A brick wall 19 cm thick has a thermal conductivity 0.811

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A brick wall 19 cm thick has a thermal conductivity 0.811 W/m °C. The outside and inside surface conductance of the wall are 16 W/m2 °C and 8 W/m2 °C respectively, then the U-value of the wall in W/m² °C is ____________

1. 4.50 to 4.95
2. 3.50 to 4.45
3. 2.30 to 2.45
4. 5.50 to 6.45

Correct Option: C

(2.30 to 2.45)
h_i = Heat transfer coefficient on inner side of the wall, W/m² °C
h_o = Heat transfer coefficient on outer side of the wall, W/m² °C
l = Thickness of the slab, m
k = Thermal conductivity, W/m °C

	1	=	1	+	1	+	1
	U		h_i		h₀		k

	1	=	1	+	1	+	.19
	U		16		8		.811

By Solving above equation, we get
U = 2.37 W/m² °C