Site Details - Ground

Location tab on model data at Site level

Ground Modelling

Ground modelling method

DesignBuilder provides 4 different ways to model the heat transfer between the ground, the external environment and the building. Select the method to be used from the options below.

 

Note: For heating and cooling design calculations, the 1-Standard method is always used regardless of the Ground modelling method selected.

Surface

Ground texture

Select the texture to be used to represent the ground in the rendered view on the Visualisation screen.

Surface solar and visible reflectance

This is a decimal number between 0.0 and 1.0 and is used to characterize the average reflectivity of the ground throughout the year. Ground reflectance data is used to calculate the ground reflected solar radiation. This fractional amount is used in the following equation:

 

GroundReflectedSolar = (BeamSolar x COS(SunZenithAngle) + DiffuseSolar) x GroundReflectance

 

Reproduced from the IBPSA BEM Book.

Important Note: Reflection from the ground is always calculated even if the Model all external reflections and shading of ground reflected solar option is not selected, in which case the ground plane is considered unobstructed, i.e., the shadowing of the ground by the building itself or by component blocks is ignored. In other words, shadowing of ground-reflected radiation by obstructions such as the building itself and component blocks is only taken into account if the Model all external reflections and shading of ground reflected solar option is set.

The ground surface reflectance is also used to quantify the reflection of visible light in the Daylighting calculations.

 

Some typical ground reflectance values are shown for various surface types below.

 

Surface Type Surface Reflectance
Water (large angle of incidences) 0.07
Coniferous forest (winter) 0.07
Bituminous and gravel roof 0.13
Dry bare ground 0.20
Weathered concrete 0.22
Green grass 0.26
Dry grassland 0.2 to 0.3
Desert sand 0.4
Light building surfaces 0.6

Ground Reflectance of Foreground Surfaces from ASHRAE 2009 HOF, Source: Adapted from Thevenard and Haddad (2006).

 

The default ground reflectance is 0.2.

Snow reflected solar modifier

A number between 0.0 and 10.0 which is used to modify the basic ground surface reflectance when snow is on the ground. Note that the value of GroundReflectanceUsed (below) must be <=1.

 

GroundReflectanceUsed  = GroundReflectance x ModifierSnow

 

During simulations, the ground is considered to be snow-covered when the SnowDepth data in the hourly weather file is > 0.

 

From the IBPSA BEM Book:

 

Site exposure Reflectance of snow-covered ground
Typical city centre 0.2
Typical urban site 0.4
Typical rural site 0.5
Isolated rural site 0.7

The default modifier is 2 for both solar and daylight radiation, giving an overall default reflectance for snow covered ground of 0.2 x 2 = 0.4, suitable for a typical urban site.

Snow reflected daylight modifier

A number between 0.0 and 10.0 which is used to modify the basic ground surface reflectance when snow is on the ground. Note that the value of DaylightingGroundReflectanceUsed (below) must be <=1.

 

DaylightingGroundReflectanceUsed  = GroundReflectance x ModifierSnow

 

During simulations, the ground is considered to be snow-covered when the SnowDepth data in the hourly weather file is > 0.  

Ground Monthly Temperatures

There are 12 fields allowing you to represent the mean ground temperature for each month of the year. These temperatures are used as the outside surface temperature for surfaces adjacent to ground.

 

Note that the depth at which these temperatures apply depends on the thickness of the ground construction (see above) added to the main floor/wall construction.

Caution: It is generally not appropriate to use the "undisturbed" ground temperatures calculated by the weather converter and provided in the header of EnergyPlus hourly weather files as these values are too extreme for the soil under typical conditioned buildings. A reasonable default value of 2°C less than the average monthly indoor building temperature is appropriate for large buildings. For smaller buildings, the ground temperatures will be somewhere between that value and undisturbed ground temperatures.

More information about determining appropriate ground temperatures is given in Ground Modelling and in the EnergyPlus Auxiliary Programs document.

Ground Deep Temperatures

There are 12 fields allowing you to represent the undisturbed deep ground temperature for each month of the year. These temperatures are used when the 1-Deep Reference ground temperature option is used in a 15-Follow ground temperature Setpoint manager.

Tip: Undisturbed deep ground temperatures can usually be obtained from the epw weather file header.

Ground Shallow Temperatures

There are 12 fields allowing you to represent the undisturbed shallow ground temperature for each month of the year. These temperatures are used when the 2-Shallow Reference ground temperature option is used in a 15-Follow ground temperature Setpoint manager.

Tip: Undisturbed shallow ground temperatures can usually be obtained from the epw weather file header.

FCFactor Ground Temperatures

Building energy code and standards like ASHRAE 90.1, 90.2 and California Title 24 require the underground wall constructions and slabs-on-grade or underground floors not to exceed certain maximum values of C-factor and F-factor, which do not specify detailed layer-by-layer materials for the constructions. If using the normal layer by layer definition method for ground constructions, users would need to create a pseudo wall or floor construction to match the thermal performance such as thermal mass effect and U-factor, and rely on the EnergyPlus Basement and Slabs tools to generate the monthly ground temperatures.

 

A simplified approach has therefore been provided to create equivalent constructions and model the ground heat transfer through underground walls and ground floors for the building energy code compliance calculations. The approach is to create constructions based on the user defined C or F factor with two layers: one concrete layer (0.15 m thick) with thermal mass, and one fictitious insulation layer with no thermal mass. This is all done internally within EnergyPlus and the user simply needs to define C-factor or F-factor values.

 

There are 2 ways to define the ground temperatures to be used with F-factor and C-factor calculations which are defined through making one of the settings for the FCFactorMethod ground temperature source: