Appendix C - Methodology for Building Envelope Trade-Off Option
Appendix C - Methodology for Building Envelope Trade-Off Option provides the mathematical methodology to calculate whether a proposed building envelope design complies with the standard through the trade-off approach. The Envelope Performance Factor (EPF) must be calculated using the procedures detailed in Normative Appendix C. This calculation is required to demonstrate compliance via the trade-off path.
The trade-off approach allows design flexibility by permitting some envelope components to perform below prescriptive requirements, provided that other components perform sufficiently above requirements to achieve overall compliance. Compliance is demonstrated when the proposed design's EPF is less than or equal to the base design's EPF.
The following steps illustrate the process undertaken on the Better Buildings project to meet the requirements of Appendix C:
Minimum Information
At the Building Level
The floor area shall be specified, broken down by space conditioning categories and building area type. Each building area type shall be selected from the classifications in the lighting power density table.
At the Exterior and Semi-exterior Surface Level
Building envelope surfaces shall be specified with the following information: assembly type, gross area, orientation, tilt, associated space conditioning category, and building area type. Each surface shall be designated as either exterior or semi-exterior.
Semi-exterior surfaces separating conditioned spaces from semi-heated spaces shall specify both associated space conditioning categories. Semi-exterior surfaces separating conditioned spaces from unconditioned spaces shall specify the associated space conditioning category and adjacency to unconditioned space.
Exterior surfaces with identical assembly type, space conditioning category, and building area type may be combined into a single surface when orientations differ by ≤22.5° and tilts differ by ≤22.5°.
For Roofs
The following shall be specified: class of construction, opaque area, U-factor, heat capacity (HC), and insulation position.
When three-year-aged test data for solar reflectance and thermal emittance of the exterior roof surface are available, the three-year-aged solar reflectance and three-year-aged thermal emittance shall be specified.
For Above-Grade Walls
The following shall be specified: class of construction, opaque area, U-factor, heat capacity (HC), and insulation position.
For Below-Grade Walls
The following shall be specified: opaque area, average depth to the bottom of the wall, C-factor, heat capacity (HC), and insulation position.
For Floors
The following shall be specified: class of construction, opaque area, U-factor, heat capacity (HC), and insulation position.
For Slab-on-Grade Floors
The following shall be specified: class of construction, perimeter length, F-factor, and heat capacity (HC).
For Uninsulated Assemblies
All uninsulated assemblies (such as projecting balconies, perimeter edges of intermediate floor slabs, concrete floor beams over parking garages, and roof parapets) shall be separately modeled.
These requirements define the minimum thermal and geometric properties needed to accurately model building envelope assemblies in the energy simulation. Specifying assembly characteristics at this level of detail ensures that the model captures the thermal performance of all envelope components, including thermal bridging effects from uninsulated assemblies that can significantly impact overall building performance.
Opaque Doors
Each opaque door shall be specified with its class of construction, area, and U-factor, and shall be associated with a surface as described in the exterior and semi-exterior surface requirements, assuming the orientation of that surface.
This requirement ensures that opaque doors are modeled with their thermal properties and proper orientation, capturing their contribution to envelope heat transfer while maintaining their relationship to the parent wall surface.
Fenestration
Each fenestration element shall be specified with its class of construction, area, assembly U-factor, assembly SHGC, visible transmittance (VT), and projection factor (PF), and shall be associated with a surface as described in the exterior and semi-exterior surface requirements, assuming the orientation of that surface.
This requirement ensures that all glazing systems are modeled with complete thermal and optical properties, capturing their impact on both heat transfer and solar heat gain while maintaining proper orientation relative to their parent surfaces.
Output Requirements
Report Identification
Name and contact information of the entity executing the simulation, and date of report
Building Location
Location of the building, including street address and climate zone
Weather Data Location
Location corresponding to the weather data used to perform the simulation
Simulation Program
Simulation program used to perform the simulation
Input Summary Tables
Tables summarising the minimum information described in the minimum information requirements
Envelope Performance Comparison
All differences between the proposed envelope performance factor and the base envelope performance factor
Peak Load Results
Peak heating and cooling loads for building classes of construction
Software Verification
The version of the software and the link to the website that contains the ASHRAE Standard 140 validation results for the version used
These output requirements ensure complete documentation and transparency of the envelope trade-off analysis, providing verification of inputs, methodology, and results necessary for code compliance review and quality assurance.
Simulation General Requirements
Simulation Program
The simulation program shall be a computer-based software program for the analysis of energy consumption in buildings. The simulation program shall include calculation methodologies for the building components being modeled.
Minimum Capabilities
The simulation program shall be approved by the adopting authority and shall, at minimum, have the ability to explicitly model all of the following:
Base Envelope Calculation
The base envelope performance factor, using only the input for the proposed envelope performance factor. The calculation procedure shall not allow the user to directly modify the building component characteristics of the base design
Annual Hourly Simulation
8760 hours per year
Hourly Schedules
Hourly variations in occupancy, lighting power, miscellaneous equipment power, thermostat set points, and HVAC system operation, defined separately for each day of the week and holidays
Thermal Mass Effects
Thermal mass effects
Thermal Zones
The number of thermal zones in the proposed design or nine thermal zones, whichever is greater
Air Economizers
Air economizers with integrated control
Daylighting Controls
Continuous daylight dimming controls and photosensors
Annual Energy Cost Calculation
The simulation program shall have the ability to determine the proposed envelope performance factor and base envelope performance factor by calculating annual energy costs.
Design Load Calculations
The simulation program shall be capable of performing design load calculations to determine required HVAC equipment capacities and airflow rates for both the proposed design building envelope and the base design building envelope.
Software Verification
The simulation program shall be tested according to ASHRAE Standard 140, except for Sections 7 and 8. The test results and modeler reports shall be posted on a publicly available website and shall include the test results of the simulation program along with the results of the other simulation programs included in ASHRAE Standard 140, Annexes B8 and B16. The modeler report in Standard 140, Annex A2, Attachment A2.7 shall be completed for results exceeding the maximum or falling below the minimum of the reference values or for missing results.
These requirements ensure that simulation programs used for envelope trade-off compliance have verified calculation capabilities, can accurately model all relevant building systems and control strategies, and have been validated against industry-standard test suites to demonstrate reliability and accuracy in predicting building energy performance.
Climatic Data
The simulation program shall perform the simulation using hourly values of climatic data, including temperature, humidity, solar radiation, and wind speed and direction from representative climatic data for the proposed design building envelope location.
For cities or urban regions for which several climatic data sources are available and for locations for which weather data are not available, the designer shall select available weather data that represent the climate at the construction site. Selected weather data shall be approved by the authority having jurisdiction.
Surface Exposure
Semi-exterior surfaces separating conditioned spaces from unconditioned spaces shall be simulated as exterior surfaces with no exposure to wind or solar radiation.
Purchased Energy Rates
The following rates for purchased energy shall be used to determine the proposed envelope performance factor and the base envelope performance factor:
Electricity
$0.1063/kWh
Heating
$0.98/therm
These requirements establish standardized climatic inputs and energy costs for envelope trade-off analysis. Using representative weather data appropriate to the building location ensures realistic thermal loads, while standardised energy rates enable fair comparison between proposed and baseline designs regardless of actual local utility rates or rate structures. Modeling semi-exterior surfaces without wind or solar exposure accurately represents their reduced environmental exposure compared to true exterior surfaces.
Compliance Calculations
The proposed envelope performance factor and base envelope performance factor shall be calculated using identical:
Simulation Program
Same software and version
Climatic Data
Same weather file
Purchased Energy Rates
Same electricity and heating rates
This requirement ensures that comparisons between proposed and baseline envelope performance are based solely on differences in envelope characteristics, eliminating any variables related to calculation methodology, climate assumptions, or energy cost assumptions that could affect compliance determination.
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