The slides provide an overview on common terminology used in the glass industry, including: Visible Light Tranmittance (VLT), Solar Heat Gain Coefficient (SHGC), Light-to-Solar Gain Ratio (LSG) and U-Value, both Winter & Summer.

1. Salt Lake City Public Library
Architect: VCBO, Architecture, LLC and Moshe Safdie and Associates
Key Glass Performance
Measures

2. Key Glass Performance Terms
Introduction
Understanding glass performance begins by understanding some key
glass performance terms. The following slides discuss some common
terminology related to the glass industry, including:
• Visible Light Transmittance (VLT)
• Solar Heat Gain Coefficient (SHGC)
• Light-to-Solar Gain Ratio (LSG)
• U-Value, both Winter & Summer

3. Key Glass Performance Terms
Visible Light Transmittance (VLT)
 Visible Light Transmittance is a percentage of the visible portion of the solar energy
spectrum coming through the glass. It is expressed as a figure between 0 (no light)
and 100 (all light).
 Measures glass’ ability to transmit light/facilitate daylighting.

4. Glass Performance & Impact of Low-e Coatings
Solar Heat Gain Coefficient (SHGC)
 Solar Heat Gain Coefficient (SHGC): The fraction of incident solar radiation
admitted through a window, both directly transmitted and absorbed and subsequently
released inward.
 SHGC is expressed as a number between 0 and 100. The lower a window's solar heat
gain coefficient, the less solar heat it transmits. As with VLT, SHGC is measured in
values from 0 to 100, with 0 representing an impenetrable wall against solar heat and
100 representing direct exposure to the sun’s heat. For example, a glass with a SHGC
of 0.27 allows 27% of the sun’s solar heat to pass through, meaning it blocks 73%.

5. Key Glass Performance Terms
Light to Solar Gain (LSG) Ratio: VLT ÷ SHGC = LSG
 The LSG Ratio is a result of a window's Solar Heat Gain Coefficient (SHGC) being
divided by its visible light transmittance (VLT) rating. The LSG Ratio in turn measures
the glass’ ability to transmit light and block heat in the form of infrared energy.
 The higher the LSG, the brighter the room is without adding excessive amounts
of heat.

6. Key Glass Performance Terms
U-Value
 A rating given to a window based on how much heat loss it allows.
 U-factors generally range from 0.2 (very little heat loss) to 1.2 (high heat loss).
 Measured as Winter U-Value and Summer U-Value.

7. Key Glass Performance Terms
Winter U-value
 Winter Nighttime U-Value measures the environmental conditions of a cold outside
temperature and no sunlight.

8. Key Glass Performance Terms
Summer U-value
 A lower Summer U-Value means the glass is better at blocking re-radiated heat,
increasing the efficiency of the air conditioner.