Possibly you might glean some useful info from the Design Builder
documentation on the subject...

Green Roof

Green roof tab Materials
Dialog

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
system
and/or
fromsite precipitation
(defined
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
precipitation.

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

Background

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 Description

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 the EnergyPlus
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 ,
1932-2000

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

"Pink
flowers"4.95

"Yellow
green leaves"3.75

"Dark
green long blades of leaves/grass"5.82

"Pinkish
red flowers"2.44

"Fern-like"6.59

"Palm
tree-like"4.41

"White
flowers with yellow center"3.21

"Small
yellow green leaves"4.08

"Long
big leaves"5.28

"Orange
stems and leaves for those which are taller"2.15

"No
special features"3.32

"Light
green edges with dark green center leave blades"5.83

"Red
yellow tulip like flowers"3.04

"Large
red leaves"2.33

"Dark
green leaf blades"~0

Tree1.69

Palm
tree2.37

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.