Roof reflectivity, emissivity and absorptance in equest

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Hi All,

ASHRAE refers to roof properties in terms of reflectance, emittance, and SRI
(Appendix G Table G3.1 5c). However, eQuest uses absorptance. I am unable to
find in the eQuest input areas (wizard and detailed) where there is a means
to input values other than absorptance. Is there a conversion factor, or a
place to input reflectance, and emittance or SRI that I am unable to find?

Many thanks for your comments.

Best regards -

Jeff Ross-Bain, PE, LEED AP BD+C, BEMP
Principal

404-220-8940 Office

404-220-8955 Direct

www.rbgb.com

Build Green. Save Time

Jeff Ross-Bain3's picture
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Jeff,

For an opaque surface, you can just convert reflectance to absorptance by:
absorptance = 1 - reflectance. This is the absorptance in the solar
spectrum. The emittance (in IR spectrum) is a little more hidden. In
detailed mode, if you select an exterior roof and look under the
"Daylighting - Shading - Other" you'll see the Outside Emissivity input.
It sounds like you know where to input the absorptance, but just in case,
it is under the roof construction in detailed mode. SRI is a calculated
value used to compare roof materials and so is not useful for energy
modeling software.

Aaron

Hi All,

ASHRAE refers to roof properties in terms of reflectance, emittance, and
SRI (Appendix G Table G3.1 5c). However, eQuest uses absorptance. I am
unable to find in the eQuest input areas (wizard and detailed) where there
is a means to input values other than absorptance. Is there a conversion
factor, or a place to input reflectance, and emittance or SRI that I am
unable to find?

Many thanks for your comments.

Best regards -

*Jeff Ross-Bain, PE, *LEED AP BD+C, BEMP

Aaron Powers2's picture
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Jeff,

Keep in mind, ASHRAE 90.1 baseline roof reflectance value is 30%. If the project design calls for a highly reflective roof (i.e. reflectance, emittance, and SRI) then a 45% reflectance is modeled in eQUEST rather than the specified reflectance per the design documents.

Thanks,

David W. Griffin II

David Griffin's picture
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For 90.1/LEED documentation purposes, this is all spelled out in Table G3.1 under the envelope piece, and that passage includes the cited SRI thresholds at which you can model an improved reflectance.

~Nick

[cid:489575314 at 22072009-0ABB]

NICK CATON, P.E.

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