[Bldg-sim] Modeling a Green Roof

Drury B Crawley dbcrawley at gmail.com
Sat Jun 26 16:26:50 PDT 2010


>From the EnergyPlus Input-Output Reference, the fields describing the green
roof top layer include:

Height of Plants {m}
Leaf Area Index {dimensionless}
Leaf Reflectivity {dimensionless}
Leaf Emissivity
Minimum Stomatal Resistance (s/m)

(this is on page 183 of the IORef PDF (page number 144):
http://apps1.eere.energy.gov/buildings/energyplus/pdfs/inputoutputreference.pdf
)


The Engineering Reference describes the equations beginning on page 114 of
the PDF (page number 90):
http://apps1.eere.energy.gov/buildings/energyplus/pdfs/engineeringreference.pdf



On Fri, Jun 25, 2010 at 10:41 AM, Nick Caton <ncaton at smithboucher.com>wrote:


>  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…
>
>
>
> ~Nick
>
>
>
> [image: cid:489575314 at 22072009-0ABB]**
>
> * *
>
> *NICK CATON, E.I.T.***
>
> PROJECT ENGINEER
>
> 25501 west valley parkway
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> direct 913 344.0036
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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 Crawley
> *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 (
> http://etda.libraries.psu.edu/theses/approved/WorldWideFiles/ETD-4616/Thesis_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.
>
>
>
> *JEREMY R. POLING, PE, LEED AP*
> Associate Vice President,
>
> Senior Sustainability Analyst
> Strategic Services
> *Site Solutions | Operations | Sustainability*
>
> *EPSTEIN*
>
> *Epstein is a firm believer in sustainability. We ask that you please
> 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.
>
>
>
> Thanks,
>
>
>
> Fareed
>
>
>
> *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
>
>
>
> Howdy!
>
>
>
> 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
>
> *Error! Filename not specified.*
>
> 6161 Savoy, Suite 1212
>
> Houston, Texas  77036
>
>
>
>  v.  713.482.2322
>
>  f.   713.482.2314
>
>  e. Jason.Boehning at ricegardner.com
>
>
>
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