Glazing typically emits heat in the form of long-wave far-infrared energy, with the wavelength varying with the temperature of the surface. Standard clear glazing has an emittance of 0.84 over the long-wave infrared portion of the spectrum, meaning that for long-wave radiation striking the surface of the glazing, 84% is absorbed and only 16% is reflected.
By comparison, low-E glazing with emittance values as low as 0.03, emits only 3% of the energy possible at its temperature and thus reflect 96% of the incident long-wave infrared radiation. Therefore, the application of low-E glazing is critical to improving thermal performance.
Often a thin noble metal coating applied to the surface in the cavity of a double-glazed unit, low-E coatings improve the radiation properties of the glazing. As about two-thirds of glazing heat losses are due to radiation, low-E coatings provide a significant reduction in heat loss and a vast improvement to the thermal insulating properties of the glazing, and It’s U-value. The U-value intern is influenced by the location of the low-E coating. Typically, to reduce external heat loads, we apply coatings on surface #1 or #2. To reduce internal heat loss, we apply coatings on surface #3 or #4.
There are two basic processes for making low-E coatings—sputtered and pyrolytic.
Sputtered coatings or soft coats are multilayered coatings only one ten-thousandth the thickness of a human hair, typically comprising metals, metal oxides, and metal nitrides and produced in a process called physical vapour deposition. Sputtered coatings often use one or more layer of silver to achieve their heat-reflecting properties but are sometimes stated to be susceptible to corrosion.
Because they offered less resistance to chemical or mechanical attack than pyrolytic coatings, they are generically referred to as ‘soft-coat low-E’. As such, they are often not sufficiently durable to be used in monolithic applications and more suited to coatings facing into a cavity within a double-glazed unit. Sputtered coatings have emittance as low as 0.02 which are substantially lower than those for pyrolytic coatings.
A typical pyrolytic coating or hard coat is a metallic oxide, most commonly tin oxide with some additives, which is bonded to the glazing while it is in a semi-molten state and produced in a process called chemical vapour deposition. Pyrolytic coatings are baked-onto a surface layer that is hard and durable, which is why they are generically referred to as ‘hard-coat low-E’. Pyrolytic coatings can be exposed to air and cleaned with traditional glazing cleaning products and techniques without damaging the coating.
The solar reflectance of low-E coatings can be manipulated to include specific parts of the visible and infrared spectrum. This is the origin of the term spectrally selective coatings, which selects specific portions of the energy spectrum so that desirable wavelengths of energy are transmitted, and others specifically reflected. A glazing material can then be designed to optimize energy flows for solar heating, daylighting, and cooling.
We typically don’t specify an emissivity value, as it is inherent in the coating and not used for procurement like other performance values. A sheet of clear glazing has a normal emissivity of 0.89, while pyrolytic coatings result in emissivity values of between 0.15 and 0.30 and high-performance magnetron coatings have values between 0.01 and 0.04.