[Bldg-sim] simulation program for green roof

hamnmegs at ozemail.com.au hamnmegs at ozemail.com.au
Sun Sep 28 14:42:52 PDT 2014

	Hi Joe, 

	A lot to think about below. In short I hadn't considered evapo
transpiration as we were trying to explain what was being observed for
a change in solar absportance on a metal deck roof - a cool roof with
a solar absportance of about 0.15 to 0.2 relaive to a more
conventional metal deck roof with probably SA>0.5 allowing for ageing.
A vegetated roof certainly would introduce the issues you refere to

	There has been some further research since the 2011 paper which has a
bit more academic rigor (my paper was based on empirical observations
and rough calculations to make a point) and it shows that the bias can
be fairly significant even with a 3-5 m/s wind. The University of
Wollongong had carried out some advanced 3D CFD studies (sponsored by
two companies) to show the bias in above roof temperatures and roof
surface temperatures (biased by the air temperature above being
higher) can be significant under many conditions. The bias occurs on
any roof, but for small roofs it is insignificant compared to other
fcators so we ignore it but it acumulates / aggregates to create an
urban heat island. However for larger shopping centre airport, and
industrial roofs the impact can become quite significant (2-5DegC). It
is on roofs such as these that AC energy savings for cool roofs have
been measured to be significant - 4-5 times higher than what typically
gets simulated with a modern energy simulation engine. 

	With truly green roofs, I agree with you that the benefits are
significant. There is an interesting debate emerging in Australia
whether it is better to cover roofs in photovoltaics or make them
green. I saw something late last year that I think Art Rosenfeld
around whether it was better to make roofs white or green. I didn't
get a chance to revisit it so can't really comment on the artricle /
paper here. 

	I will try to remember to loop back on this forum / thread when the
research gets into the public realm. 


----- Original Message -----
 From:"Joe Huang" 
Sent:Fri, 26 Sep 2014 12:47:20 -0700
Subject:Re: [Bldg-sim] simulation program for green roof

Reposted with better subject line.

On 9/26/2014 11:24 AM, Joe Huang wrote:  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

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 paper
[1] ).

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
[2] ). 


	White Box Technologies, Inc. 346 Rheem Blvd., Suite 205A Moraga CA
94556 yjhuang at whiteboxtechnologies.com [3]
http://weather.whiteboxtechnologies.com [4] for simulation-ready
weather data (o) (925)388-0265 (c) (510)928-2683 "building energy
simulations at your fingertips" 
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 [5]  



Tel: 416.644.4226 [6] • Toll Free: 1.888.425.7255 [7] x 527  


	FROM: Bldg-sim [mailto:bldg-sim-bounces at lists.onebuilding.org [8]] ON
BEHALF OF hamnmegs at ozemail.com.au [9]
SENT: Friday, September 26, 2014 4:28 AM
TO: Fred Betz; shahryar habibi; bldg-sim at lists.onebuilding.org [10]
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 -----    


	"Fred Betz"   



	"shahryar habibi" , "bldg-sim at lists.onebuilding.org [13]"    




	Thu, 25 Sep 2014 20:23:22 +0000   


	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



Senior Sustainable
Systems Analyst   


5802 Research Park Blvd. | Madison, WI  53719

P: 608.236.1175 [15] | F: 608.238.2614 [16]  
fbetz at aeieng.com [17]  |  WWW.AEIENG.COM [18]   



	FROM: shahryar habibi [mailto:shahryar_habibi at yahoo.com [19]] 
SENT: Thursday, September 25, 2014 9:00 AM
TO: bldg-sim at lists.onebuilding.org [20]
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  





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