Reading documentation thermal absorptance = thermal emissivity, but that solar absorptance is inverse of reflectance, and the same for visible.  For visible, I think that one a decent estimate is to subtract 0.1 from solar. Â
From http://energy.lbl.gov/coolroof/ref_01.htm "Most building materials (including glass!) are opaque in this part of the spectrum, and have an emittance of roughly 0.9. Materials such as clean, bare metals are the most important exceptions to the 0.9 rule. Thus clean, untarnished galvanized steel has a very low emittance, and aluminum roof coatings have intermediate levels of emittance."Â
Personally I see very little reason to ever change the emmisivity because bare metals are not used very often due to corrosion issues or high cost of metals that will not corrode- basically unless you have a stainless steal building element there is no reason to diverge from the default.  So then-- I tend to keep thermal absorptance set to 0.9 (default value) for simplicity.Â
Here is what I typically use:Â
| Name | OutsideRoof_BaselihneLowSlopeRoof_SRI78 | OutsideRoof_LowSlopeRoof_SRI85 | OutsideRoof_BaselineSteepSlopeRoof_29-SRI | OutsideRoof_SteepSlopeRoof_49-SRI |
| Roughness | VeryRough | VeryRough | VeryRough | VeryRough |
| Thickness | 0.005 | 0.005 | 0.005 | 0.005 |
| Conductivity | 0.5 | 0.5 | 0.5 | 0.5 |
| Density | 500 | 500 | 500 | 500 |
| Specific Heat | 500 | 500 | 500 | 500 |
| Thermal Absorptance | 0.9 | 0.9 | 0.9 | 0.9 |
| Solar Absorptance | 0.36 | 0.3 | 0.72 | 0.58 |
| Visible Absorptance | 0.36 | 0.3 | 0.72 | 0.58 |
Below is a help desk ticket:Â
The physicists may quibble, but for EnergyPlus purposes we are using grey radiant exchange and emit=absorb within a given range of wavelenghts. What you typically see, however is a difference between solar absorptance and thermal absorptance (emittance) because they cover a different mix of wavelengths.
Mike
> The I/O Reference, on PDF page 122, tells me that:
> "Field: Thermal Absorptance
> The thermal absorptance field in the Material input syntax represents
> the fraction of incident
> long wavelength radiation that is absorbed by the material. This
> parameter is used when
> calculating the long wavelength radiant exchange between various
> surfaces and affects the
> surface heat balances (both inside and outside as appropriate). For
> long wavelength radiant
> exchange, thermal emissivity and thermal emittance are equal to
> thermal absorptance..."
>Â
> â??Emitâ?? and â??absorbâ?? have different definitions in my
> vocabulary, so I'm writing to ask for confirmation that the descrption
> above is correct. Thanks in advance!
Michael J. Witte, GARD Analytics, Inc.
Mike
> The I/O Reference, on PDF page 122, tells me that:
> "Field: Thermal Absorptance
> The thermal absorptance field in the Material input syntax represents
> the fraction of incident
> long wavelength radiation that is absorbed by the material. This
> parameter is used when
> calculating the long wavelength radiant exchange between various
> surfaces and affects the
> surface heat balances (both inside and outside as appropriate). For
> long wavelength radiant
> exchange, thermal emissivity and thermal emittance are equal to
> thermal absorptance..."
>Â
> â??Emitâ?? and â??absorbâ?? have different definitions in my
> vocabulary, so I'm writing to ask for confirmation that the descrption
> above is correct. Thanks in advance!
Michael J. Witte, GARD Analytics, Inc.
And E+ documentation indicating that emmisivity = absorbence.Â
Field: Thermal Absorptance
The thermal absorptance field in the Material input syntax represents the fraction of incident
long wavelength radiation that is absorbed by the material. This parameter is used when
calculating the long wavelength radiant exchange between various surfaces and affects the
surface heat balances (both inside and outside as appropriate). For long wavelength radiant
exchange, thermal emissivity and thermal emittance are equal to thermal absorptance.
Values for this field must be between 0.0 and 1.0 (with 1.0 representing â??black bodyâ??
conditions).
Field: Solar Absorptance
The solar absorptance field in the Material input syntax represents the fraction of incident
solar radiation that is absorbed by the material. Solar radiation includes the visible spectrum
as well as infrared and ultraviolet wavelengths. This parameter is used when calculating the
amount of incident solar radiation absorbed by various surfaces and affects the surface heat
balances (both inside and outside as appropriate). If solar reflectance (or reflectivity) data is
available, then absorptance is equal to 1.0 minus reflectance (for opaque materials). Values
for this field must be between 0.0 and 1.0.
Field: Visible Absorptance
The visible absorptance field in the Material input syntax represents the fraction of incident
visible wavelength radiation that is absorbed by the material. Visible wavelength radiation is
slightly different than solar radiation in that the visible band of wavelengths is much more
narrow while solar radiation includes the visible spectrum as well as infrared and ultraviolet
wavelengths. This parameter is used when calculating the amount of incident visible radiation
absorbed by various surfaces and affects the surface heat balances (both inside and outside
as appropriate) as well as the daylighting calculations. If visible reflectance (or reflectivity)
data is available, then absorptance is equal to 1.0 minus reflectance (for opaque materials).
Values for this field must be between 0.0 and 1.0.
â??â??
Jeremiah D. Crossett  | Senior Analyst  | LEED Green Associate   ![]()
On Thu, Jul 10, 2014 at 4:32 AM, Asit Mishra asitkm76@xxxxxxxxx [EnergyPlus_Support] <EnergyPlus_Support@xxxxxxxxxxxxxxx> wrote:
ÂIn other words, all the values are being kept high (> 0.7) ?I was incorrectly interpreting then that Absroptance = 1 - reflectance and hence I was giving low values to these properties.Thanks for correcting my erroneous assumption.Regards,asitOn Thu, Jul 10, 2014 at 4:48 PM, Jean Marais jeannieboef@xxxxxxxxx [EnergyPlus_Support] <EnergyPlus_Support@xxxxxxxxxxxxxxx> wrote:
ÂVisual = ? ca. 0.75 to 0.95 (very little energy in this bandwidth so errors can be tolerated as they have comparitively small effect)Eg.
Cool paint AK-103Solar Absorbtance = 0.7324 (Solar Direct Reflectance is incorrectly named)Thermal Absorbtance = 0.939
2014-07-07 14:46 GMT+02:00 Asit Mishra asitkm76@xxxxxxxxx [EnergyPlus_Support] <EnergyPlus_Support@xxxxxxxxxxxxxxx>:
ÂDear Jeremiah,             I am very thankful for the two spreadsheets. They will be very useful in my work.ÂI was in particular referring to the paints that have been called cool roof paints.These are a few values for solar direct reflectance as quoted from a database provided by US-India Joint Center for building Energy Research and Development. (I am attaching the original document as well)Thermatek Heat Reflective Paint:  0.8973Cool paint AK-103, Aroma paints: 0.7324Sun cool - LHP coating: 0.9097Actually, when I started out, I did what Jean had mentioned - change the reflectivity/solar absorptance etc. of the outermost layer for a wall or roof. The introduction of a thin filmÂwas just for convenience so that I could make it an outermost layer in any kind of wall without having to change properties of standard layers.ÂEither way, results still did not show much change in the year round indoor temperatures, i.e. before and after use of the reflective coats.ÂRegarding emmisivity, I wanted to know exactly which property to control to change emissivity.The typical cool roof paints have a high reflectivity and a high emissivity as well.ÂLooking at a typical material specification, I am unable to see how I change this for a surface/the outermost layer.example Material,  A1 - 1 IN STUCCO,     !- Name  Smooth,          !- Roughness  2.5389841E-02,      !- Thickness {m}  0.6918309,        !- Conductivity {W/m-K}  1858.142,         !- Density {kg/m3}  836.8000,         !- Specific Heat {J/kg-K}  0.9000000,        !- Thermal Absorptance  0.9200000,        !- Solar Absorptance  0.9200000;        !- Visible AbsorptanceI am sorry if my question is not making full sense. And thank you both for your immediate response and help.Regards,
asitOn Mon, Jul 7, 2014 at 9:46 AM, Jeremiah Crossett jcrossett@xxxxxxxxxxxxxxx [EnergyPlus_Support] <EnergyPlus_Support@xxxxxxxxxxxxxxx> wrote:
ÂI  my experience unfinished metal is the only thing that warrants using anything far off from default, and only modify absorbence for parametric studies.  Attached is a spreadsheet you can find online that can be used to fit product data into E+, just remember that solar [and visible] absorbence is the inverse of the reflectance info you can get from manufactures.  Also attached is a spreadsheet where  absorbence / reflectance = albedo..Hope this helpsÂâ??â??Jeremiah D. Crossett  | Senior Analyst  | LEED Green Associate Â
On Sun, Jul 6, 2014 at 6:59 PM, Asit Mishra asitkm76@xxxxxxxxx [EnergyPlus_Support] <EnergyPlus_Support@xxxxxxxxxxxxxxx> wrote:
ÂasitRegards,Please adviseSecondly, normally what I have seen in material specifications, while values for absorptivity can be specified, there was no obvious way to specify a high emissivity (apart from indirectly doing so using surface roughness).This did not seem to produce desired resultsHello,        I would like to have some idea on how you all might have approached simulating the effect of a reflective coat of paint. The way I went about it is defined a material with low absorptivity and added a thin layer (0.5 mm) to the roof/wall construct.
__._,_.___
Posted by: Jeremiah Crossett <jcrossett@xxxxxxxxxxxxxxx>
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