Glass performance

VLT, LSG & the selectivity ratio

The whole trick of modern glass is letting the daylight in while keeping the heat out, and these three numbers are how you measure whether a product actually pulls it off. Learn what they mean and you can compare two glazings honestly, spot the marketing spin, and hit energy code without guessing.

6 min read Intermediate NFRC metrics

The three core metrics

VLT
Visible Light Transmittance
Fraction of visible-spectrum light (380–780 nm) that passes through the glass. Higher = brighter interior.
SHGC
Solar Heat Gain Coefficient
Fraction of total solar energy (UV + visible + near-IR) that enters as heat. Lower = less cooling load.
LSG
Light-to-Solar-Gain Ratio
VLT divided by SHGC. Higher ratio means more light per unit of heat gain, the "selectivity" index.

The key insight is that visible light (what humans see and need for tasks) is only one portion of the solar spectrum. Solar energy also includes ultraviolet (UV) radiation and near-infrared (near-IR) radiation, both of which contribute to heat gain but provide no useful illumination. A selective glass coating blocks the near-IR while allowing visible light to pass, increasing LSG above 1.0.

Solar spectrum, visible window vs. total solar range
UV (280 nm)Visible (380–780 nm)Near-IR (780–2500 nm)

Light-to-solar-gain ratio in practice

If you only remember one number from this page, make it LSG. For a daylit building that has to keep its cool, it is the single best "is this glass any good?" figure you can look at, and the math behind it could not be friendlier:

LSG = VLT ÷ SHGC

Sample product comparison, LSG visualization
Clear double-pane (no coating)VLT 0.79 / SHGC 0.70 → LSG 1.13
Standard Low-E (moderate performance)VLT 0.60 / SHGC 0.38 → LSG 1.58
High-performance selective Low-EVLT 0.68 / SHGC 0.27 → LSG 2.52
Dark tinted glass (solar control only)VLT 0.20 / SHGC 0.20 → LSG 1.00
Bar length proportional to LSG ratio (max bar = LSG 3.0). Values illustrative.

Dark tinted glass blocks heat and light in roughly equal measure, hence an LSG near 1.0. A well-designed selective Low-E coating transmits proportionally more light than heat, pushing LSG above 2.0. For most occupied commercial buildings where daylighting reduces electric lighting energy, higher LSG is generally preferable.

Energy code requirements

The IECC (International Energy Conservation Code) and ASHRAE 90.1 set maximum SHGC and minimum VLT requirements for fenestration by climate zone. Both metrics matter independently:

  • In hot and mixed climates (IECC Climate Zones 1–4), maximum SHGC limits are the binding constraint, often 0.25 or below for certain orientations.
  • Minimum VLT requirements (typically VLT ≥ 0.27 in ASHRAE 90.1) ensure that meeting the SHGC limit doesn't lead to excessively dark glass that defeats daylighting intent.
  • The effective LSG must be ≥ 1.1 in many code compliance paths, meaning you cannot simply use dark tinted glass with VLT and SHGC both at 0.20 and meet code without an exception.
Whole-window vs. center-of-glass
VLT and SHGC values reported by NFRC are whole-window values, they include the effect of the frame, edge of glass, and spacer, not just the center of the glass lite. Products report significantly different values between COG (center-of-glass) and whole-window ratings. Always use whole-window NFRC values when checking energy code compliance.

Using LSG in product selection

Building type / goalTypical targetReasoning
Office building, hot climate, daylitLSG ≥ 2.0, SHGC ≤ 0.25Maximize daylight, minimize cooling load
Residential, mixed climateLSG ≥ 1.4, balance SHGC by orientationSouth/west needs SHGC control; north may allow higher VT
Museum / galleryHigh VLT + UV blocking (not LSG alone)Artwork protection requires <1% UV transmittance; LSG secondary
Cold climate heating-dominatedHigher SHGC on south (0.40+) for passive solarSolar gain is beneficial for heating; low U-factor more critical than low SHGC
High LSG doesn't mean low glare
A glass product with VLT of 0.72 and LSG of 2.4 will deliver substantial daylight, but may also cause glare on work surfaces near the perimeter. High VLT and high LSG are generally desirable, but interior shading design (blinds, fritted glass, overhangs) is needed to manage glare separately from the glass specification.
Disclaimer: Performance values used in comparisons are illustrative. Always obtain current NFRC-certified values from the manufacturer for compliance documentation.