Coating surfaces & designations
In an IGU, a coating on "surface 2" and the same coating on "surface 3" are two different products with two different jobs, and mixing them up is one of the easiest and most expensive mistakes in glass specification. This little numbering system is how the industry keeps that straight. Learn it once and you will never order a mirror-image unit by accident.
The surface numbering system
The rule is refreshingly simple: start outside the building and count every glass face as you walk inward. For a standard double-pane IGU in an exterior wall, that gives you four surfaces:
| Surface | Location | Typical coating use | Notes |
|---|---|---|---|
| 1 | Exterior lite, outer face | Self-cleaning pyrolytic coatings; rare | Exposed to weather; must be durable |
| 2 | Exterior lite, gas-space face | Soft-coat Low-E (most common); primary thermal performance coating | Protected inside gas space; allows high-performance coatings |
| 3 | Interior lite, gas-space face | Second Low-E in triple-pane; solar control in high-SHGC-rejection specs | Also protected; used when Surface 2 alone is insufficient |
| 4 | Interior lite, interior face | Hard-coat Low-E (durable for exposed use); anti-reflective coatings | Exposed to room; only durable coatings viable here |
Hard coat vs. soft coat
Low-E coatings come in two fundamental types based on how they are deposited, and this determines which surfaces they can be placed on:
- Chemical vapor deposition (CVD) at ~600°C during glass manufacture
- Fused into the glass surface, extremely durable
- Can be placed on any surface including exposed surfaces (1, 4)
- Lower emissivity performance than soft coat
- Can be heat-treated after coating without damage
- Magnetron sputtering at room temperature; multiple thin metallic layers
- High performance, very low emissivity achievable
- Delicate: must be protected inside an IGU gas space (Surface 2 or 3)
- Cannot be placed on Surface 1 or 4 in most applications
- Must be edge-deleted before IGU sealing
Reading coating specifications
A coating specification on a drawing or in a spec section typically identifies the product, the glass thickness, and the surface designation. Common notation formats:
- "1/4" clear + 1/2" argon + 1/4" clear, Low-E on Surface 2", explicit and preferred.
- "Low-E IGU, coating #2", shorthand, acceptable if the unit configuration is clearly defined elsewhere.
- "Low-E glass" alone, ambiguous; does not specify surface, coating type, or performance target. Avoid this in formal specs.
Field verification of coating orientation
A Vacucheck gap gauge or a simple reflectance check can confirm the coating is on the correct surface after installation. A glass-surface locator tool (small pen-like device with multiple flame reflections) allows a technician to identify which surface has a coating in the field by observing whether one flame reflection is dimmer or missing compared to the others, the coated surface alters reflectance in a way that is visible in the pattern.