Friday, April 25, 2014

Development of an Integrated Cool Roof Energy Savings Calculator

Posted by Webmaster on 14. May 2007 20:25

Memo to Calculator Advisory Group (PDF 44 KB), May 2007
Download Adobe Acrobat.

Posted By: Tech Admin from Technologists Inc

May 14, 2007 8:25 PM

Currently there are both a DOE (ORNL) and EPA (LBNL) on-line energy savings calculator for cool roofs. The DOE model is based on measured energy savings at the ORNL facilities. The EPA model is based on LBNL parametric DOE-2 simulations that utilize a validated algorithm for heat transfer through the roof. The input and output features of the LBNL and ORNL models are somewhat different. In FY 2006, DOE convened an industry group to assist in the development of a new integrated calculator that will replace the existing DOE and EPA calculators. Unfortunately, the process to update the calculators relied too much on voluntary contributions. Recently, the California Energy Commission funded a joint LBNL/ORNL project to assist with the new calculator development. Attached please find a copy of the developmental plan to conduct this work. Your comments are welcome and should be posted to this web page by June 30, 2007. Warm Regards Andre Desjarlais, ORNL; Hashem Akbari, LBNL; Marc LaFrance, US DOE

Replied By: Martha VanGeem from CTLgroup

20 - Jun - 2007 1:03 PM


I commend ORNL for taking on this effort. However, consistent with Marc LaFrance's June 2007 presentation (posted) and work performed at ORNL, I recommend that the modeling inputs included in the calculator include thermal mass. This could be implemented as thickness and density of concrete, pavers, or ballast, for instance. Perhaps this is covered in the "roof type."

Replied By: from

20 - Jun - 2007 5:04 PM

Development of an integrated cool roof energy savi...

I also commend ORNL and LBNL for their efforts in this field. I would encourage the use of empirical models and correlations for the sake of simplicity. Empirical models tend to do well under certain circumstances and can save considerable computational time while still provide adequate accuracy. In the case of roof insulation, I recently demonstrated that empirical models fit subsequent experimental data well (see Alvarado, J. L., E. Martinez, Passive Cooling of Concrete Roofs in Tropical Climates, Energy and Buildings, 2007. In press). Also, a roof calculator should be flexible enough to account for new materials.


Jorge L. Alvarado, Ph.D, P.E.
Assistant Professor
Department of Engineering Technology
and Industrial Distribution
Texas A&M University
3367 Tamu
College Station, TX, 77843-3367
Tel. (979) 458-1900 Fax (979) 862-7969

Replied By: from

22 - Jun - 2007 10:49 AM

non-vented roof...

Is non-vented roof going to be considered as a part of the software's input parameter?

Replied By: from

28 - Jun - 2007 8:31 AM


I have tried both calculators, but use LBNL by far the most. It is more user friendly and offers a format that our contractors and sales reps can relate to better. Because of its ease of use, I have been able to get more people to learn to use it and understand the results. The following are suggestions for improvements to LBNL:
1) Currently fields for insulation R-value and reflectivity only allow selecting the pre-set values from the pop-down menu. Change to allow selecting either from the pop-down menu or entering any value manually.
2) We use the LBNL calculator almost exclusively and have found that the actual savings experienced by building owners are typically even better than the calculated estimates. This is good because the building owners are pleasantly surprised rather than being disappointed.
3) Don't take down the old calculator until the new one is up and running and completely debugged. Better yet, keep the old one available indefinitely, but relabel it so it can easily be differentiated from the new one.

Replied By: Ken Bettger from EverSealed Windows, Inc.

29 - Jun - 2007 11:09 AM

Integrating the calculator tools, each of which originated separately from the other is a difficult task and very likely may be accurately described as a new integrated calculator. I applaud the efforts of each individual from the respective participating organizations, working to produce a flexible and comprehensive calculator. While it is tempting to attempt to find consensus and make progress through the efforts of volunteers, most likely it will take the concentrated efforts of dedicated individuals to produce a calculator that provides the desired practical and valuable functionality.

I agree with previous comments pointing out some of the specific parameters and techniques which should be considered. In addition to the points already made, I also encourage the team developing the cool roof calculator to ensure that all of the significant physical variables are included in the model. Since one of the goals for the effort is integration, the team should be aware of and should integrate the performance parameters for significant roof penetrations. For instance, the current high level of interest in day-lighting as a replacement for powered lighting will surely result in increased use of skylights. The energy use vs. natural light benefit tradeoff for skylights can be very attractive today and continues to improve as the result of implementation of electrochromic glass coatings for radiant energy modulation, along with translucent aerogel insulation and the imminent vacuum insulated glazing (VIG) for mitigation of conductive heat losses.

Ken Bettger



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