The minimum thickness (m) of wall required to prevent condensation is

Question

Building has to be maintained at 21⁰C (dry bulb) and 1.5⁰C (wet bulb). The dew point temp under these conditions is 10.17⁰C. the outside temp is –23⁰ (dry bulb) and internal and external surface heat transfer coefficients are 8 W/m²K and 23 W/m²K respectively. If the building wall has a thermal conductivity of 1.2 W/mK, the minimum thickness (m) of wall required to prevent condensation is                   

      (a) 0.471                                                             (b) 0.407

      (c) 0.321                                                             (d) 0.125