Consider an external wall with given structure and geometry (Figure). a) Calculate the U-value of the wall applying the 'U-value method' presented in lec- ture slides. b) Calculate the temperature under the plywood at the joist, when the indoor tempera- ture is +21C and the outdoor temperature is -26C. The internal and external thermal resistances are 0.13 mK/W and 0.04 mK/W, respective- ly. The thermal resistances of the air gap and the vapor barrier are 0.09 mK/W (assume slightly ventilated air layer) and 0.02 mK/W, respectively. The thermal conductivities of the building materials are listed below. (W/mK): Thermal conductivities - Brick: 0.70 - Wind shield: 0.055 - Joist: 0.10 - Mineral wool: 0.055 - Plywood:0.13 Brick 130 mm Air gap 40 mm Wind shield 13 mm Mineral wool 150 mm US1 barrier Vapor + plywood 12 mm 13 160 T,=? Joist (150 mm)