Frequently Asked Questions

FAQs » EMISSIVITY

Question: What are emissivity and thermal emittance?

Answer:

Emissivity is a measure of the thermal emittance of a surface. Emissivity is defined as the ratio of radiant heat flux emitted by a material to that emitted by a blackbody radiator at the same temperature. Emissivity values range between 0 and 1. Emissivity may be thought of as the ability of a material to emit heat (via infrared radiation) to the surrounding atmosphere. Higher numbers indicate faster heat transfer. There are standardized methods to measure the thermal emittance of roofing materials like ASTM C1371 and ASTM E408.

Question: What are reflectivity and solar reflectance?

Answer:

Reflectivity is a measure of the solar reflectance of a surface. Reflectivity is defined as the ratio of the reflected solar radiation flux to the incident flux. Reflectivity values range between 0 and 1 with the higher number indicating higher reflectivity. This is also sometimes expressed as a percentage from 0% to 100%. A more reflective roof will reflect the solar radiation away from the surface rather than absorbing it. Absorbed solar radiation will increase the surface temperature. There are standardized methods to measure the solar reflectance of roofing materials like ASTM E903 and ASTM E1918.

Question: Give me some practical examples of roofing materials that are cool, warm and hot in a warm climate.

Answer:

Table I Roof Surface Temperature in Oak Ridge, TN August 14, 2000

Material	Reflectivity	Emissivity	Max. Surface	Temp. (°F)
White prepainted metal roof	.64	.83	123	Cool
Unpainted metal roof	.64	.08	142	Warm
Asphalt Shingle	.09	.91	164	Hot

Remember that emissivity and reflectivity both play a part in the surface temperature of a roof.

Question: Why does a white prepainted metal roof have a much higher emissivity than an unpainted metal roof?

Answer:

Question: Does metal thickness affect emissivity?

Answer:

No. This is a property of the roofing surface itself.

Question: Should emissivity be a part of requirements in such programs as Energy Star?

Answer:

The EPA does not currently require emissivity as part of their program. This follows the logic of Oak Ridge National Laboratory that the desired radiation properties for a roof should be based on an analysis of the local situation, rather than on the generality that "high reflectivity, high emissivity" is good for everyone, everywhere. With the variations achievable with metal roofing, properties can be "tailored" to make economic, energy-efficient sense for the specific location.

Question: Can you comment on the disadvantages of non-metal roofing systems compared to prepainted metal roofing?

Answer:

The success of cool roofs relies on maintaining the reflectivity of the roofing system. Energy savings are not necessarily constant over the lifetime of the roof system, as the solar reflectivity of a roof surface changes. Color changes caused by aging, dust, air-borne pollutants, water ponding, and the effects of biological attack can all reduce the performance of non-metal Cool Roofing (e.g. EPDM, sprayed-on coatings). High levels of maintenance and/or \cleaning are critical to maintaining a highly reflective, cool surface for non-metal roofing systems. In contrast, unpainted and prepainted metal roofing systems shed dirt and do not have problems with biological attack. Unpainted and prepainted metal roofs are able to maintain their reflectivity over longer periods of time with minimal maintenance. In addition, metal roofing has a significant recycled content, is recyclable, sustainable, and has a much longer life span compared to most non-metal roofing systems.