ASHRAE-90.1-2022- - Page 276

Extracted Content

The impact of the thermal bridging associated with the outside of the girt and the insulation is to reduce the thermal resistance of the insulation. The reduction is calculated using Equation A9.4-38:

(A9.4-38)

1 + ---------------------------- Lf

ROP = ROPI

1

Step 6 : Add the R-values inside and outside the girt including air films. The total thermal resistance associated with the girt is the sum of the R-values inside and outside the girt as shown in Equation A9.4-39:

RTP = RUP + ROP

(A9.4-39)

The next calculation is to add the inside and outdoor air film coefficients using Equation A9.4-40:

RTP + air = RTP + RAB + RAT

Step 7: Calculate the overall insulation assembly using the R-values in Steps 3 and 6. The overall insulation system R-value is determined using Equation A9.4-41:

Rinsul - sys

= --------------------------------------------------------------  2 L 1 + Lf  RBP + airRTP + air - 2 L 1 RTP + air + LfRBP + air

Step 8: Calculate the U-factor from the finite element analysis results. The overall U-factor for the insulation assembly is determined using Equation A9.4-42:

(A9.4-40)

(A9.4-41)

(A9.4-42)

Uadj

1 = ---------------------------------------------------------- - 0.8627 Rinsul - sys + 1.132

where Uadj = adjusted overall U-factor represented by the correlation with the finite element modeling in Btu/ h·ft [2] ·°F. Step 9: Calculate the overall U-factor for any continuous insulation if present. If there is any continuous insulation present, first calculate the R-value adjacent to the flange using Equation A9.4-43:

= R ----------------------------

RBFci = Rci

1 + 2--------------  hci 

1

(A9.4-43)

• ------------- Lf

where RBFci = thermal resistance of continuous insulation adjacent to the flange, h·ft [2] ·°F/Btu Rci = thermal resistance of the continuous insulation, h·ft [2] ·°F/Btu hci = thickness of the continuous insulation, ft Next, calculate the area-weighted R-value for the continuous insulation using Equation A9.4-44:

Roci

=  ---------------------------------------------2 L 1 + Lf  RBPciRci 2 L 1 RBPci + LfRci

where Roci = overall thermal resistance of continuous insulation in h·ft [2] ·°F/Btu Finally, calculate the overall U-factor using Equation A9.4-45:

(A9.4-44)

(A9.4-45)

Uo

1 = -------------------------1 -------- Uadj - + Roci

A9.4.7 Insulated Metal Panels. U-factors of insulated metal panels shall be determined by two- or threedimensional finite difference or finite volume computer models or by testing in accordance with Section A9.3.2 and shall include panel side joints.

A10. THERMAL BRIDGING CHI FACTORS AND PSI FACTORS

A10.1 Determination of Psi-Factors and Chi-Factors. Psi-factor (ψ) and chi-factor (χ) values representative of an as-designed thermal bridging condition shall be determined in accordance with one of the following:

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