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

[Vikram Sami]

My 2 cents:

 

*         I would be careful of taking too much credit for the
evapotranspiration effect of green roofs. This really depends on the
plant selection since different plants have different rates of
transpiration. Furthermore, a lot of green roofs (especially on LEED
projects) use a plant palette of drought resistant plants (might be
different in the NW) - sedums and succulents, which typically have a
much lower rate of transpiration (some of them shut down in the really
hot summer months). 

*         Our office typically details green roofs with 3-4 inches of
polyiso because the soil does not really provide thermal resistance, (it
does provide thermal capacitance). 

 

 

Vikram Sami, LEED AP 
Direct Phone 404-253-1466 | Direct Fax 404-253-1366 

LORD, AECK & SARGENT ARCHITECTURE
1201 Peachtree Street NE, Suite 300, Atlanta, GA 30361 
Responsive Design * Technological Expertise * Exceptional Service 
Please visit our new website | www.lordaecksargent.com
<file:///\\www.lordaecksargent.com\> 

P Think GREEN before you print.

 

 

From: bldg-sim-bounces at lists.onebuilding.org
[mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Alex Chapin
Sent: Monday, May 18, 2009 4:51 PM
To: bldg-sim at lists.onebuilding.org
Subject: [Bldg-sim] Modelling an eco-roof / green roof

 

I have two suggestions for you:

1.       Use the GreenSave Life-Cycle Cost Benefit Calculator
(http://www.tremcoroofing.com/greensave.asp) found on Tremco's website.
Tremco is a roofing company.  They provide an on-line tool that
calculates the life-cycle cost of green roofs.  It explains the energy
savings methodology briefly and gives resources.

2.       Use DesignBuilder (interface for EnergyPlus;
http://www.designbuilder.co.uk/) or EnergyPlus directly
(http://energyplus.gov/).  The newest release of DesignBuilder, Version
2, now has the capability of modeling Green Roofs.  If you go their "new
functionality" page and click on "Green roofs" link, it will give you a
more detailed description of the modeling methodology.  DesignBuilder
also allows you to account for irrigation control of the green roof and
precipitation (entered into the model separately from the weather file).

 

Hope this helps!  Good luck. 

 

Here is an excerpt from the GreenSave site:

Annual Savings/sq.ft. of Roof Area

City

Heating
(therms/sq.ft.)

Cooling+Fans
(kWh/sq.ft.)

Phoenix, AZ

0.016

0.41

Washington, DC

0.020

0.29

Portland, OR

0.020

0.25

Source: Allen Lee, Quntec LLC

Annual energy savings for three different cities for a "typical" green
roof on a single story building. Savings estimates per sq.ft. of roof
are fairly constant with building height, though they vary a moderate
amount for heating energy. The analysis assumes natural gas heating and
electric cooling.

Other studies have indicated a 1% decrease in heating energy and a 6%
decrease in cooling energy associated with green roofs.

 

 

Alex Chapin, E.I.T., LEED AP

Energy Engineer

2rw Consultants

T: 434-296-2116 ext. 101

F: 434-977-1862

 

From: "Nathan Miller" <nathanm at rushingco.com>

Subject: Re: [Bldg-sim] Modelling an eco-roof / green roof

To: "'Erin Rowe'" <Erowe at peci.org>, <bldg-sim at lists.onebuilding.org>

Message-ID: <00d101c9d7e3$4fdf44b0$ef9dce10$@com>

Content-Type: text/plain; charset="us-ascii"

 

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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