# Modeling a Green Roof

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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

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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

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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/Th
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.

JEREMY R. POLING, PE, LEED AP

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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/

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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
(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 CATON, E.I.T.

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There's also been some interesting research done in California on this.

Here's a paper published by Pablo La Roche at Cal Poly Pomona
Here's another out of USC.

Vikram Sami, LEED AP

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>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

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