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Re: [EnergyPlus_Support] Properties of roof sheets
The EnergyPlus datasets are primarily based on ASHRAE Handbook data which gives
very little guidance regarding thermal emissivity and solar absorpance. In the
datasets, the thermal emissivity is 0.9 for all materials. Beware of using
values for "new" galvanized steel. Things weather and get dirty, so the new
value does not apply for the long term.
The Florida Solar Energy Center (FSEC) has published several reports regarding
roof properties and their effect on cooling loads. A list of publications may
be found at:
http://www.fsec.ucf.edu/bldg/resources/publications/index.htm
Two especially pertinent reports are:
http://www.fsec.ucf.edu/bldg/pubs/cr670/index.htm - Lots of measured property
data
http://www.fsec.ucf.edu/bldg/pubs/cr1336/cr1336.htm - Table 1 lists solar
reflectances for six new and weathered materials
Ned and Agas, would you please send me citations for the sources you are using
for property data?
Mike
On 4 May 2004, at 15:13, Edward Lyon wrote:
> I have previously noted that there are lot of "differences" between
> properties in material libraries and available reference data. Most
> often I find that insulation values are not adjusted for thermal
> bridging elements. I would check manufacturer data for specific
> products. There is also an LBL web site with tested values for
> different types of roofing materials. You are on the right track,
> always question the reasonableness of the library data....
>
> Ned Lyon
>
> Senior Project Manager
> Simpson Gumpertz & Heger Inc.
>
> 41 Seyon Street
> Building #1, Suite 500
> Waltham, MA 02453
>
> Phone: 781-907-9000
> Fax: 781-907-9009
>
>
> >>> agas@xxxxxxx 5/3/2004 10:41:06 AM >>>
> Could someone clear up the different thermal properties of roof
> sheets.
> I have data from a McGraw-Hill book with a chart indicating the
> following for new galvanised sheet:
>
> Solar reflectance 0.35
> Solar absorbtance (inverse of refl.) 0.65
> Far IR emissivity 0.15-0.23
> Far IR reflectance (inverse of emissivity) 0.77--.85
>
> BLASTMaterials.IDF (included in E+ DataSets directory) has the
> following:
>
> MATERIAL:Regular,METAL - GALVANIZED STEEL 1 / 16 IN, !- Material
> Name
> Smooth, !- Roughness
> 1.6000000E-03, !- Thickness {m}
> 45.31000 , !- Conductivity {w/m-K}
> 7833.020 , !- Density {kg/m3}
> 500.0000 , !- Specific Heat {J/kg-K}
> 0.9000000 , !- Thermal Emittance
> 0.2300000 , !- Solar Absorptance
> 0.2300000 ; !- Visible Absorptance
>
> This seems to be quite different. What are the correct figures to use?
>
> What are the relevant figures for different coloured painted steel
> corrugated roof sheets, e.g. white, light green ?
>
> regards,
>
>
>
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========================================================
Michael J. Witte, GARD Analytics, Inc.
EnergyPlus Testing and Support
EnergyPlus-Support@xxxxxxxx
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