The ability of a material to radiate energy is known as emissivity. In general, highly reflective materials have a low emissivity and dull darker coloured materials have a high emissivity. Low emissivity (low-e) refers to a surface condition that emits low levels of radiant thermal infrared (heat) energy.
Surface emissivity is the amount of infrared energy emitted by a specific material and is often expressed as a ratio or percentage of “1” with the “1.0” value being a perfect “black body”. A material that emitted heat energy at half that rate, like polyester film, would have a rating of .50 or 50%.
Objects like aluminium or copper, with low surface emissivity (low-e) do not transmit/radiate heat well because most of the infrared energy is reflected instead of absorbed. Aluminium (shiny foil faced sisalation) only absorbs around 3% of the radiant energy hitting it, reflecting the other 97% away from its surface.
In the insulation industry, surface emissivity is a major factor in the effectiveness of any product, yet one often overlooked and overshadowed by thermal conductivity or “k values”. Both are important in that they address different methods of heat transfer (infrared radiation vs conduction) but in some applications, like roof insulation in warm climates, infrared radiation plays a bigger role than conduction, and low surface emissivity is key.
Low-emissivity coatings (silver or tin oxide) are also used to minimise the amount of ultraviolet and infrared light that can pass through glass without compromising the amount of visible light that is transmitted. All materials, including windows, radiate heat in the form of long-wave, infrared energy depending on the emissivity and temperature of their surfaces. Radiant energy is one of the important ways heat transfer occurs with windows. Reducing the emissivity of one or more of the window glass surfaces improves a window’s insulating properties (albeit minimally in the greater context of holistic whole-building performance).
Low emissivity products are also useful in roof insulation by minimising the amount of infrared energy that is radiated downwards into roof spaces and into the habitable spaces of dwellings. Sisalation is a product that we are familiar with in the South African context although foil-faced insulatory products are commonly used worldwide (Kingspan, Celotex, etc).
Solar Horizontal Irradiance across South Africa: