3.5 Performance Approach


Under the performance approach, energy use of the building is modeled by compliance software approved by the Energy Commission. The compliance software simulates the time-dependent value (TDV) energy use of the proposed building, including a detailed accounting of envelope heat transfers using the assemblies and fenestration input, and the precise geometry of any exterior overhangs or side fins. The most accurate tradeoffs between different envelope components – and among the envelope, the space-conditioning system, and the installed lighting design – are accounted for and compared with the standard design version of the building. The proposed design has to have TDV energy less than or equal to the standard design.

This section presents some basic details on the modeling of building envelope components. A discussion on the performance approach, and fixed and restricted inputs, is included in Chapter 11. The following modeling capabilities are required by all approved nonresidential compliance software. These modeling features affect the thermal loads seen by the HVAC system model. More information may be found in the ACM Reference Manual and the CBECC-Com User Guide.

3.5.1          Compliance Modeling      Mass Characteristics

Heat absorption, retention, and thermal transfer characteristics associated with the heat capacity of exterior opaque mass surfaces such as walls, roofs and floors are modeled.

Typical inputs are:



   Standard U-factor

   Reference Joint Appendix JA4 table

   Framed cavity R-value

   Proposed assembly U-factor.

The heat capacity of concrete masonry unit walls and solid concrete walls is provided in Tables 4.3.5 and 4.3.6 of Reference Appendix JA4. Effective R-values for interior and exterior insulation are provided in Table 4.3.13 of Reference Appendix JA4.      Opaque Surfaces

Heat gains and heat losses are modeled through opaque surfaces of the building envelope. The following inputs or acceptable alternative inputs are used by this modeling capability:

Surface areas by opaque surface type.

Surface orientation and slope.

Thermal conductance of the surface. The construction assembly U-factor is developed by specifying a construction as a series of layers of building materials, each of which represents insulation, framing, homogenous construction material, or a combination of framing and cavity insulation.

Surface absorptance = 1-solar reflectance. Surface absorptance is a restricted input (except for roofs).

Note for roofs: Surface absorptance and emittance are variable inputs in the proposed design for roofs to provide design flexibility where a cool roof is not specified. The roof reference design is set with a cool roof surface absorptance for nonresidential buildings in all climate zones. The difference in surface absorptance creates a credit that can be used with the whole-building performance method. For more information on cool roofs, see Section 3.2.4.

To model the proposed design as a cool roof, the roofing product must be listed in the directory of the CRRC. If the roof is not rated, a default aged reflectance of 0.08 is used for asphalt or composition shingles and 0.10 for other roofing products. If the proposed design does not have a cool roof, the performance method may be used to trade off with other measures, such as increased insulation or HVAC equipment efficiency, so that the TDV energy of the proposed design does not exceed that of the standard design.      Fenestration

Heat transfer through all fenestration surfaces of the building envelope are modeled using the following inputs:

      Fenestration areas. The glazing width and height dimensions are those of the rough-out opening for the window or fenestration product. Window area of the standard design is limited to the prescriptive limit of 40 percent of the gross exterior wall area (that is adjacent to conditioned space) or six times the display perimeter, whichever is greater. If the proposed design window area exceeds this limit, a trade-off may be made with measures such as increased envelope insulation or increased equipment efficiency to offset the penalty from fenestration.

      Fenestration orientation and slope. Vertical windows installed in a building located in any of the four cardinal orientations, north, south west, and east. Skylights are considered less than 60o from the horizontal, and all windows and skylights provide solar gain that can affect the overall energy of the building unless they are insulated glass. 

      Fenestration thermal conductance (U-factor). The overall U-factor shall be taken from NFRC rating information, default values in Table 110.6-A of the Energy Standards, or from Reference Nonresidential Appendix NA6 if less than 200 ft².

      Fenestration solar heat gain coefficient (SHGC). The SHGC shall be taken from NFRC rating information default values in Table 110.6-B of the Energy Standards or from Reference Nonresidential Appendix NA6 if less than 200 ft². The baseline building uses a SHGC equal to the value from Tables 140.3-B, 140.3C, or 140.3-D. The baseline building has no overhangs, but overhangs can be modeled in the baseline building.

      Visual Transmittance        Overhangs and Vertical Shading Fins

Approved compliance software programs are able to model overhangs and vertical fins. Typical inputs for overhangs are:

    Overhang projection.

    Height above window.

    Window height.

    Overhang horizontal extension past the edge of the window.

o If the overhang horizontal extension (past the window jambs) is not an input, then the program assumes that the extension is zero (that is, overhang width is equal to window width), which results in fewer benefits from the overhang.

Vertical fins are modeled with inputs of horizontal and vertical position relative to the window, the vertical height of the fin and the fin depth (projection outward perpendicular to the wall).      Slab-on-Grade Floors and Basement Floors

Heat transfer through slab-on-grade floors and basement floors is modeled by calculating perimeter heat losses and interior slab heat losses. The heat losses from the perimeter and the interior are modeled by the use of an F-factor that accounts for the rate of heat transfer from the slab to the soil. Reference Joint Appendix JA4 contains F-factors for common insulation conditions (vertical insulation outside or a combination of the two). The insulation depth and insulation R-value affect heat loss through basement floors.