This page contains questions about emissivity and how it relates to cool roofing. If you would like additional information or have other questions, please contact MBMA by clicking here.

Emissivity questions:

What are emissivity and thermal emittance?

Whate are reflectivity and solar reflectance?

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

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

When a roof panel emits infrared radiation, does it emit only to the sky or does it also emit radiation in the other direction, that is, into the building?

Does metal thickness affect emissivity?

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

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

Should energy codes for roofing be the only governing factor regarding what kinds of roofing material can be installed on a building.

What are emissivity and thermal emittance?

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.

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What are reflectivity and solar reflectance?

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.

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

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

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

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

The physical characteristics of paint made from organic and inorganic materials are much different than the characteristics of a metallic material.  White paint is a better emitter of radiation than bare metal.  The controlling characteristic is the surface finish and not the underlying material.

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When a roof panel emits infrared radiation, does it emit only to the sky or does it also emit radiation in the other direction, that is, into the building?

The roof panel emits radiation in both directions.

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Does metal thickness affect emissivity?

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

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

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.

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

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.

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Should energy codes for roofing be the only governing factor regarding what kinds of roofing material can be installed on a building.

No.  There are many other factors that should be considered when developing mandatory codes or incentive programs that influence what types of roofing material can be used for buildings.  Recycled content, recyclability, sustainability and durability should be included as part of a balanced approach to any building code or incentive program.  Otherwise, one may fill landfills with non-recyclable roofing materials that are "cool," but have short life spans.  Metal roofing is recyclable and sustainable.  Metal roofing’s life span is much longer compared to most non-metal roofing systems.

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