Solar gain

Solar gain (also known as solar heat gain or passive solar gain) is the increase in thermal energy of a space, object or structure as it absorbs incident solar radiation. The amount of solar gain a space experiences is a function of the total incident solar irradiance and of the ability of any intervening material to transmit or resist the radiation.

Objects struck by sunlight absorb its visible and short-wave infrared components, increase in temperature, and then re-radiate that heat at longer infrared wavelengths. Though transparent building materials such as glass allow visible light to pass through almost unimpeded, once that light is converted to long-wave infrared radiation by materials indoors, it is unable to escape back through the window since glass is opaque to those longer wavelengths. The trapped heat thus causes solar gain via a phenomenon known as the greenhouse effect. In buildings, excessive solar gain can lead to overheating within a space, but it can also be used as a passive heating strategy when heat is desired.

Solar gain is most frequently addressed in the design and selection of windows and doors. Because of this, the most common metrics for quantifying solar gain are used as a standard way of reporting the thermal properties of window assemblies. In the United States, The American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE), and The National Fenestration Rating Council (NFRC) maintain standards for the calculation and measurement of these values.

The shading coefficient (SC) is a measure of the radiative thermal performance of a glass unit (panel or window) in a building. It is defined as the ratio of solar radiation at a given wavelength and angle of incidence passing through a glass unit to the radiation that would pass through a reference window of frameless 3 millimetres (0.12 in) Clear Float Glass. Since the quantities compared are functions of both wavelength and angle of incidence, the shading coefficient for a window assembly is typically reported for a single wavelength typical of solar radiation entering normal to the plane of glass. This quantity includes both energy that is transmitted directly through the glass as well as energy that is absorbed by the glass and frame and re-radiated into the space, and is given by the following equation:

${\displaystyle F(\lambda ,\theta )=T(\lambda ,\theta )+N*A(\lambda ,\theta )}$

Here, λ is the wavelength of radiation and θ is the angle of incidence. "T" is the transmissivity of the glass, "A" is its absorptivity, and "N" is the fraction of absorbed energy that is re-emitted into the space. The overall shading coefficient is thus given by the ratio:

${\displaystyle S.C.=F(\lambda ,\theta )_{1}/F(\lambda ,\theta )_{o}}$

The shading coefficient depends on the radiation properties of the window assembly. These properties are the transmissivity "T" , absorptivity "A", emissivity (which is equal to the absorptivity for any given wavelength), and reflectivity all of which are dimensionless quantities that together sum to 1. Factors such as color, tint, and reflective coatings affect these properties, which is what prompted the development of the shading coefficient as a correction factor to account for this. ASHRAE's table of solar heat gain factors provides the expected solar heat gain for 1⁄8 in (3.2 mm) clear float glass at different latitudes, orientations, and times, which can be multiplied by the shading coefficient to correct for differences in radiation properties. The value of the shading coefficient ranges from 0 to 1. The lower the rating, the less solar heat is transmitted through the glass, and the greater its shading ability.