ASHRAE-90.1-2022- - Page 277

Extracted Content

a. From simulation models compliant with ISO 10211 using details representative of the actual construc tion and modeling assumptions consistent with generally accepted architectural and engineering practice. b. From ISO 14683. c. From testing of the assembly in accordance with ASTM C1363 with and without the presence of the

thermal bridge condition to determine a linear transmittance value or point transmittance value for the thermal bridge condition. d. From application of heat transfer theory in accordance with generally accepted engineering practice and

where approved by the a uthority having jurisdiction . e. As indicated in Table A10.1. The default column shall be used where the thermal bridge meets prescrip tive requirements. The unmitigated column shall be used where the thermal bridge does not meet the prescriptive requirements.

( Informative Note: In Table A10.1, the values for thermal bridge details prescribed in Section 5.5.5 are based on data from ASHRAE Research Project 1365 and the BC Hydro Thermal Bridging Guide listed in Informative Appendix E.)

A10.2 Assembly U-Factor Adjustment for Simulation of Thermal Bridges. For the purpose of incorporating the effects of thermal bridges in simulations as required by Section 12 and Normative Appendix G, where a thermal bridge is not modeled as a separate element, the clear-field U-factors of modeled assemblies shall be modified in accordance with Equation A10.2. This modification shall be achieved in the simulation model by altering the conductance value assigned to any one or more insulation layers within the modeled assembly without altering the properties of modeled building material layers.

Utot = {[(∑ψ i × Li ) + (∑χ j × nj )]/ Atotal ) + Uo } (A10.2)

where Utot = overall thermal transmittance, including the effect of linear thermal bridges and point thermal bridges not included in the construction assembly Uo -factor, Btu/(h·ft [2] ·°F) Uo = clear-field thermal transmittance of the construction assembly as determined in accordance with Section 5, Btu/(h·ft [2] ·°F) Atotal = total opaque projected surface area of the construction assembly, ft [2]

ψ i = psi-factor, thermal transmittance for each type of linear thermal bridge, Btu/(h·ft [2] ·°F) Li = length of a particular linear thermal bridge as measured on the outside surface of the building envelope, ft [2]

χ i = chi-factor, thermal transmittance for each detail type of point thermal bridge, Btu/(h·°F) ni = number of occurrences a particular type of point thermal bridge

ANSI/ASHRAE/IES Standard 90.1-2022 (I-P) 275