Back in the 1800s a British-German astronomer named William Herschel told the world about infrared radiation. Then a few other guys like Josef Stefan and Ludwig Boltzmann figured out the energy exchange between two surfaces had a constant, which found its way into a now famous law called the Stefan-Boltzmann equation.ii A form of this formula can be used to calculate the net exchange of radiative heat (Qrad, Watts) between, for example, the roof of a building and the sky.
Qrad = εσA(∆T4)
ε = surface emissivity of the roof
A = roof area, m²
σ = the Stefan-Boltzmann constant (5.6704 × 10 -8 W / m² ∙ K4)
ΔT = temperature differential between the sky and roof, K [Kelvin = ( TºC + 273,15)]