Building energy code and standards, such as ASHRAE 90.1 used for LEED in DesignBuilder, require underground wall constructions (aka below-grade walls) and slab-on-grade floors not to exceed certain maximum values of C-factor and F-factor.
EnergyPlus provides a simplified approach to create equivalent constructions and model the ground heat transfer through below-grade walls and slab-on-grade floors for the building energy code compliance calculations. The approach taken behind the scenes in EnergyPlus is to create constructions based on the user-defined C-factor 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. So it is only necessary to set up C-factor and F-factor construction input settings and allow EnergyPlus to take care of the details.
ASHRAE 90.1 defines below-grade wall, which needs to use C-factor construction, as the portion of a wall in the building envelope that is entirely below the finish grade and in contact with the ground. C-factor constructions differ from the usual wall constructions in that they describe an entire construction rather than individual layers. This object is used when only the wall height (depth to the ground) and the C-factor are available.
ASHRAE 90.1 defines slab-on-grade floor, which needs to use F-factor construction, as the portion of a slab floor of the building envelope that is in contact with the ground and that is either above grade or is slightly below the grade (less than or equal to 24 inch, or 600 mm, below the final elevation of the nearest exterior grade). The F-factor represents the heat transfer through the floor, induced by a unit temperature difference between the outside and inside air temperature, on per linear length of the exposed perimeter of the floor.
Both C-factor and F-factor constructions exchange heat on their outer boundary with the ground temperature which is defined by the site level monthly Ground > FCfactorMethod ground temperatures and which can be accessed from the site details section at site level in DesignBuilder. During the simulation these C-factor and F-factor surfaces exchange heat with the FCFactor ground temperatures only. These FCfactorMethod ground temperatures can be either read from the header of the hourly weather file used or taken from user’s manual input.
Note: Basement ground floors were introduced into DesignBuilder for ASHRAE 90.1 models only in order to distinguish floors from slab-on-grade floors and are normally outside the scope of slab-on-grade floor definition described in ASHRAE 90.1 and are not regulated by the 90.1 Standard. The baseline model takes the same construction for basement ground floor as that used in the proposed model, where a standard layered floor construction is normally used.
C Factor walls differ from usual walls in that they describe an entire construction rather than individual layers. C Factor walls are used when only the wall height (depth to the ground) and the C-factor are available. They access a model that creates an equivalent layer-by-layer construction for the underground wall to approximate the heat transfer through the wall considering the thermal mass of the earth soil.
C-Factor is the time rate of steady-state heat flow through unit area of the construction, induced by a unit temperature difference between the body surfaces. The C-Factor unit is W/m2·K. The C-factor does not include soil or air films. ASHRAE Standard 90.1 and California Title 24 specify maximum C-factors for underground walls depending on space types and climate zones.
This field describes the height of the underground wall, i.e. the depth to the ground surface (in m or ft).
F Factor floors differ from the usual ground floor constructions in that they describe an entire construction rather than individual layers. F Factor floors are used when only the floor area, exposed perimeter, and the F-factor are available. They access a model internal to EnergyPlus that creates an equivalent layer-by-layer construction for the slab-on-grade or underground floor to approximate the heat transfer through the floor considering the thermal mass of the earth soil.
Note: F-Factor constructions should only be selected for Ground adjacent surfaces with exposed perimeter greater than zero.
The area of the slab-on-grade or underground floor and the exposed perimeter length (direct contact with ambient air) are both calculated automatically by DesignBuilder behind the scenes.
F-Factor represents the heat transfer through the floor, induced by a unit temperature difference between the outside and inside air temperature, on the per linear length of the exposed perimeter of the floor. The unit for this input is W/mK. ASHRAE Standard 90.1 and California Title 24 specify maximum F-factors for slab-on-grade or underground floors depending on space types and climate zones.
Note: F-Factor constructions cannot be used in ground surfaces which are adjacent to a Kiva Foundation. Generally F-Factor and Kiva are separate and non-compatible methods to account for ground 2-D/3-D conduction and should not be used together.