Salt Lake City Public Library

Architect: VCBO, Architecture, LLC and Moshe Safdie and Associates

Key Glass Performance

Measures

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

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.

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%.

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.

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.

Key Glass Performance Terms

Winter U-value

Winter Nighttime U-Value measures the environmental conditions of a cold outside

temperature and no sunlight.

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.