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Community and Q&A

Metal Roof SRI ratings

Peter L | Posted in Energy Efficiency and Durability on

If a “cool roof” design has a SRI of 40 would going for a SRI of 54 make that much more of a difference?

Does having a cool roof help with the R-Value of roof assembly? According to the manufacturer they quote ORNL as saying it does help with summer time A/C loads, especially in high-desert climates.

Replies

  1. GBA Editor
    Martin Holladay | | #1

    Peter,
    A "cool roof" saves energy when installed on a poorly designed home. It won't really save any energy when installed on a well-designed home.

    The classic situation that might call for a "cool roof" is a hot-climate house with ductwork in an unconditioned attic. If you have such a house, you might consider installing a "cool roof."

    If you follow GBA recommendations -- and all of your ductwork is located inside your home's conditioned envelope, and you have ceiling insulation that at least meets minimum code requirements -- you will see hardly any benefit from a cool roof.

  2. Charlie Sullivan | | #2

    As an addendum to Martin's correct answer, a cool roof also make a contribution to lowering the urban heat island effect. If you live in a hot climate urban area, using a cool roof is an important contribution to mitigation of that problem.

  3. D Dorsett | | #3

    A cool roof also has the intended consequence of wintertime lower roof deck temps, which becomes a (very) modest increase in space heating load, but a more significant increase in high wintertime moisture accumulation in the roof decks for unvented insulated roof assemblies.

    With very low pitch roofs there is still a cooling season energy use advantage to having a cool roof even at an IRC code minimums, but from 3:12 or higher that advantage fades very quickly. On large flat roof commercial buildings it's still "worth it", even in cold climates.

  4. Peter L | | #4

    Would a cool roof with a high SRI help with RADIANT HEAT? For example, say there was a fire nearby and the heat from the flames creates radiant heat. Would a cool metal roof with a high SRI help to mitigate that radiant heat from penetrating into the roof sheathing/building?

  5. Expert Member
    Dana Dorsett | | #5

    The cool roof absorption/emissivity profiles are in the deep infra-red- the same spectrum as say, a forest fire. Only about half the solar spectrum energy is in the deep infra-red, the rest is in the visible and UV spectrum. Cool roof's work by re-radiating the deep infra-red back into the cold radiant-temp sky, while reflecting a good chunk of the near-infra-red, UV, & visible spectra.

    So the short answer is, no, a cool roof with an SRI of only 10 won't reflect very much of the heat radiating from a nearby fire. It is not a low-E coating with broad infra-red reflectance- it is designed to be a deep infra-red absorber & emitter. A titanium-white high SRI finish would reflect a good portion of forest or house fire spectrum though.

    But any metal roof has a lot to offer for keeping burning embers falling from that fire from igniting your roof!

    http://www.intechopen.com/source/html/38093/media/image6.jpeg

    http://science.nasa.gov/media/medialibrary/2000/09/05/sunspectrum_resources/spectrumgif.gif

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