Radiation and Thermal Control

Solar radiation falls at varying intensities across a range of wavelengths:

About 5% of solar energy that hits the earth does so as destructive UV light — this means that coatings need to be as transparent as possible to UV
Around 45% of solar energy falls as visible light and is absorbed or reflected by the coating. We see the reflected light as the colour and brightness of the surface.
Approximately 50% of solar energy is in the near infrared region. This, too, is partially reflected or absorbed. The absorbed energy heats the coating which then re-emits part of this energy in the longer wave length thermal infrared.
The remainder is conducted into the body of the roof.

Solar energy which is not reflected, re-emitted or carried away by convection currents is conducted into the interior.
If absorbed heat is not re-emitted efficiently externally, the roof temperature rises so that more heat is transmitted internally and conducted through to the interior surface.
The interior organic coating will behave like the exterior coating when the conducted energy from outside meets it. The coating will first absorb the energy and then re-emit it into the inside of the room as energy in the thermal or mid infrared region.
The greater the heat flux into the interior, the harder air conditioning must work to maintain a steady temperature.

Thermal properties of typical roofing materials
	Initial Solar Reflectance	Infrared Emittance
Metal (unpainted)	0.60 – 0.80	0.04 – 0.10
Metal (painted and granular coated)	0.10 – 0.75 *	0.75 +
Comp asphalt shingles	0.05 – 0.25	0.90
Modified bitumen	0.05 – 0.25	0.90
Built-up roofing	0.05 – 0.80	0.90
Concrete/clay tile	0.20 – 0.70	0.90
White single-ply membrane	0.70 – 0.80	0.85+
	* depending on color
Source: ORNL and LBNL, Cool Metal Roofing Coalition www.coolmetalroofing.org

Unpainted metal reflects well — but gets very hot because it is poor at re-emitting heat: paint is vital to keep roofs cool.