The following is assembled to explain by example the effect of emittance on surface temperature.

The emittance of a surface is the ratio of the radiant heat flow of the surface to the radiant heat flow of an ideal surface. Scientists call such an ideal surface a "black body". A shiny metal surface can have an emittance less than 0.1. A dark, dull coating can have an emittance above 0.9. Most of Childers colored jacketings have emittances near 0.8. The emittance of Galvalume has been measured as 0.06, while T-304 S.S. - stucco embossed was measured as 0.17.

A 6 inch horizontal pipe at 400°F insulated with 3 inches of foamglas with Galvalume jacketing would have a surface temperature of about 150°F in still 100°F air. That same pipe with a federal blue coated jacket would have a surface temperature of 123°F - a difference of 27°F and under the OSHA critical temperature for worker protection.

A 6 inch horizontal pipe at -250°F insulated with 3 inches of Koolphen K and Galvalume jacketing would have a surface temperature of 63°F in still 80°F air. That same pipe would have a 73°F surface temperature with humble grey coated jacketing - a difference of 10° in condensation protection.

The higher emittance (darker) material will have a surface temperature closer to the air temperature. The difference is due to a higher level of radiant heat transfer between the surface and its surroundings. The lower surface temperature of high emittance materials in hot services are obtained at the expense of insulation efficiency, and vice versa for cold service. The selection strategy depends on the reason for the insulation. If condensation prevention is the main reason for the insulation, then by all means use a black mastic. If cost of refrigeration is paramount, then use metal jacketing. In the examples above, the energy "costs" of using the coated jacketing were 8% and 4% respectively. Remember that an insulation system with a higher "r", or with a significant air velocity will be less effected by the emittance of its surface.

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