the measure of how well the window or door blocks heat flow. It is measured in Btu/h*ft2*°F/BTU/h*ft2*°F, which is the rate that heat moves through a window or door.

The R-value is how well insulation prevents heat transfer. It is measured in BTU/h*ft2*°F, which is the rate at which insulation prevents heat from moving through an opening between an inside and outside surface of a building material like drywall or glass.

How to Improve U-Values:

  • Use IGUs that are double or triple-glazed.

  • In the cavity between the glass lites, use a noble gas such as argon.

  • Optimize the cavity size between glass lites—for both air and argon, a 12" spacing between lites is the most common.

  • Warm-edge spacers should be specified to establish an effective thermal barrier

  • Low-e coatings should be applied (coatings can be used on more than one surface)

Considering Low-E Glass

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