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

Alex Chapin alexc at 2rw.com
Mon May 18 13:51:23 PDT 2009


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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