ASHRAE-90.1-2022- - Page 273
Extracted Content
a thermal spacer block or continuous insulation present. Air spaces may also exist depending on the specific drape profiles.
There are nine steps in the calculation process:
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Step 1—Characterize the thermal conductivity of the mineral fiber insulation.
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Step 2—Define the parabolic profiles for each insulation layer.
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Step 3—Calculate the R-values for insulation and air spaces in cavity both outside and inside insulation layers, including air films.
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Step 4—Calculate the R-value inside the girt and adjacent to the web.
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Step 5—Calculate the R-value outside the girt.
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Step 6—Add the R-values inside and outside the girt, including air films.
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Step 7—Calculate the overall insulation assembly using the R-values in Steps 3 and 6.
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Step 8—Calculate the U-factor from the finite element analysis results.
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Step 9—Calculate the U-factor for any continuous insulation if present.
Step 1: The thermal conductivity of the mineral fiber insulation is represented by a thermal curve of the form in Equation A9.4-22:
(A9.4-22)
= A + B --------- o - o + C --------- y - y o o
k = A + B
o o
where k = thermal conductivity, Btu·ft/h·ft [2] ·°F - = nominal density, lb/ft [3]
- = nominal thickness, ft y = thickness of insulation, ft A = 0.014917 B = 0.0004377 C = 0.0056897 Step 2: Assume that each layer of mineral fiber has a parabolic profile defined by Equation A9.4-23:
---------------- y - Yo - = ----- x 2 - ----- x Ym - Yo xm xm
(A9.4-23)
where x = distance from edge of girt, ft y = distance from edge of wall panel, ft Yo = insulation thickness at x = 0, ft Ym = insulation thickness at x = Xm, ft Step 3: Calculate R-values for the insulation and air spaces in the cavity both inside and outside insulation layers, including air films.
Because the configuration can possibly consist of both mineral fiber insulation and an air space, the composite is given by Equation A9.4-24:
R = ---- x 1 a x 0 a k ---- ya + Y ------------- mk – y
xa ---- y + Y ------------- m - y - dx + ------------ Ym -
0 ka k k Ym
1 xa
= ----
---- y + ------------- m - dx k k
(A9.4-24)
where ka is the thermal conductivity of air in Btu·ft/h·ft [2] ·°F.
The trapezoidal integration method is used to evaluate the integral and calculate R and is given by Equation A9.4-25:
b
f x dx
a
12 [-] [-] Nk = 1 xk + 1 – xk yk + yk + 1
(A9.4-25)
where xk = point to analyze along the x -axis, ft xk +1 = point ahead of the point being analyzed, ft yk = thickness at point being analyzed, ft yk +1 = thickness at point ahead of the point being analyzed, ft
ANSI/ASHRAE/IES Standard 90.1-2022 (I-P) 271