Answer to Question #267770 in Molecular Physics | Thermodynamics for Maheshi

Question

6. A house wall consists of a 10 cm thick layer of brick (𝜅 = 0.68 𝑊𝑚−1𝐾

−1

), a 15 cm 

thick layer of fiberglass insulation (𝜅 = 0.038 𝑊𝑚−1𝐾

−1

), and a 1 cm thick layer of 

gypsum board (𝜅 = 0.48 𝑊𝑚−1𝐾

−1

). The interior surface temperature is 22℃ and exterior 

surface temperature is 5 ℃.

(a) What is the overall R-value of this wall?

(b) What is the heat flux through this wall?

Answer 1

a)

$R=R_1+R_2+R_3=\frac{\sigma_1}{\kappa_1}+\frac{\sigma_2}{\kappa_2}+\frac{\sigma_3}{\kappa_3}=4.115~\frac{m^2\cdot °C}W,$

b)

$Q=R\Delta T=70~\frac W{m^2}.$