[Equest-users] Vegetated roof

[Nick Caton]

Hi Sambhav,

Best single piece of advice I can offer is to invest some time to peruse the [equest-users] and [bldg-sim] archives.  There have been many discussions centered around green roofs, but nobody has established a "best practice" to my knowledge.  You will have to ultimately define for yourself what course of action is most appropriate for your current project.

Limitations to how eQuest handles constructions have led many to recommend EnergyPlus and it's brethren (such as Designbuilder), where approximations such as assuming a constant soil dampness are unacceptable.  If a specific end-goal is to model a green roof as accurately as possible, it seems reasonable to consider other software options.  I can't speak much further to exactly how to get started with EnergyPlus - something I'm still working on myself.

If you're comfortable with approximations inherent to using eQuest, you could optionally account for the shading effects of vegetation with a building shade + shading schedule appropriate to the seasonal shading effects.  Thermal mass/conductivity of the soil, assuming a certain constant dampness, should be represented in a custom layered construction.  For that construction, reflectivity should be representative of the soil surface.  Consider increasing the roof surface area beyond its actual boundaries to account for thermal conduction of the vegetation.  Hmm - maybe I've just set a standard?

Some have suggested the net thermal effects of green roofs, inclusive of nuances eQuest cannot account for, are rendered negligible when the layers & thermal mass comprising a green roof are situated external to (above) a significant layer of insulation.  I haven't heard of any studies investigating this, but it's very much worth considering from an energy modeling perspective.  If the net thermal effects turn out to be negligible, that's reasonable grounds for deciding to not include the green roof at all in the model - it may be time-efficient to build a separate model to explore this specific possibility for your project's local climate and green roof properties.

~Nick

NICK CATON, P.E.
SENIOR ENGINEER

Smith & Boucher Engineers
25501 west valley parkway, suite 200
olathe, ks 66061
direct 913.344.0036
fax 913.345.0617
www.smithboucher.com 


-----Original Message-----
From: equest-users-bounces at lists.onebuilding.org [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of sambhav tiwari
Sent: Tuesday, April 24, 2012 11:56 PM
To: equest-users at lists.onebuilding.org
Subject: [Equest-users] Vegetated roof

Hi All,
          Ho to model vegetated roof  whether the effect of vegetation shall be considered in roof reflectivity or in U value of roof.

If someone did it before kindly can share the method.


Thanks
Sambhav.
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