What is thermal radiation?
Thermal radiation is the transfer of heat by electromagnetic waves, mostly in the infrared range, emitted by every object according to its surface temperature. Along with conduction and convection it is one of the three ways heat moves. The key difference is that radiation needs no medium: that is how the sun’s heat reaches the earth through empty space. The Dutch term is warmtestraling.
How much an object radiates depends on its surface temperature and on the material, expressed as its emissivity ε (between 0 and 1). Most building materials, such as brick, concrete and timber, have a high emissivity of around 0.9 and so radiate heat readily. Polished aluminium is around 0.1, oxidised aluminium around 0.4. The ideal radiator is a black body, with ε = 1.
The emitted heat flux follows the Stefan-Boltzmann law: q = ε × σ × T⁴, where σ ≈ 5.67 × 10⁻⁸ W/m²K⁴ and T is the absolute temperature in kelvin. Because of the fourth power, radiation rises steeply as temperature goes up.
Radiation in buildings
- Comfort. How warm you feel depends not only on air temperature but also on the temperature of the walls, windows and floor around you. Sitting next to a cold window feels chilly even when the air is warm.
- Infrared panels heat mainly by radiation, warming people and surfaces directly.
- Reflective foil cuts radiant heat loss, but only when it faces an air gap.
In practice
- Face reflective foil towards an air gap. Pressed against another material, its low emissivity no longer does anything.
- Keep foil clean and bright. Dust and oxidation raise emissivity and reduce the benefit.
- Warm up cold surfaces with insulation or better glazing; that often improves comfort more than turning up the thermostat.
- Take care with thermal imaging. Shiny metal surfaces reflect and emit little, so an infrared camera shows a misleading temperature.
Common mistake: treating reflective foil as a full insulation layer. Without an air gap, or with a dusty surface, it adds little thermally.
Related terms
- Conduction
- Convection
- Reflective foil
- Reflection, absorption, transmission
- Infrared panel
- Infrared photo
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