Green Roof

Green roof tab Materials Dialog

 

To create a green roof in DesignBuilder, follow these steps:

 

  1. Create a green roof material using data on the Green roof tab of the material dialog.
  2. Create a roof construction using the green roof material as the outermost layer.
  3. Specify the green roof construction on roof surfaces as appropriate.

 

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 (day 0 of the simulation) are defined on the Green roof tab of the Materials dialog.

 

EnergyPlus documentation states: "This model was developed for low-sloped exterior surfaces (roofs). It is not recommended for high-sloped exterior surfaces (e.g., walls)." However it may be possible to approximately model green walls, though we are not yet aware of any validation of the underlying EnergyPlus model and certainly the irrigation would have to be treated differently to roofs as walls will not naturally trap much precipitation.

Tip: EnergyPlus documentation suggests that green roofs may require "more timesteps" than normal simulations.

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

Green roof data

Moisture diffusion calculation method

Two moisture diffusion models can be selected:

 

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.