Prevention is better than cure

The Sun is constantly flooding the earth with huge amounts of energy. This energy comprises of 4 main bands; UV (ultraviolet), visible light, near infra-red (shortwave) radiation and infra-red (longwave) radiation. Of the energy that reaches the earth it is primarily made up of visible light and near infra-red (shortwave) radiation with only 2-3% UV radiation.

When the suns near infra-red (shortwave) radiation strikes glass, or any object for that matter, 3 key things happen.

1. Some of the suns’ energy is directly transmitted through
2. Some of the suns’ energy is reflected
3. Some of the sun’s energy is absorbed

The level of transmission, reflection and absorption will depend on the type of glass.

The energy that is transmitted through the glazing, when it hits an object inside the room such as a chair, desk, wall etc is absorbed and re-radiated as longwave infra-red 'heat' rays. The same applies to the sun’s energy which was absorbed by the glass, it is converted to infrared 'heat' rays and some of this is re-radiated into the building, the rest back into the atmosphere.

The total solar ‘heat’ radiation entering the building is a combination of what is directly transmitted through the glass (ie is going to be converted to heat) and the element of radiation that is absorbed by the glass and re-radiated inwards.

Clear glass is virtually transparent to the incoming shortwave radiation but when this radiation is converted by objects to longwave 'heat' rays, glazing becomes opaque to it, causing the ‘heat’ rays to remain trapped in the building, warming it up, which happens particularly quick in all glass buildings such as conservatories, orangeries etc and is why this process is known as the greenhouse effect.

How external blinds prevent…

Unlike glazing which is virtually transparent to the suns radiation, external blind fabrics are not.

When an external blind is employed in front of the glazing, it blocks a large portion of the incoming shortwave solar radiation as the majority of the suns energy is either reflected, in the case of lighter colour fabrics (hence why cricketers wear white) or absorbed by the material (darker colours). The radiation that is absorbed by the material and re-radiated as longer infra-red ‘heat’ rays towards the glazing does not transmit through the glass and this heat between the glass and fabric is simply dissipated as hot air rises.

By stopping shortwave radiation reaching the glazing, excessive solar heat gain is prevented, with the resultant effect of a naturally air conditioned internal environment, with minimum load on any installed air conditioning units and a lower energy bill along with a reduction in a buildings carbon emissions. In many cases, air conditioning of any description becomes superfluous.

See examples of just how much solar gain can be prevented by exterior blinds in combination with various glazing types in our related article here.