Solar Control

More than half of the sun's energy is invisible to the eye. Most reach us as near-infrared with a few per cent in the ultraviolet (UV) spectrum. Thus, solar radiation accounts for only a small proportion of the spectrum of electromagnetic waves, between 300 – 2,500 nm. Ultraviolet, visible and infrared (shortwave) wavelengths comprise the solar spectrum and relate to heat or the total incident solar radiation.

When radiation strikes the external surface of the glazing, radiation components are reflected, absorbed and transmitted depending on the properties of the glazing, its surface, the respective wavelength and the angle of incidence.

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Solar Reflectivity

Just as light is reflected or absorbed, so is heat. As a fraction of the total heat transfer, reflectivity is generally a low fraction of the total solar energy transmittance.

In commercial buildings, it is often desirable to consider increasing the reflectivity of the glazing, subsequentially reducing the solar transmittance and the glass body colour. Tints are often added to the glazing to absorb a larger portion of the solar heat and block daylight but profoundly impact the appearance of the glazing.

The biggest challenge with increased solar reflectivity is its potential to impact the surrounding areas where the glazing system is installed. With glazing acting as a mirror due to specular reflection, efforts that increase solar reflective glass also increase the amount of heat reflected. This should be avoided in urban locations because of its impact on adjacent buildings. It is also important to remember that increasing solar reflectivity significantly reduces our ability to view internally and externally at different times of the day.

Solar Absorptivity

Glazing is very absorptive of long-wave infrared energy. This property is best illustrated in the use of clear glazing for greenhouses. It allows the transmission of intense solar energy but blocks the retransmission of the low-temperature heat energy generated inside the greenhouse and radiated back to the glazing.

Solar energy not transmitted through the glazing or reflected off its surfaces is absorbed. Once absorbed, the energy is transformed into heat, raising the glazing temperature and eventually reradiating heat internally and externally.

When ‘heat-absorbing’ glazing is exposed to the sun, it feels much hotter to the touch than clear glazing. Tints are generally grey, bronze, or blue-green and are traditionally used to lower the solar heat gain coefficient and control glare. Since they block some of the sun's energy, they reduce the cooling load on the building and its air-conditioning equipment. However, the effectiveness of heat-absorbing single glazing is significantly reduced if cool. Conditioned air flows across the glazing and, therefore not the most efficient way to reduce cooling loads.

Solar Transmittance

When we refer to glazing and solar energy, we refer to total solar energy transmittance.

While some construction markets use alternative units to describe the total solar energy transmittance, it is generally referred to as the ‘solar heat gain coefficient’ or simply SHGC. Accounting for the total energy transmitted through the glazing system, including frames, the SHGC is expressed as a dimensionless number from 0 to 1. Influenced by the number of panes and types of coatings, a high SHGC signifies high heat gain, while a low coefficient means low heat gain.

A clear double-glazed unit has an SHGC of around 0.78. This value decreases somewhat by adding a low-E coating and substantially when adding a glazing tint or reflective coating. Since the area of a frame can have a low SHGC, the overall window SHGC is often lower than the centre-of-glazing value. However, this is not always true for poor-performing framing systems with high thermal transmittance.

	U-value	SHGC	VT	LSG
Clear 6 mm glazing	5.8	0.82	0.88	1.07
Standard clear double-glazed unit	2.5	0.71	0.78	1.1
Single low-E clear (#2) double glazed unit	1.7	0.56	0.61	1.09
Higher Performance Triple low-E clear (#2) double glazed unit	1.4	0.27	0.46	1.7

In commercial buildings, SHGC through glazing systems is a significant factor in determining the cooling load. It must be considered with the heating load to be optimised for the best annual performance.