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25 Marks Question 1 The following bottom to top construction is given for a flat roof: • Internal surface convective coefficient 8 W/m2K. 100 mm reinforced concrete slab (k = 2.3 W/mK) • 50mm concrete screed (k = 0.41W/mK) • 100mm of rigid polyurethane insulation (k = 0.022 W/mK). 10mm of bitumen felt layers (k = 1.15 W/mk). • External surface convective coefficient 25W/m²K. . (a) Find the conductance “U” value for the roof if there is no thermal bridging. (10 Marks) (b) Find the conductance "U" value for the roof, if 10% of the insulation layer is thermally bridged by the concrete screed. (10 Marks) (c) How much additional insulation is required for this roof to have a "U" value of 0.20 W/m²K. (5 Marks)