What Is U-Value?
When it comes to evaluating the energy performance of architectural glass, one of the most important performance measures is U-value. Also known as U-factor, this figure measures the insulating characteristics of the glass, or how much heat flow or heat loss occurs through the glass due to the difference between indoor and outdoor temperatures.
U-values can tell you how well an insulating glass unit (IGU) will hold in heated or cooled air. The lower the number, the better the insulating performance. U-values generally range from 0.1 (very little heat loss) to 1.0 (high heat loss). The U-value of a window is measured by the number of BTUs that will pass through each square foot of area per degree of temperature difference from one side of the window to the other.
While you may have heard of a similar performance metric called R-value, U-value is used to measure the performance assemblies of IGUs whereas R-value is used to measure the performance of most other parts of the building envelope, such as walls, floors and roofs. Additionally, these figures are mathematical reciprocals of each other. For this reason, lower U-values indicate better insulating performance whereas higher R-values indicate better thermal resistance.
Can I Lower the U-Value of My Glass Selections?
Thanks to advances in low-emissivity (low-e) and insulating technologies for IGUs, architects have more options than ever to lower U-values wherever increased thermal performance is needed. These options can double or even triple thermal performance, yielding yearly energy savings and a positive return on carbon.
Some of the traditional ways U-values can be improved are:
- Add a low-e coating
- Use double- or triple-glazed IGUs
- Use a noble gas, such as argon, in the cavity between lites of glass
- Optimize the cavity size between lites of glass—a ½ " space between lites is the most widely used for both air and argon
- Specify warm-edge spacers that create an effective thermal barrier, and
- Use low-e coatings (coatings can be used on more than one surface)
However, in recent years the architectural glass market has introduced new technologies that have allowed for even lower U-values than ever before, including fourth-surface low-e coatings and vacuum insulating glass (VIG).
Vacuum Insulating Glass (VIG)
Vacuum insulating glass is an innovative architectural glass that combines vacuum technology with a low-e coating in an IGU to provide exceptional thermal performance that rivals the performance of traditional walls. VIG units can be used alone to replace monolithic (single-pane) glass without needing to replace the framing system or as a substitute for the interior lite in any double- or triple-glazed IGU, where it forms a second airspace and creates a hybrid IGU.
With R-values up to R20, VIG units can deliver thermal performance that is 5 times better than conventional insulating glass and up to 20 times better than monolithic glass. To learn more about the science of vacuum insulating glass, read Vitro's Glass Education Center article on the topic.
Fourth-Surface Low-e Coatings
Fourth surface low-e coatings are engineered specifically for use on the interior surface of an IGU that can dramatically improve U-values when paired with solar control low-e glass, typically applied on the second surface.
In addition to offering enhanced durability and insulation performance, fourth-surface low-e coatings are designed to allow for a high rate of visible light transmittance (VLT) and to retain the indoor temperature by slowing down heat transfer through the IGU to reduce winter heating costs. This results in up to a 20% improvement in the U-value over using solar control low-e glass alone on the second surface.
What Impact Can These Technologies Have on U-Value?
Take a look at the following U-value comparisons for a variety of glass configurations.
Note: Measurement of improved U-values are calculated using a baseline of a Double Low-e 1” IGU Solarban® 70 (2) Clear + Clear with argon from Vitro Architectural Glass, with a U-value of 0.24.
It’s clear from these configurations that new technologies like VIG and fourth-surface low-e coatings represent the next generation of thermal glazing, building on the improvements that double- and triple-glazed IGUs have offered for decades. In years to come, U-value optimized IGUs likely will be used to replace monolithic glazing in older buildings, especially as building code standards demand more energy-efficient products. To learn more about these developments, read Vitro’s Glass Education Center article Codes and Climate Zones.
For more information about Vitro’s fourth-surface low-e coating, visit vitroglazings.com/sungate-therml. To learn more about Vitro’s vacuum insulating glass, visit vacumaxvig.com. On both product pages, you can browse informational brochures, download technical documentation and order free glass samples.
