What Is Low-E Glass? The Technology Behind Energy-Efficient Windows

Almost every energy-efficient window today uses Low-E glass. But what is it, and how does it work?

Low-E Defined

According to Lawrence Berkeley National Laboratory, Low-E stands for "low emissivity." An emissivity rating measures how well a surface radiates heat—lower emissivity means the surface reflects heat rather than emitting it.

Clear glass has an emissivity of about 0.84, meaning it absorbs and re-radiates 84% of infrared energy. According to LBNL, Low-E coatings reduce this to 0.04-0.15—blocking most heat transfer by radiation.

How Low-E Coatings Work

According to the U.S. Department of Energy, Low-E coatings are microscopically thin metallic layers (usually silver-based) applied to glass surfaces. They work by:

1. Reflecting infrared heat back toward its source
2. Transmitting visible light (you can still see through clearly)
3. Blocking UV radiation that damages furnishings

In winter, they reflect your home's heat back inside. In summer, they reflect solar infrared heat back outside.

The Performance Impact

According to NFRC testing data:

| Glass Type | U-Factor | Heat Loss Reduction |
|------------|----------|---------------------|
| Clear single-pane | 1.04 | Baseline |
| Clear double-pane | 0.49 | 53% |
| Low-E double-pane | 0.30 | 71% |
| Low-E double with argon | 0.27 | 74% |

Low-E makes more difference than adding a second pane of clear glass.

Visual Appearance

According to LBNL, modern Low-E coatings are nearly invisible:

• Slight color tint (usually gray or green)
• No mirror effect
• Full daylight visibility

The Bottom Line

Low-E coatings are the single most impactful window technology. According to DOE, they're now standard in virtually all energy-rated windows—and the reason modern double-pane windows dramatically outperform older clear-glass double-pane.

*For complete Low-E information, see: [Low-E Glass Coatings](/guides/low-e-glass-coatings)*