Low-E Coatings, Argon Fill and Spacers: How Window Glass Performance Works
Summary
U-factor and SHGC are produced almost entirely by the glass package: the low-E coating, the gas fill, and the spacer. This page explains what each component does and how to read an NFRC label.
- Author
- Written by: OCWR Research Desk
The glass package produces the numbers
A window's rated performance is mostly a property of the insulating glass unit, not the frame material. Three components do the work.
Low-E coatings
A low-emissivity coating is a microscopically thin metallic layer, usually sputtered onto an inner glass surface. It reflects long-wave infrared radiation while transmitting visible light.
- Single-silver low-E reduces heat transfer and moderately reduces solar gain.
- Double- and triple-silver (spectrally selective) low-E substantially reduces solar heat gain while keeping visible transmittance high. This is how manufacturers reach the California SHGC 0.23 prescriptive limit.
- Coating position matters: surface 2 (outboard lite, inner face) is typical for solar control in cooling climates like Orange County.
Gas fill
Argon replaces air in the cavity and conducts heat less readily, improving U-factor modestly. Krypton allows the same performance in a narrower cavity and costs more; it is uncommon in residential replacement here. Gas fill degrades slowly over decades; the seal is what determines whether it stays put.
Spacers
The spacer separates the lites and carries the sealant. Traditional aluminum spacers conduct heat around the edge of the unit, producing cold edges and condensation. Warm-edge spacers — stainless steel, structural foam or thermoplastic — reduce that edge conduction, improving condensation resistance and slightly improving whole-window U-factor.
Reading an NFRC label
| Label value | Meaning | Orange County target |
|---|---|---|
| U-factor | Heat transfer rate; lower is better | 0.30 or lower (Title 24 prescriptive) |
| SHGC | Fraction of solar heat admitted; lower admits less | 0.23 or lower (Title 24 prescriptive) |
| Visible transmittance | Fraction of visible light admitted | Higher preserves daylight |
| Air leakage | cfm per sq ft at test pressure | Lower is better; 0.3 or below is common |
The label applies to the exact configuration tested, including grids and glass options. Adding a grid or changing the coating changes the rated values.
What matters most in Orange County
In both Climate Zone 6 and Zone 8, cooling and glare dominate. SHGC is the number to optimize, especially on west and south exposures. A compliant-but-marginal SHGC on a west-facing wall in Anaheim will feel materially worse than a 0.20 unit at the same U-factor.
Sources & Citations
Citations for this reference are being catalogued.
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