Every window and glazed door ships with an NFRC label showing three numbers: U-factor, Solar Heat Gain Coefficient (SHGC) and Visible Transmittance (VT). These three values determine how a glazing system performs in your climate and in our experience, they're consistently misread or underweighted in the specification process.

U-Factor: The Insulation Number

U-factor measures how quickly heat moves through the entire assembly, glass and frame, in BTU/hr·ft2·°F. Lower is better. A single-pane aluminum window has a U-factor around 1.3. A code-minimum double-pane window is around 0.30. Our thermally broken aluminum systems with dual-pane low-E glass achieve 0.20-0.25. Triple-pane configurations reach 0.12-0.15.

The frame matters as much as the glass. An aluminum frame without a thermal break conducts cold at U ≈ 1.3, close to a single-pane window, regardless of what glass is in it. The thermal break is what makes the whole system perform to spec.

SHGC: The Solar Number

Solar Heat Gain Coefficient measures how much solar radiation passes through the glass into the interior, on a scale of 0 to 1. Higher SHGC means more passive solar heating, beneficial in cold climates with south-facing glass, a liability in hot climates where you're fighting cooling loads.

The right SHGC for your project depends on climate zone, orientation and shading. A west-facing wall of glass in Phoenix needs a very different SHGC than south-facing glass in Minneapolis. Energy Star requirements set maximum SHGC by climate zone: 0.25 for hot climates, higher for mixed and cold zones.

Don't fall into the trap of specifying the darkest available tint to minimize SHGC, very low SHGC coatings also reduce visible transmittance, making your interior darker and requiring more artificial lighting. The optimization is zone- and orientation-specific.

VT: The Light Number

Visible Transmittance is the fraction of visible light that passes through the glass. A clear dual-pane window has VT around 0.70. Tinted and low-SHGC coatings typically reduce VT to 0.40-0.55. For most residential applications, we recommend VT no lower than 0.50, below that, the interior reads noticeably darker on overcast days.

How to Specify Right

Start with your climate zone and the orientation of each glazed wall. Browse our thermal window and door systems to see the glass packages and U-factor ratings we offer. Your architect or energy consultant can run a load calculation that determines the target SHGC and U-factor for each facade. Bring those numbers to us and we'll identify the glass package, coating type, pane count, gas fill, spacer, that hits the targets within your budget. We provide the full NFRC data sheet for permit submittals and energy modeling.