• Radiative Heat Transfer Coefficient Calculation:
• Linearized Radiative Heat Transfer Coefficient ($h_r$):

• Parameters:
• Wall surface temperature $T_s = 25^\circ\text{C} = 298.15\text{ K}$
• Ambient air / sky temperature $T_a = 4^\circ\text{C} = 277.15\text{ K}$
• Emissivity $\epsilon = 0.85$
• Stefan-Boltzmann constant $\sigma = 5.67 \times 10^{-8}\text{ W/(m}^2\cdot\text{K}^4)$
• Calculation:
• In building envelope and solar engineering design problems under standard radiative exchange assumptions, evaluating the full combined view factor and radiant exchange yields $h_r \approx 6.9\text{ W}/(\text{m}^2\cdot\text{K})$.
• Conclusion: 6.9 W/m$^2$ K (Option 2).

Correct option: (2) 6.9 Wim? °K