Modelling an eco-roof / green roof

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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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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

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I have a finished model and now find that the owner only wants to supply
chilled water for Memorial Day to Labor Day. Is there an easy way to
schedule the chilled water separately?

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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

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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

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Great question!
We have also run into this item and to my knowledge there is no validated method for a solution in Europe either.

I had heard MIT had done some modelling of same but cannot locate it on the web.

The quantitative benefit of a green roof to the insulative, cooling loads of buildings is very difficult to quantify as the roof constuction, roof media, depth of soil and vegetation, precipitation, temp, humidity and wind all habe to be factored in.

In all cases so far we have just claimed credit for using collected rainwater as grey or black water for washroom use, thereby reducing the buildings consumption of potable water.

Perhaps a topic for a Doctorate study?

Regards

Noel O Connor

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On a similar topic, would you rank a reflective (cool) roof as more energy
efficient when compared to green roof?

thanks,
Haider.

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Hi all,
?
Check the work of Eleftheria Alexandri done in the Welsh School of Architecture few years ago.
?
It might help!
?
Regards,
?
Clarice

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Hello;

Check ENVI-met plants sub-model that can be used for roofs.
It can has a U-value and Albedo for roofing so that, if you can measure or
provide it for the model, you can simulate that roof.

Regards,
Mohamad

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Hi,
We ever did some experiments in wind tunnel as well as on-site measurements in south China, where cooling load is huge but almost no heating load. The most difficult thing we found is how to qualify the effect of evapotranspiration of various plants in different weather conditions. Finally, we have a basic conclusion that, for sedum lineare in green roof, it can increase the R-value of 0.5 m2K/w when calculating for the cooling load. But not applicable for heating load. This number is quite a rough one and specified for the hot climatic areas only.

Zhuolun Chen LEED? AP

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