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
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
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
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 ----------
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 CATON, E.I.T.
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 another out of USC.
Here's
Vikram Sami, LEED AP
>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