Green Roof - eQuest style

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OK, my fellow Raiders of the Lost BTU..

What would be your advice for developing a green roof in eQuest? I am sure some of you have gone through the USGBC with hopes of getting this approved.

I eagerly await your suggestions with thanks.

John Aulbach

John Aulbach's picture
Joined: 2011-09-30
Reputation: 1

I advise you to use a more modern software that has the ability to
correctly simulate the effects of a Green Roof rather then some wacky
eQuest work around. Professor David Sailor of Portland State
University developed
the Green Roof object for Energy Plus that uses a
a finite difference algorithm and works well. The problem with eQuest/DOE2
is that it is based on a conduction transfer algorithm, and can not really
emulate the effects of a Green Roof. If you need the easy to use inputs, I
suggest Design Builder to generate the Energy Plus file.

Best of luck, and feel free to ask if you need more about EP/DB..

CleanTech Analytics's picture
Joined: 2012-02-09
Reputation: -1

I suppose there is probably some benefit to a green roof, but I've never
tried to claim credit for it.

At least in Seattle's damp climate, when you would most benefit from the
additional insulation of a green-roof-layer (winter) is the same time of
the year the growth medium will be saturated, and likely provide you with
the least insulative value. In the summer, when you would actually want
the heat to escape, the medium will dry out (unless you are irrigating

Soil has an insulative value of only around R-0.104/in (at least according
to a table in the Seattle Energy Code), so even ignoring moisture levels,
it doesn't seem like there is much benefit there.

I suppose in a heavy cooling climate the reflectivity of the plants might
get you some benefit, as well as the evaportative cooling effect if you
are irrigating. Is that where the credit would be? Is there a thermal mass

Nathan Miller, PE, LEED AP BD+C

Nathan Miller's picture
Joined: 2011-09-30
Reputation: 200

So Jeremiah:

Do you any business relationship with Design Builder?

Paul Diglio, CEM, CBCP

Paul Diglio's picture
Joined: 2011-09-30
Reputation: 400


While dumping eQuest entirely and (re?)starting a project using different software may certainly be a viable solution, I think it's reasonable to assume someone posting a "how do I do this in eQuest" question in forum titled [Equest-users] is probably not looking for such answers first! I personally understand EnergyPlus is often the tool-of-choice for green-roof specific research, but let's avoid telling a carpenter to throw away his hand saw just because power tools exist ;).


eQuest can certainly *approximate* green roofs, to varying degrees of accuracy. There were a couple long, informative, and unfortunately disjointed conversations on this exact topic held over the [bldg-sim] list some time ago. I'm attaching those messages including various attachments and white papers here in a zip file - if you can move these into a folder in outlook or similar you may find it easier to follow the flow of the conversation...

Read up on what's been discussed so far, and I'm certain you'll be able to decide for yourself how comfortable you are using eQuest for your current project ;).


[cid:489575314 at 22072009-0ABB]


Nick-Caton's picture
Joined: 2011-09-30
Reputation: 805


I used eQuest intensively for a time and recently moved most of my modeling
to EnergyPlus. In addition to having more capabilities and being more
flexible, EnergyPlus gets regular updates and bug fixes. EnergyPlus is
also a total bear, runs slowly compared to eQuest and doesn't have a fully
functional user interface. I wouldn't recommend it to everybody.

eQuest is very good at modeling buildings that use fairly normal
constructions with fairly normal HVAC systems. I have run into problems
when modeling water to water heat pumps, central ventilation systems (with
heat recovery), high thermal mass buildings, and dessicant wheels, to name
a few. While I've run into problems with EnergyPlus, I've either been able
to resolve the issue with help from the EnergyPlus helpdesk, or, when my
issue illuminated an underlying problem with source code, a bug fix was
incorporated into the next version.

I understand that eQuest/DOE2 is not receiving funding for updates and bug
fixes of eQuest. A few years ago I tried to start a conversation around
updates, bug fixes and feature improvements, and also suggested that we in
the modeling community fund some of this effort. There were quite a bit of
responses from users to the "modeling wishlist," but few if any responses
to the funding idea.

To extend your carpentry analysis, I think that the eQuest community needs
to face the fact that the eQuest saw just isn't capable of cutting some of
the species of wood coming into the shop these days.


No Username provide's picture
Joined: 2011-09-30
Reputation: 200


Thank you very much for the feedback. I personally don't have the cross-platform experience necessary to share such perspective, and it is not my goal to pretend that I do, just trying to keep our community's mutual efforts to help each other constructive =).

As well as I can predict the weather, I see myself adding EnergyPlus to my toolbox sooner or later as well. UI issues and other items collectively affecting the time-efficiency of EnergyPlus are what I see as the greatest hurdle for myself and others... more easily dismissed if you have the time to burn, but harder to do so where time is a scarcer resource.

In the meantime, myself and others are in the camp of needing to make do with the tools we are able to use with both accuracy and speed. I do not mean to argue against learning new tools by sharing suggestions/means to approximate.

Thanks again for the input!


[cid:489575314 at 22072009-0ABB]


Nick-Caton's picture
Joined: 2011-09-30
Reputation: 805

As I am an Equest user for appropriate projects, an Energy Plus user for
projects that can not be done well in eQuest, and as a representative of
Design builder my advise is to use Design builder, (free for a month) to
develop your Green Roof project. You will find that design builder has a
Green Roof feature built in and that it is not much harder then eQuest.
You can export your project from Design Builder to Energy Plus, in a
fashion similar to how you might start with the wizards in eQuest then move
on to the detailed mode at the tail end of the project.

Then you can benchmark whatever work around(s) you decide to use in eQuest
with the Design Builder/Energy Plus results, or can just use the Design
Builder/Energy Plus simulation for your project.

There has been a lot of work put into the Green roof simulation tools of
Energy Plus/Design Builder, and I think that using work around in eQuest
simply for fear of the unknown land of Energy Plus should not deter you
from the prospect, as Design Builder has simplified Energy Plus down to a
comparable ease of use to that of eQuest.

Below is the documentation from Design Builder on modeling a Green Roof for
your review..

Green Roof

Green roof tab Materials

Green roofs can be modelled in DesignBuilder by creating a roof construction
using a Green roof material as the outer layer. The green roof can receive
water during the simulation from an irrigation
from site precipitation
separately from the hourly weather data). The initial properties of the
soil layer are defined on the Green roof tab of the Materials dialog.

Green walls can also be modelled though in this case the irrigation must be
treated differently to roofs as walls will not naturally trap much

Note that specifying a green roof material as the material for a *component
block* will not work - these only use materials for their reflective


Use of green roofs (aka ecoroofs or vegetated roofs) is becoming
increasingly common for both new and retrofit buildings. There is
widespread recognition and a growing literature of measured data that
suggest green roofs can reduce building energy consumption. The EnergyPlus
Green Roof capability can assist developers and architects in assessing the
likely magnitude of energy savings associated with various implementation
options (e.g., soil type/depth, irrigation options, plant type). It
provides a quantitative and physically-based building energy simulation
tool that represents the effects of green roof constructions and
facilitates more rapid spread of green roof technologies and make it
possible to account for green roof benefits in state energy codes and
related energy efficiency standards such as LEED.

The green roof model accounts for:

- Long wave and short wave radiative exchange within the plant canopy,
- Plant canopy effects on convective heat transfer,
- Evapotranspiration from the soil and plants, and
- Heat conduction (and storage) in the soil layer

The ability to track moisture-dependent thermal properties is not
implemented yet due to stability issues in the CTF scheme, but is under
development for use with the finite difference solution scheme made
available in EnergyPlus starting in version 2. As implemented in EnergyPlus
the green roof module allows the user to specify ?ecoroof? as the outer
layer of a rooftop construction. The user can then specify various aspects
of the green roof construction including growing media depth, thermal
properties, plant canopy density, plant height, stomatal conductance
(ability to transpire moisture), and soil moisture conditions (including
irrigation). The model formulation includes the following:

- Simplified moisture balance that allows precipitation, irrigation, and
moisture transport between two soil layers (top and root zone).
- Soil and plant canopy energy balance based on the Army Corps of
Engineers? FASST vegetation models (Frankenstein and Koenig), drawing
heavily from BATS (Dickenson et al.) and SiB (Sellers et al.).
- Soil surface (Tg) and foliage (Tf) temperature equations are solved
simultaneously each time step, inverting the CTF to extract heat flux
information for the energy balance calculation. The detailed energy balance
analysis and resulting equations, being rather complicated, are summarized
here. The interested reader is referred to the FASST documentation cited
herein for the complete development. The end result is a set of two
simultaneous equations for temperature?one for the soil surface and the
other for the foliage.

Green Roof Model

As with a traditional roof, the energy balance of an green roof is
dominated by radiative forcing from the sun. This solar radiation is
balanced by sensible (convection) and latent (evaporative) heat flux from
soil and plant surfaces combined with conduction of heat into the soil
substrate. This energy balance is illustrated in the diagram below. The
variables introduced in this figure are defined in theEnergyPlus
Engineering Document

*The energy balance for a green roof.*

The energy budget analysis follows the Fast All Season Soil Strength
(FASST) model developed by Frankenstein and Koenig for the US Army Corps of
Engineers. FASST was developed, in part, to determine the ability of soils
to support manned and unmanned vehicles and personnel movement. In order to
accomplish this, however, FASST tracks the energy and moisture balance
(including ice and snow) within a vegetated soil. It is a one-dimensional
model that draws heavily from other plant canopy models including BATS
(Dickinson et al.) and SiB (Sellers et al.). FASST is implemented in
EnergyPlus with only a few modifications to adapt it for use with a
relatively thin soil layer.
Green roof data Height of Plants

The average height of plants in the green roof.
Leaf Area Index (LAI)

This is the projected leaf area per unit area of soil surface. It is a
dimensionless number between 0.001 and 5.0. The tables below gives some
typical values for LAI.

The table below is reproduced from Global Leaf Area Index Data from Field
Measurements ,

The table below is reproduced from the PhD Thesis of Chen Yu entitled The
intervention of plants in the conflicts between buildings and climate - A
case study in Singapore
Plant descriptionMean Leaf Area Index (LAI) Picture"White flowers, spider
lily" 3.07

"Yellow green leaves"
"Dark green long blades of leaves/grass"
red flowers"2.44



flowers with yellow center"
yellow green leaves"
big leaves"5.28

stems and leaves for those which are taller"
special features"3.32

"Light green edges with dark green center leave blades"
yellow tulip like flowers"
red leaves"2.33

"Dark green leaf blades"


Leaf Reflectivity

The fraction of incident solar radiation that is reflected by the
individual leaf surfaces. Solar radiation includes the visible spectrum as
well as infrared and ultraviolet wavelengths. Values for this field must be
between 0.1 and 0.4.
Leaf Emissivity

This field is the ratio of thermal radiation emitted from leaf surfaces to
that emitted by an ideal black body at the same temperature. This parameter
is used when calculating the long wavelength radiant exchange at the leaf
surfaces. Values for this field must be between 0.8 and 1.0 (with 1.0
representing ?black body? conditions).
Minimum Stomatal Resistance

This field represents the resistance of the plants to moisture transport.
It has units of s/m. Plants with low values of stomatal resistance will
result in higher evapotranspiration rates than plants with high resistance.
Values for this field must be in the range of 50.0 to 300.0.
Max volumetric moisture content of the soil layer (saturation)

Maximum volumetric moisture content of the soil depends on the properties
of the soil and in particular the porosity.
Min (residual) volumetric moisture content of the soil layer

The minimum possible volumetric moisture content of the soil layer.
Initial volumetric moisture content of the soil layer

The volumetric moisture content of the soil layer at the start of the
simulation. The moisture content will be updated during the course of the
simulation based on surface evaporation, irrigation and precipitation.

Design Builder USA's picture
Joined: 2012-03-05
Reputation: 0

So I have a 2 part question for the group:

1. Has anyone compared the results before and after adding a green roof (assuming similar insulation levels in both - because you have to have insulation in a green roof) in eQUEST and in DB/E+? Also - has anyone compared the delta that you get in eQUEST with the Delta that you get in E+?

2. If so - did you see a significant difference? My guess is no - but I am willing to be convinced. For a well-insulated roof - especially in a building more than a couple of stories, your thermal load through the roof is probably not what's breaking the bank (residential users might be different - I am assuming people who are running Energyplus & eQUEST models aren't doing this for a small house, but a larger building).

3. As far as I know - the positive effect of green roof constriction is mainly for the lower surface temperature in the summer (the soil material - especially when its wet - is pretty conductive. It does provide thermal mass, but that's less effective outside your insulation). Furthermore - green roofs often use drought resistant plants that resist drought by not evapotranspiring in hot dry months - so you lose some of the benefit there.

Are we obsessing about getting pinpoint accuracy on the answer when the more pertinent question might be - does it really matter & by how much?

Apologies for stirring the pot, but its Friday afternoon and I can hear the beer bottles clinking.

Vikram Sami, LEED AP BD+C

Sami, Vikram's picture
Joined: 2011-09-30
Reputation: -1

Dear Vikram & eQuest list,

I have recently taken a position with Design Builder USA, and want to make
it clear that I am an agent of the company.

Also I want to be clear that I am an eQuest user, that I learned building
energy simulation with a combination of eQuest and Energy Plus and that I
joined the eQuest and Energy Plus lists before I was hired at Design
I apologise if there was any confusion of my role, when posting and
referencing design builder, and in the future if I am going to advocate for
design builder I will send the email from my design builder USA email
It is not my intention to use the list as a sales method for design
builder, rather when I see posts that I may have useful insight to I
attempt to help by posting my thoughts. Sometimes my thoughts may advocate
for Design Builder, Energy Plus, Ope Studio, AECOSIM, ECOTECH, TRNSYS,
ESP-r, or some other simulation tool, tho from my research Design Builder
in tandem with Energy Plus is the best tool on the market for many uses. I
am personally teaching myself all of the simulation tools, attempt to
advocate for the right tool for the job, and try very hard not to allow my
affiliations effect my advise. I am also an Energy Modeler, currently the
energy modeler for 3 registered LEED projects, and the dedicated energy
modeler for Phase Change Energy Solutions working various projects globally
that are modeling their BioPCM product.

I am on all of the SIM lists, and sometimes see projects, or modelers that
would be a better fit for eQuest, and do at some points advise people to
start with eQuest, (for instance when someone is just geting started, or
when the model is for EEM's only)

Like many people on this list I where many hats, and will from this point
forward be sure to be very clear of my perspective when posting, and if I
mention design builder will use my company email...

Thank you to all in the Simulation community, and happy modeling!

Design Builder USA's picture
Joined: 2012-03-05
Reputation: 0