Re: [EnergyPlus_Support] SRI/Reflectivity study

Thanks!

On Thu, Oct 4, 2012 at 1:07 PM, Jim Dirkes <jim@xxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

Brent brings up an excellent point. You don?t want to model the roof?s performance for ?day 1? of its life. ASHRAE is saying that you should degrade the performance to reflectance of .45 when the published value is .70. Section 5.5.3.1 (mentioned by the excerpt below) cites several ASTM standards that I?m unfamiliar with, but I think the idea is to make sure the product passes an independent performance test ? in order to qualify for using the .45 reflectance.

This does not address how to model the performance of differing SRI values; if I learn anything on that topic, I?ll share it here.

?For exterior roofs other than roofs with ventilated attics, the roof surface may be modeled with a reflectance of 0.45 if the reflectance of the proposed design roof is greater than 0.70 and its emittance is greater than 0.75. The reflectance and emittance shall be tested in accordance with the Exception to Section 5.5.3.1. All other roof surfaces shall be modeled with a reflectance of 0.3.?

James V Dirkes II, PE, BEMP, LEED AP
www.buildingperformanceteam.com
Energy Analysis, Commissioning & Training Services
1631 Acacia Drive, Grand Rapids, MI 49504 USA
616 450 8653

From: EnergyPlus_Support@xxxxxxxxxxxxxxx [mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx] On Behalf Of Griffith, Brent
Sent: Thursday, October 04, 2012 3:53 PM
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Subject: RE: [EnergyPlus_Support] SRI/Refelectivity study

For a so-called ?cool roof? change the Material object that is outermost in the Construction object for the roof. The values to use depend on dirt accumulation, age, type of coating etc, but I usual values are a solar absorptance of 0.45 and a thermal absorptance of 0.75 ( from ASHRAE 90.1-2007 Table 11.3.1 No. 5.c) .

From: EnergyPlus_Support@xxxxxxxxxxxxxxx [mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx] On Behalf Of Javed Iqbal
Sent: Thursday, October 04, 2012 1:24 PM

To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Subject: Re: [EnergyPlus_Support] SRI/Refelectivity study

Thanks for your response. I found a document on Cool roof and it says:

"The most important feature of an ideal cool roof is that its surface strongly reflects sunlight.

The surface of an ideal cool roof should also efficiently cool itself by emitting thermal radiation.

Thus, a cool roof should have both high ?solar reflectance? (ability to reflect sunlight, measured

on a scale of 0 to 1) and high ?thermal emittance? (ability to emit thermal radiation, also

measured on a scale of 0 to 1). The solar reflectance and thermal emittance of a surface are

called its ?radiative? properties because they describe its abilities to reflect solar radiation and

emit thermal radiation".

http://coolcolors.lbl.gov/assets/docs/fact-sheets/Cool-roof-Q%2BA.pdf

It means cool roof has both the properties i.e. Thermal reflectance and Solar Reflectance so according to this i need to consider both the values for cool Roof. Please guide me on this..

Regards!
On Thu, Oct 4, 2012 at 12:09 PM, Jeremiah Crossett <jcrossett@xxxxxxxxxxxxxxx> wrote:

I thought Solar Absorptance was the key input for SRI-

Jeremiah D. Crossett | Senior Analyst | Phase Change Energy Solutions

120 E. Pritchard St. | Asheboro, NC 27203 | Mobile 503-688-8951
www.phasechange.com

On Thu, Oct 4, 2012 at 11:22 AM, Javed Iqbal <eee.javed@xxxxxxxxx> wrote:

Hi,

I'm trying to see effect of roof reflectivity on Energy consumption and as per I/O guide there are two parameters Thermal absorptance and Visible absorptance which accounts the impact of reflectivity. Can anybody guide me on which value needs to be considered for high SRI/Reflectivity study.

Thanks in Advance!

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Javed Iqbal, LEED AP, CEA
Energy Analyst

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Javed Iqbal, LEED AP, CEA

Energy Analyst

Javed Iqbal, LEED AP, CEA

Energy Analyst