ASHRAE-90.1-2022- - Page 269
Extracted Content
C k = A + B +
[—] [-]
where k = thermal conductivity, Btu/h·ft·°F = density, lb/ft [3]
A = 0.014917 B = 0.0004377 C = 0.0056897 Step 2: Assume the fiberglass batt forms a parabolic profile defined by Equation A9.4-3:
X
Y = Yo + Ym - Yo [X] [---] L 2 - [---] L
Determine the cavity U-factor ( Uc ) using Equation A9.4-4:
(A9.4-2)
(A9.4-3)
(A9.4-4)
B oto
Uc
oto 2 YoYm
Ym - Yo 1 + ---------------- - --------------------------- Ym 1 - Y ---------------- m - Y - o Y
1 + Y ---------------- m - Y - o
--------------------------- Ym
1 - Y ---------------- m - Y - o
- ---------------- -
--------------------------- Ym
- Y ---------------- m - Y - o Ym
where o = reference density of the fiberglass, lb/ft [3]
to = reference thickness of the fiberglass, ft The properties of fiberglass insulation are presented in Table A9.4.6.1. Include the thermal resistance s of the interior ( Ri ) and exterior ( Re ) air films to calculate the overall cavity U-factor ( Uco ) using Equation A9.4-5:
Uco
1 = -----------------------------1 ---- U - c + Ri + Re
(A9.4-5)
Step 3: Determine the U-factor ( Ufo ) over the structural framing member. The variable Yo represents the total combined thickness of the thermal spacer block and the compressed insulation. The density of the compressed insulation is determined by Equation A9.4-6:
c
= --------- oto tc
where
c = density of the compressed insulation over the framing member, lb/ft [3 ] tc = thickness of the compressed insulation over the framing member, ft Determine the thermal resistance of the compressed insulation ( Rc ) using Equation A9.4-7:
(A9.4-6)
(A9.4-7)
Rc
= ------------------------------------- tc - A + B c + C c
Determine the overall framing U-factor ( Ufo ) at the structural framing member, including the air film resistances, using Equation A9.4-8:
Ufo
1 = -------------------------------------------RTB + Rc + Ri + Re
(A9.4-8)
where Ufo = U-factor over the structural framing member, Btu/h·ft [2] ·°F RTB = R-value of the thermal spacer block, h·ft [2] ·°F/Btu Rc = R-value of the compressed insulation, h·ft [2] ·°F/Btu
ANSI/ASHRAE/IES Standard 90.1-2022 (I-P) 267