In the previous post, types of heat entering a building was discussed, of which solar heat comprises around 80% and the rest is non-solar heat. It becomes very important to control non-solar heat as well even though it contribute to only 20% heat entering a building, especially in buildings where there is 24 x 7 operations and households, so as to bring down the energy consumption during night time. In this post, non-solar heat and how all to control it will be discussed in detail.
Non-solar heat is mainly transferred in three ways- conduction, convection and radiation, and is measured in terms of U-value (W/m2.K).U-value could be defined as the amount of heat in watts per unit of time, transferred through 1 sq.m of glass per 1 degree Kelvin temperature difference between exterior and interior. When there is a difference in temperature across a material, heat gets transferred through the process of conduction, just as what happens when a metal rod gets heated. Glass is considered to be a good conductor of heat, and the coefficient of thermal conductivity of glass is 1.05 W/m2.C.
When hot air molecules collides with glass, glass gets heated up and results in heat transfer through conduction, also when the cold air molecules inside collides with heated glass, again a heat transfer happens. This heat transfer through air molecules is called convection.
The most important and the larger part amongst non-solar heat transfer happens through radiation. Short Wave Infra Red radiations from the sun gets absorbed by various objects and these objects get heated up to 80 degrees. The objects then re-emit Long Wave Infra Red radiation, which amounts to 70% of heating caused by non-solar heat transfer.
To control non-solar heat transfer through conduction, the best and easiest solution is to use a thicker glass. A 12 mm clear glass has a U-value of 5.5 W/m2.K, whereas a 6 mm clear glass has a U-value of 5.7 W/m2.K.
So as to control the non-solar heat control caused by convection, the ideal solution would be to use an Insulated Glass Unit / Double Glazed Unit (IGU/DGU). Larger the air gap in the IGU, lesser would be the U-value. A further reduction in the heat transfer caused by convection could be achieved by using inert gases inside the IGU (Argon and Kripton).
The best solution to control non-solar heat caused by Long Wave Infra Red radiation is to use a low-e coated or thermal insulating glass. The thermal insulating properties of such glasses block Long Wave Infra Red radiation.
Non-solar heat control is secondary, as it is most important to decide on the required light transmission and then go for maximum solar heat protection. A very low U-value glass configuration might also result in a heat trap inside the building, so it is ideal to go for a moderate U-value, especially in hot and humid climatic conditions.