[Bldg-sim] *****SPAM***** Re: simulation program for green roof

Joe Huang yjhuang at whiteboxtechnologies.com
Fri Sep 26 11:24:50 PDT 2014

Thanks for providing this link to the paper, which I've read with interest.  However, I'm 
puzzled because  my primary concern with respect to modeling green roofs is whether and 
how evapotranspiration has been handled and, unless I missed it in my quick read, I don't 
see that mentioned in the paper.

In brief, evapotranspiration is the process by which vegetation sheds excessive heat gain 
through transpiration, a process akin to sweating in animals for the same reason.  When I 
had worked with the Urban Heat Island Group at LBNL (before the group got formalized), my 
main interest was the effect of vegetation on counteracting the urban heat island, and 
very quickly I got enmeshed with evapotranspiration models both at the individual tree 
level (too detailed) or at the macro urban level (which I ended up applying in this 1987 

In another project that I was then doing on modeling foundation heat flows, I worked with 
another researcher at the Univ. of Minnesota who had a foundation model including the heat 
balance of the surrounding soil, who told me that he could get any ground surface 
temperature he wanted simply by playing around with the evapotranspiration rate  (which is 
really just the conversion of incoming solar radiation to latent heat).

Since a green roof is basically a shallow slice of ground with vegetation, wouldn't it 
make sense to add evapotranspiration to the model (if it doesn't handle it already) before 
looking for other explanations why measured and modeled results differ?  In my view, 
attributing these differences to the urban heat island effect is unsatisfying for these 
reasons: (1) it doesn't explain the cause and effect (although i would suspect that it's 
precisely the latent cooling of evapotranspiration), (2) it confounds a building-level 
strategy with a neighborhood-level phenomenon, i.e., you can be in an urban heat island 
(or oasis) without a green roof, as well as having a green roof somewhere with no urban 
heat island, (3) I am skeptical that a single green roof would have an appreciable effect 
on reducing the air temperature above the roof, unless the air is completely still (see 
http://heatisland.lbl.gov/publications/effects-roof-reflectance-air-temperat ).


White Box Technologies, Inc.
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On 9/26/2014 5:45 AM, Jones, Christopher wrote:
> Link to the paper referred to below:
> http://www.ibpsa.org/proceedings/BS2011/P_1927.pdf
> *Christopher Jones**,***P.Eng./
> /Tel: 416.644.4226 • Toll Free: 1.888.425.7255 x 527
> *From:*Bldg-sim [mailto:bldg-sim-bounces at lists.onebuilding.org] *On Behalf Of 
> *hamnmegs at ozemail.com.au
> *Sent:* Friday, September 26, 2014 4:28 AM
> *To:* Fred Betz; shahryar habibi; bldg-sim at lists.onebuilding.org
> *Subject:* Re: [Bldg-sim] simulation program for green roof
> But do all of these programs account fro the impact on the roof micro climate? None to 
> my knowledge do. When you get a large roof surface the above roof air temperature bias 
> can be 2-5DegC between a cool roof (or green roof) and more historical roof (higher 
> absoprtance). These second order effects become essential to account for if you have 
> mechanical equipmnt on the roof as ventilation air and condesning temperatures are 
> impacted. We have been doing some research into this area in Australia. Think about the 
> urban heat island effect at a micro scale, becomes very important with big box retail, 
> shopping centres, industrial facilities, airports, maybe hospitals etc. for shopping 
> centres in Australia with AHUs on the roof, E+, DOE-2.2 and IES only account for 20-25% 
> of the cool roof benefits via th first order effects. The second order effects due to 
> the micro climate bias are 3-4 times what is accounetd for within these programs ...
> We haven't been able to get software developers to bring attention to this so if others 
> can shake the tree it would be good as the workarounds while possible are quite involved 
> with a high degree of uncertainty. There is a paper I wrote for IBPSA BS 2011 I can 
> forward when I get back from leave in a few weeks for those interested.
> Graham Carter
>     ----- Original Message -----
>     *From:*
>     "Fred Betz" <fbetz at aeieng.com <mailto:fbetz at aeieng.com>>
>     *To:*
>     "shahryar habibi" <shahryar_habibi at yahoo.com <mailto:shahryar_habibi at yahoo.com>>,
>     "bldg-sim at lists.onebuilding.org <mailto:bldg-sim at lists.onebuilding.org>"
>     <bldg-sim at lists.onebuilding.org <mailto:bldg-sim at lists.onebuilding.org>>
>     *Cc:*
>     *Sent:*
>     Thu, 25 Sep 2014 20:23:22 +0000
>     *Subject:*
>     Re: [Bldg-sim] simulation program for green roof
>     EnergyPlus has a model for green roofs
>     I’m not sure it handles acoustics, but it will simulate energy and water.
>     *Fred Betz***PhD., LEED AP ®BD&C
>     Senior Sustainable
>     Systems Analyst
>     5802 Research Park Blvd. | Madison, WI  53719
>     P: 608.236.1175 | F: 608.238.2614
>     fbetz at aeieng.com <mailto:fbetz at aeieng.com>  | *www.aeieng.com <http://www.aeieng.com/>*
>     *From:*shahryar habibi [mailto:shahryar_habibi at yahoo.com]
>     *Sent:* Thursday, September 25, 2014 9:00 AM
>     *To:* bldg-sim at lists.onebuilding.org <mailto:bldg-sim at lists.onebuilding.org>
>     *Subject:* [Bldg-sim] simulation program for green roof
>      Dear Colleagues,
>     Would you tell me, please, which simulation program can simulate performance of
>     green roofs.I would be grateful if you
>     could kindly guide me by offering any information . I am planning to simulate
>     performance of one the building in terms of
>     energy and acoustic through green roof.
>     Thanks in advance for any help and advice
>     */Shahryar Habibi/*, architect
>     PhD candidate in  Architectural Technology
>     University of Ferrara, Department of Architecture
>     *
>     <mailto:francesco.guidi at unife.it>*
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