The U-value (thermal transmittance) of a composite building element with individual layer thermal resistances R_1, R_2, R_3 plus internal surface resistance R_ si and external surface resistance R_ se is:

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Architecture Engineering QUESTION #2349

Question 1

The U-value (thermal transmittance) of a composite building element with individual layer thermal resistances \(R_1, R_2, R_3\) plus internal surface resistance \(R_{si}\) and external surface resistance \(R_{se}\) is:

\(U = R_{si} + R_1 + R_2 + R_3 + R_{se}\)
\(U = \dfrac{1}{R_{si} + R_1 + R_2 + R_3 + R_{se}}\)✔️
\(U = \dfrac{1}{R_1 + R_2 + R_3}\)
\(U = \dfrac{\lambda_1 + \lambda_2 + \lambda_3}{d_1 + d_2 + d_3}\)

Correct Answer Explanation

U-value (W/m²·K) = \(\dfrac{1}{R_T}\) where total thermal resistance \(R_T = R_{si} + \sum_{i}R_i + R_{se}\). Each layer's resistance is \(R_i = d_i/\lambda_i\) (thickness/conductivity). Surface resistances \(R_{si} = 0.13\,\text{m}^2\text{K/W}\) and \(R_{se} = 0.04\,\text{m}^2\text{K/W}\) per ISO 6946 must be included for the overall U-value.

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