[Bldg-sim] Modelling an eco-roof / green roof

[Nathan Miller]

I assume you are referring to so-called green roofs, that is vegetated
roofs. I too am in the NW, and have been asked several times about whether
we can take credit in the energy model for the benefits of a green roof. I
have usually erred on the side of caution, and advised that we not claim
benefit. 

 

This is partially due to the fact that when you most want the additional
R-value that the growth media might provide, during the winter, the soil
is most likely to be saturated with water and provide the least insulative
value. There obviously will still be some benefit compared to a bare roof,
but it would be pretty hard to quantify. There would also be great
variability in the effect depending the depth of the media, is it in trays
or directly on the roof, etc. 

 

I suppose that if you intend on watering the roof during the summer the
evapo-transpiration might help keep the roof area cooler and result in
lower cooling loads, though again, that would be difficult to quantify. 

 

I'd be interested if others out there have found some reputable data on
energy advantages to green roofs. Otherwise, I'd stick with the stormwater
and habitat benefits they provide as a selling point. 

 

Nathan Miller

Senior Energy Engineer/Mechanical Engineer

direct: 206.788.4577

fax: 206.285.7111 

From: bldg-sim-bounces at lists.onebuilding.org
[mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Erin Rowe
Sent: Monday, May 18, 2009 10:58 AM
To: bldg-sim at lists.onebuilding.org
Subject: [Bldg-sim] Modelling an eco-roof / green roof

 

I was wondering if anyone has any recommendations on how to model an
eco-roof.  How are the U-values / R-values determined?  Are there any
standards referenced or typical assumptions made?  Are there substantial
energy savings associated with a eco-roof?

I've read a few studies that claim that there are and are not significant
energy savings associated with an eco-roof, and I was under the impression
that the major benefit of an eco-roof is storm water mitigation and
possible sewage savings.  I was hoping that the bldg-sim group can shed
some light on this topic. The projects I'm working on are typically in the
Pacific NW, where I would think the energy savings associated with this
technology would be minimal since the climate is fairly temperate.  I
would appreciate any thoughts you might have on this technology, it's
associated energy savings and modelling assumptions.

Thanks, 

Erin 

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