Guardian Insights

Tips for Specifying Glass

Open Access Technology International building

Pushing Performance

There are multiple measurements to meet performance goals:

U-Factor measures heat gain or loss through glass due to difference between indoor and outdoor temperatures. A lower number means better performance.

Solar Heat Gain Coefficient (SHGC) measures how well a product blocks heat caused by sunlight. The lower the SHGC, the less solar heat it transmits, and the more comfortable occupants are. The right SHGC can help preserve warm interior air in cold climates and help retain expensive air-conditioned air in hot climates. Specify 0.4 or below in the North, and 0.25 or below in the South.

Visible Light Transmission (VLT) is the percent of visible light that passes through glass. VLT can help facilitate daylighting and, if designed thoughtfully, can aid in offsetting electric lighting and cooling loads. A higher VLT can enhance daylighting, and a lower one can add privacy. Managing VLT will help avoid glare.

Light to Solar Heat Gain Ratio (LSG) compares the VLT to the SHGC. A higher LSG ratio can make rooms brighter.


Building orientation plays a tremendous role in energy performance and occupant productivity. Solar heat gain and daylighting can vary greatly depending on the path of the sun in the summer vs. winter. Overlooking these can result in excessive heat gain or glare issues that may compromise energy performance and occupant comfort. The sun’s arc impacts each façade differently:

  • North: All-day indirect sunlight
  • South: Solar exposure throughout the day
  • East: Direct light at low angles in morning
  • West: Direct light at low angles in afternoon

Low-E glass façades can maximize natural light with minimal impact on solar heat gain for those façades that have long-term sun exposure. Different coatings can impact glare and solar heat gain in different ways.

For example, in this project below, a transparent, low-E coating on a clear glass substrate (left side of image) is used in meeting rooms to harvest light, while reflective glass lowers visible light transmission to diminish glare for people working in cubicles (middle and right).

Guardian SunGuard® SuperNeutral® 68 (left side) and AG 50 (right side) coated glass on OATI. Photo by Mitchell Stier.

Seeing Samples

When evaluating glass samples:

  • View samples outdoors on an overcast day.
  • View triple silver low-E coatings at a 40- or 50-degree angle.
  • Hold about 10 feet away.
  • Turn a sample over to see the reflectivity of the glass, as well as the interior reflected aesthetic.
  • Visit the job site in different lighting conditions and times of day.
  • View with a black background to replicate a punched window application without lighting.
  • View with a white background to demonstrate a nighttime application or looking through an all-glass, corner elevation.

The decision to build cubicle walls or create open interiors can affect the way glass looks from the outside. Consider all of these factors. Talk to your manufacturer early so that the team can forecast your needs. Equipped with a glass selection plan and resources, you can surpass your project goals.