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Thermal Insulation - The Effect of Surface Emittance on Heat Flow

EMITTANCE: The ability of a material's surface to release it's heat to an adjacent air space.

AIR SPACE
HIGH-EMITTANCE SURFACE: Most building materials, unpolished metals, painted surfaces, mass-type insulations, fibers, non-metalized plastics, wood, etc.

 

Readily releases MOST (80% to 90%) of it's heat to the adjacent air space.

AIR SPACE
LOW-EMITTANCE SURFACE: Polished pure aluminum, silver, gold.

 

Releases VERY LITTLE (2% to 5%) of it's heat to the adjacent air space.

In the above illustration the heat source can be a solar-heated roof or siding, sheet rock, heated water, cement block, etc. Because they are in contact, heat will conduct from the source to the emitting surface causing the surface temperature of the emitter to rise. But the low emittance surface will release very little of this heat to the adjacent air space.

For that reason, AIR-TO-AIR thermal values are far more important than surface temperatures when measuring the performance of a reflective insulating system.