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 from site 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 description |
Mean 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 |
|
| Tree |
1.69 |
|
| Palm tree |
2.37 |
|
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.
