[Bldg-sim] Modeling a Green Roof

Nick Caton ncaton at smithboucher.com
Fri Jun 25 07:41:29 PDT 2010

Open question:


I've ascribed to the "tack an R / thermal mass value onto a new  layer"
approach in the past... but would appreciate a more nuanced approach if
it could be done quickly enough.


I'm curious:  Does EnergyPlus or any other option/model out there
account the variable shading effects of the foliage on the roof
construction, as a user might define it?  I've speculated this element
could be handled relatively simply in eQuest using a geometrically
adjacent shading surface with an annual fractional schedule, but eQuest
(to my experience) doesn't have a clean approach to account for the
varying thermal mass and insulative properties of soil that varies in
moisture over time...








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From: bldg-sim-bounces at lists.onebuilding.org
[mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Drury B
Sent: Friday, June 25, 2010 9:25 AM
To: bldg-sim at lists.onebuilding.org
Subject: [Bldg-sim] Modeling a Green Roof


EnergyPlus has a model for green roofs built in -- based on experimental
measurements and experience of researchers at Portland State University,
Prof David Sailor:  http://web.cecs.pdx.edu/~sailor/


---------- Forwarded message ----------
From: Jeremy Poling <jpoling at epsteinglobal.com>
Date: Fri, Jun 25, 2010 at 10:04 AM
Subject: Re: [Bldg-sim] Modeling a Green Roof
To: Jason.Boehning at ricegardner.com, bldg-sim at lists.onebuilding.org

Penn State University (http://horticulture.psu.edu/cms/greenroofcenter/)
and Michigan State University (http://www.hrt.msu.edu/greenroof/) both
have significant research programs on green roofs.  I would recommend
looking at the results of the Penn State study on energy transfer in
Green Roofs (
esis_Paulo_Tabares_revised.pdf).  In general, green roofs handle energy
through the insulating effects of soil, the cooling effects of
evapotranspiration in the plants, and water flows within the green roof
assembly.  This means that a calculated R-value will be different
depending on the amount of plant material participating in
evapotranspiration, the wetness of the soil, and whether the roof is
intensive or extensive.  A straight R-value approach is very simplified
and should be used with the understanding that model results are much
less likely to be accurate.  The Penn State study gives a review of
published literature showing extensive green roofs have R-values between
1.8 and 4.8 while intensive green roofs have R-values between 5 and 20.


In general, the average R-value of soil is 0.25 per inch, so for a 12"
soil depth the R-value of the soil would be R-3 excluding assumptions on
soil wetness and evapotranspiration.  We typically go conservative and
just model the R-value of the additional soil since you would need a
TRNSYS or MatLab model to take into account the other energy benefits of
a green roof.


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Senior Sustainability Analyst
Strategic Services
Site Solutions | Operations | Sustainability


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consider the environment before printing this e-mail.


From: bldg-sim-bounces at lists.onebuilding.org [mailto:
bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Fareed Syed
Sent: Friday, June 25, 2010 8:05 AM
To: Jason Boehning; bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] Modeling a Green Roof


I heard from our director of sustainability to use R-5 for a typical
green roof. It was confirmed by few senior architects.






From: bldg-sim-bounces at lists.onebuilding.org [mailto:
bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Jason Boehning
Sent: Friday, June 25, 2010 8:46 AM
To: bldg-sim at lists.onebuilding.org
Subject: [Bldg-sim] Modeling a Green Roof




Has anyone modeled a green roof? I have a standard roof with 12 inches
of soil and grass. I have been searching the web for thermal properties
such as U values, thermal absorptivity and so on. Are there any good
references to find these values or does anyone have tips to model a
green roof? I appreciate your help.



Jason Boehning, EIT

Mechanical Engineer

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6161 Savoy, Suite 1212

Houston, Texas  77036


 v.  713.482.2322

 f.   713.482.2314

 e. Jason.Boehning at ricegardner.com
<mailto:Jason.Boehning at ricegardner.com> 


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