Low-E Glass and Code Compliance
Low-emissivity (Low-E) glass has a microscopically thin metallic coating that reflects infrared radiation while allowing visible light to pass through. It is the primary technology enabling windows to meet modern energy code requirements, and it is now standard in virtually all code-compliant replacement windows.
How Low-E Works
Without Low-E coating, glass transmits heat readily in both directions — warm air inside in winter radiates through the glass to the cold outside, and solar radiation in summer passes through and heats the interior. Low-E coating reflects this long-wave infrared radiation back into the room in winter (reducing heat loss) and reflects solar infrared back outside in summer in hot climates (reducing heat gain). The result is a significantly lower U-factor and, in some configurations, a lower SHGC.
Types of Low-E Coatings
| Type | Process | Best For | Notes |
|---|---|---|---|
| Hard-coat (pyrolytic) | Applied during float glass manufacturing | General use, single-pane storm windows | Durable; can be tempered; slightly lower performance |
| Soft-coat (sputtered) | Applied after glass formation in a vacuum | IGU (insulated glass unit) inner surfaces | Higher performance; must be sealed in IGU; cannot be exposed to air |
| Triple-silver Low-E | Multiple soft-coat layers | High-performance Zones 5–8 | Achieves U-0.20 and below; maintains high VT |
Low-E Position in Insulated Glass Units
In a double-pane IGU, the coating position matters. Coating on surface 2 (inside face of outer pane) is optimal for cold climates — maximizes solar gain while reducing heat loss. Coating on surface 3 (inside face of inner pane) is better for hot climates — reduces solar gain. Most manufacturers optimize this for the target climate zone market.
Historic Districts and Low-E
Low-E coated glass is generally acceptable in historic districts — the coating is not visible and does not change the appearance of the glass. However, some very strict commissions may object to the slightly different reflectivity of modern coated glass compared to original single-pane glass. This is unusual but worth confirming with your preservation officer.