Low-E Coating Position: Why Surface 2 vs. Surface 3 Matters

In a double-pane window, there are four glass surfaces: the exterior face of the outer pane (Surface 1), the interior face of the outer pane (Surface 2), the exterior face of the inner pane (Surface 3), and the interior face of the inner pane (Surface 4). According to Lawrence Berkeley National Laboratory, where the Low-E coating is placed dramatically affects performance.

Surface Numbering

According to the Efficient Windows Collaborative:

```
Outside | Pane 1 | Gap | Pane 2 | Inside
← [1] [2] ← [3] [4] →
```

Surface 1 faces the weather. Surface 4 faces your room.

Position and Performance

According to LBNL research:

Surface 2 Low-E (Most Common for Cold Climates)

• Reflects interior heat back into the room
• Maximizes insulation value (lowest U-factor)
• Reduces solar heat gain (lower SHGC)
• **Best for:** Idaho and other heating-dominated climates

Surface 3 Low-E (Sometimes Used for Mild/Hot Climates)

• Reflects solar radiation before it enters
• Slightly higher U-factor
• Can allow more solar gain when desired
• **Best for:** Passive solar applications, mixed climates

Why It Matters for Idaho

According to the U.S. Department of Energy, in heating-dominated climates like Idaho:

1. Your primary concern is heat loss, not solar gain
2. Surface 2 Low-E maximizes insulation
3. Most windows sold here use Surface 2 position
4. This is the correct choice for most applications

Passive Solar Exception

According to LBNL, if you're specifically optimizing for passive solar:

• South-facing windows may benefit from Surface 3 Low-E
• Allows more solar heat into the home
• Must balance with higher U-factor tradeoff
• Typically a specialty specification

The Bottom Line

For Idaho homes, Surface 2 Low-E is almost always correct. Unless you're specifically optimizing south-facing windows for passive solar gain, don't overthink this—standard windows from quality manufacturers will have the coating in the right position for cold climates.

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