Glazing - Low-E Coatings

Glazing surfaces emit heat primarily in the form of long-wave far-infrared radiation, with the wavelength determined by the surface temperature. Standard clear glass has an emittance of around 0.84 in this spectrum, meaning it absorbs 84% of incident long-wave radiation and reflects just 16%.

In contrast, low-emissivity (low-E) glazing dramatically reduces radiant heat transfer. With emissivity values as low as 0.03, low-E coatings reflect up to 96% of incident long-wave infrared energy, significantly enhancing the thermal performance of the glazing.

How Low-E Coatings Work

Low-E coatings are usually made from thin layers of noble metals and are applied to one of the inner surfaces of double- or triple-glazed units. Since about two-thirds of heat loss through glazing occurs via radiation, adding a low-E coating reduces this heat transfer, substantially improving the glazing’s U-value.

The location of the coating within the glazing unit influences its performance:

• To reduce external heat gain (cooling load), coatings are typically applied to surface #1 or #2 (the exterior-facing layers).

• To reduce internal heat loss (heating load), coatings are applied to surface #3 or #4 (the interior-facing layers).

Types of Low-E Coatings

There are two main types of low-E coatings, based on how they are manufactured:

Sputtered Coatings (Soft-Coat Low-E)

• Made using physical vapour deposition, where layers of metals (often including silver) are applied in an ultra-thin format

• Provide very low emissivity (as low as 0.02)

• More susceptible to corrosion and physical damage, best suited to sealed cavities within double-glazed units

• Not typically used in monolithic (single-pane) applications due to durability concerns

Pyrolytic Coatings (Hard-Coat Low-E)

• Created using chemical vapour deposition during the glass manufacturing process

• Commonly use tin oxide and form a durable, bonded surface layer

• More robust and resistant to mechanical and chemical wear, can be exposed and cleaned using standard methods

• Typical emissivity values range from 0.15 to 0.30

Spectrally Selective Coatings

Advanced low-E coatings can be engineered to reflect or transmit specific wavelengths of solar energy. These spectrally selective coatings allow desirable wavelengths, like visible light for daylighting, to pass through, while reflecting infrared wavelengths responsible for heat gain or loss. This allows glazing systems to be tailored for optimal performance in both heating and cooling climates.

Emissivity in Specification

While emissivity is a fundamental property, it's generally not specified directly in procurement. Instead, it is an inherent property of the coating type. For reference:

• Clear glass: ~0.89

• Pyrolytic low-E: 0.15–0.30

• Sputtered (magnetron) low-E: 0.01–0.04