Re: [EnergyPlus_Support] simulating a coat of reflective paints (cool roofs) [1 Attachment]

DearÂ Jeremiah,

Â Â Â Â Â Â Â Â Â Â Â Â Â I am very thankful for the two spreadsheets. They will be very useful in my work.Â

I 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 aÂ databaseÂ provided by US-India Joint Center for building Energy Research and Development. (I am attaching the original document as well)

Thermatek Heat Reflective Paint: Â 0.8973

Cool paint AK-103, Aroma paints: 0.7324

Sun cool - LHP coating: 0.9097

Actually, 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 filmÂ

was just forÂ convenienceÂ so that I could make it an outermost layer in any kind of wall without having to change properties of standard layers.Â

Either way, results still did not show much change in theÂ yearÂ round indoorÂ temperatures, i.e. before and after use of the reflective coats.Â

Regarding 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.Â

Looking at a typical material specification, I am unable to see how I change this for a surface/the outermost layer.

example Material,

Â Â A1 - 1 IN STUCCO, Â Â Â Â !- Name

Â Â Smooth, Â Â Â Â Â Â Â Â Â !- Roughness

Â Â 2.5389841E-02, Â Â Â Â Â !- Thickness {m}

Â Â 0.6918309, Â Â Â Â Â Â Â !- Conductivity {W/m-K}

Â Â 1858.142, Â Â Â Â Â Â Â Â !- Density {kg/m3}

Â Â 836.8000, Â Â Â Â Â Â Â Â !- Specific Heat {J/kg-K}

Â Â 0.9000000, Â Â Â Â Â Â Â !- Thermal Absorptance

Â Â 0.9200000, Â Â Â Â Â Â Â !- Solar Absorptance

Â Â 0.9200000; Â Â Â Â Â Â Â !- Visible Absorptance

I am sorry if my question is not making full sense. And thank you both for your immediate response and help.

Regards,

asit

On Mon, Jul 7, 2014 at 9:46 AM, Jeremiah Crossett jcrossett@xxxxxxxxxxxxxxx [EnergyPlus_Support] <EnergyPlus_Support@xxxxxxxxxxxxxxx> wrote:

Â

I Â my experience unfinished metal is the only thing that warrants using anything far off from default, and only modify absorbence for parametric studies. Â 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. Â Also attached is a spreadsheet where Â absorbence / reflectance = albedo..

Hope this helpsÂ

â??â??
Jeremiah D. CrossettÂ Â | SeniorÂ AnalystÂ Â |Â LEED Green AssociateÂ
â??â??
120 E. Pritchard St. Â | Asheboro, NC 27203Â
â??â??
Â | MobileÂ 503-688-8951
www.phasechange.comÂ

Â Â

On Sun, Jul 6, 2014 at 6:59 PM, Asit Mishra asitkm76@xxxxxxxxx [EnergyPlus_Support] <EnergyPlus_Support@xxxxxxxxxxxxxxx> wrote:

Â

Hello,
Â Â Â Â Â Â Â Â 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.

This did not seem to produce desired results
Secondly, 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).

Please advise

Regards,

asit