Thermal modeling for this guide was undertaken using HEAT3.1. 1 HEAT3 is a three-dimensional finite-element thermal analysis software tool commonly used by the building industry to analyze building enclosure assemblies in three dimensions, which two-dimensional analysis tools (such as THERM) cannot accurately analyze. It allows for a more detailed analysis of building enclosure assemblies, including the impact of fasteners, masonry ties and discrete clips, and other construction realities. Modeling can determine effective R-values/U-factors from the heat flow measured through the building enclosure assembly.
The boundary conditions used for this guide’s modeling are industry standard ASHRAE winter exterior and interior boundary conditions with temperatures of 0°F and 70°F and surface films 0.17 ft²·°F·hr/Btu and 0.68 ft²·°F·hr/Btu respectively. The material conductivities used for the modeling are provided in Table A-1.
Additional modeling parameters include the following:
- Material properties used are based on the following references: – ASHRAE Handbook
– Fundamentals2 – ASHRAE Standard 90.13
– Product testing data
– NFRC 1014 - All thermal modeling and resulting R-values are for standard wall assemblies, including the floor line where applicable, but do not account for additional framing and resulting heat flow around penetrations (e.g., windows and doors) unless otherwise noted.
- Modeling was completed for cladding support clips, masonry ties, and shelf angle support options based on common products available to the market but does not necessarily reflect the exact design and dimensions of these products.
- All air spaces, including vented air spaces behind masonry cladding, are assigned R-values based on values given for unventilated plain air spaces in the ASHRAE Handbook – Fundamentals.2 This approach is consistent with numerous studies showing that air cavities, including vented air cavities, provide measurable resistance to heat flow. However, some energy codes may require air cavities to be treated differently or neglected entirely from the R-value determination of an assembly. Consult the local energy code and the authority having jurisdiction for additional requirements.
- All steel-stud back-up walls include fiberglass-reinforced exterior-grade gypsum sheathing and interior gypsum board.
- Steel studs are not modeled with conduit cutouts in the web of the stud.
References
- Blocon USA. “HEAT3 – Heat transfer in three dimensions.” https://buildingphysics.com/heat3-3/ (retrieved June 14, 2018).
- American Society of Heating, Refrigerating and AirConditioning Engineers. 2017 ASHRAE Handbook – Fundamentals (Atlanta: ASHRAE, 2017).
- American Society of Heating, Refrigerating and AirConditioning Engineers. ANSI/ASHRAE/IES Standard 90.1-2016 Energy Standard for Buildings Except Low-Rise Residential Buildings, IP ed. (Atlanta: ASHRAE, 2016).
- National Fenestration Rating Council Incorporated. NFRC 101-2017 Procedure for Determining Thermophysical Properties of Materials For Use in NFRC-Approved Software (Greenbelt: National Fenestration Rating Council Incorporated, 2016).
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