5.1.2.1 General
Glazed areas in buildings should be designed so that account is taken of the overall energy balance in relation to the effects on the thermal comfort of occupants and the total annual energy implications of solar gain and energy loss.
These should be examined separately, since factors affecting one aspect of performance might have no effect on the other. Energy gain is due to transmission of solar radiant energy through the glass into the building. Solar energy is all at relatively short wavelengths and is controlled by using the glass to absorb or reflect the energy. Energy loss is due to the transfer of energy by conduction, convection and long wavelength radiation and can be influenced by a number of factors, for example incorporating air cavities, including gases of low thermal conductivity and/or low emissivity glass.
5.1.2.2 Thermal comfort
Energy transmission through glazing can significantly influence occupier comfort by raising or lowering room air temperatures. For example hot sunny weather can give rise to excessive energy gains; and energy losses can occur through the glazing during cold weather or at night. Thermal comfort can also be influenced by direct radiation through the glazing and/or by radiation exchange between the glazing and the occupants.
5.1.2.3 Solar energy gain
Factors that can influence the amount of solar gain through the glazing include,
a) orientation;
b) glazed area;
c) shading devices, both internal and external; and
d) properties of the glazing.
5.1.2.4 Total solar energy transmittance
5.1.2.4.1 Total solar energy transmittance is the proportion of solar radiation at normal incidence transferred through the glazing. It is composed of the direct transmittance (short wave component) and the part of the solar absorptance dissipated inwards by long wave radiation and convection (long wave component).
The total solar energy transmission properties of solar control glazing can be described by their shading coefficients. The shading coefficient is derived by comparing the properties of the solar control glazing with a clear float glass having a total solar energy transmittance of 0.87 (that is clear glass between 3 mm and 4 mm thick).
NOTE — Total solar energy transmittance is also known as ‘g’ value.
5.1.2.4.2 Solar control glass
Solar control glass can be manufactured in several forms. The function of solar control glass is to reduce the total solar energy transmittance, which usually leads to a decrease in the transmission of the visible part of the solar spectrum. However, some tints and coatings are able to attenuate preferentially non-visible solar radiation, leaving the transmission of the greater proportion of the visible radiation largely unchanged.
The application of ceramic frit-fired into the surface of the glass can also be used to modify the energy and light transmission of the glass. The effect can be varied across a pane depending on the screen print pattern. Manufacturers should be consulted for specific details.
Solar control glass can be used in single glazed or double glazed unit depending upon the site requirements. The relationship between light transmission and total solar energy transmission is expressed as the light/energy ratio.
There is no optimum light/energy ratio. The selection of an appropriate product depends on the requirements of the building. Solar gain may be relatively high — for making use of passive solar gains, or low — to reduce air conditioning loads. Light transmission will depend on the extent to which daylight is used to obviate artificial lighting.
Some solar control glasses can be toughened or heat strengthened and gives a means of raising the design stress and ensuring safety from thermal fracture. The following are the common varieties.
NOTE — Some manufacturers use this as a descriptive code, quoting a light transmission figure followed by a total solar energy transmission figure.
a) Body tinted glass (for increased absorption) — Solar control properties and colour vary with the thickness of the glass. When used in insulating glass units, body tinted glass should be positioned as the outer pane because the energy due to the absorbed radiation is more easily dissipated to the outside.
b) Reflective coated glass — It uses the principle of increasing the direct reflection to maximize solar energy attenuation. In comparison with clear glass, its absorption of solar energy is also increased. An informative annex on test requirements is given at Annex B.