Making Sense of Glass: A Guide to Basic Performance Data Interpretation
Glass is more than just a window to the world — it’s a key performance element in any building. From energy efficiency and comfort to safety and acoustics, the way glass performs directly influences how a space feels and functions.
At Viridian, we’re often asked: “What do all these numbers mean?” – referring to the data in performance charts and product specifications. Understanding these values is essential for making the right choice for your project. This guide breaks down the basics of glass performance data and how to interpret them with confidence.
Light Transmission (VLT – Visible Light Transmission)
What it Means
VLT measures the amount of visible light that passes through the glass, expressed as a percentage, with the higher the number, the more daylight enters the space. The visible light wavelength spectrum ranges from 380 to 780 nanometres (nm).
Why it Matters
High VLT glass maximises natural light, reducing the need for artificial lighting. Lower VLT options may be chosen for glare control, privacy, or to complement architectural design.
Example: A glass with 75% VLT allows three-quarters of natural daylight through, while 40% VLT glass will appear noticeably darker.
Solar Heat Gain Coefficient (SHGC)
What it Means
SHGC represents the proportion of total solar radiation that passes through the glass at normal incidence. It comprises the direct solar transmission and the part of the solar absorption dissipated inwards by radiation and convection.
Why it Matters
A lower SHGC means less solar heat enters the building, helping to keep interiors cooler in summer. A higher SHGC can be beneficial in colder climates where passive solar heating is desirable.
Tip: Pairing SHGC data with your building orientation helps optimise comfort and energy use.
U Value (Thermal Transmittance)
What it Means
The U Value measures the rate of heat transfer through the glazing caused by temperature differences between the indoor and outdoor environments. Lower U Values indicate better insulating performance. It is expressed in watts per square metre per kelvin (W/m²·K).
Why it Matters
Low U Values improve thermal comfort and reduce energy costs by minimising heat loss in winter and heat gain in summer.
Example: Single glazing may have a U Value around 5.8, while a high performance double glazed unit could be 1.6 or lower.
Reflectance and Appearance
What it Means
Visible Light Reflectance describes how much visible light is reflected by the glass surface – both externally and internally. It influences how the building looks from outside and how reflections appear inside.
Why it Matters
Designers use reflectance values to achieve aesthetic balance between transparency, brightness, and privacy.
Example: A glass with 20% external reflectance will have a slight mirrored look, while 8% will appear more neutral and transparent.
Acoustic Performance (Rw / STC Ratings)
What it Means
The Rw (Weighted Sound Reduction Index) or STC (Sound Transmission Class) values indicate how well the glass reduces sound transmission. Higher numbers mean better acoustic control.
Why it Matters
Acoustic ratings are key for projects near roads, rail, or high-density areas. Combined with frame and seal performance, they define the overall sound insulation of a window system.
Safety and Strength Ratings
What it Means
Performance data may also indicate compliance with AS 1288 or AS/NZS 2208 – standards that specify glass safety, impact resistance, and breakage behaviour.
Why it Matters
Selecting the right safety glass (toughened, or laminated) ensures compliance and occupant protection in all applications.
Bringing It All Together
Interpreting glass performance data is about finding balance – between light and heat, visibility and privacy, aesthetics and efficiency. Each number tells a part of the story, but together they describe how a glazing system will perform in real-world conditions.
At Viridian, we make performance data transparent and accessible, so you can specify with confidence. Our Glass Performance Charts and technical team are here to help you match products to your project’s climate zone, design intent, and sustainability goals.
Ready to make an informed choice?
Explore the Viridian Glass Performance Calculator, Viridian Interactive WERS Performance Tool or speak with our team to understand how your glass selection will perform – in light, heat, sound, and safety.