Careful attention needs to be paid to choose the right glass. The choice of the right glass can confer thermal insulation and thereby contribute to comfort for all seasons, day and night!
Whenever differences in temperature exist between surfaces, heat will migrate from the warmer area to the cooler area. This is true of all surfaces. However, a glazed surface is special in that it is also transparent to solar radiation which results in free heat gain.
Heat Exchanges through a surface
Heat is exchanged through a surface and hence lost in any of 3 different ways:
The heat flow between the two faces of a sheet of glass depends on the temperature differences between the faces and the thermal conductivity of the material.
The thermal conductivity of glass is λ = 1.0 W/(m.K)
- Conduction is the transfer of heat within a body or between two bodies in direct contact. No material is physically moved during this type of transfer.
- Convection is the transfer of heat between the surfaces of a solid and a liquid or a gas. This type of transfer involves movement via circulation.
- Radiation is the transfer of heat by radiation between two bodies at different temperatures.
At ambient temperature, this radiation takes place in the infra-red band of the spectrum, at wavelengths above 5 µm. It is proportional to the emissivity of these bodies.
Emissivity is related to the surface characteristic of a body. The lower the emissivity, the weaker the heat transfer
The normal emissivity εn of glass is 0.89. Certain types of glass can be modified by means of a low emissivity coating, in which case εn can be less than 0.10.
Thermal transmittance of a surface
Heat transmittance through a surface by conduction, convection and radiation is expressed by its U-value.
This is the rate of heat loss per square metre for a temperature difference of 1 degree Kelvin, or Celsius, between the interior and exterior.
It is possible to calculate a specific U-value by using design values of the surface exchange co-efficients, which will take into account environmental variants, such as wind speed.
The lower the U-value, the lower the heat loss.
The U-value of glazing
The principle behind double glazing is that by enclosing a cavity of dry still air between two sheets of glass, heat exchange by convection is reduced and the low thermal conductivity of the air limits heat loss by conduction.
For eg: 6 mm clear glass in single glazing configuration has a U-value of 5.7 W/sq.m K. If the clear glass is used in a standard double glazing configuration of 6 mm clear glass – 12 mm air filled cavity – 6 mm clear glass, the U-value drops to 2.8 W/sq.m K
Improving the U-value of windows
Improving the U-value means reducing the transfer of heat by conduction, convection and radiation.
Since it is impossible to alter the internal and external heat transfer co-efficients, any enhancements are brought about by reducing heat exchange between the two glass components of the double-glazed-unit:
- Radiated heat transfer can be reduced by using glass with a low-emissivity or low-e coating.
- Capitalizing on this concept, Saint-Gobain has developed a range of low-emissivity coated glasses which provide enhanced thermal insulation:
- SGG Planitherm
- SGG Nano
- SGG Nano Silver
- SGG Envision
- SGG Xtreme
- Heat loss by conduction and convection can be further reduced by replacing the air in the cavity with a gas with lower thermal conductivity (generally argon)