An architect wishes to limit the nightly energy loss through a single exterior wall…
PHYSICS · Multi-part · CBSE · Physics · Thermal Properties of Matter · Thermal Properties of Matter
Problem
An architect wishes to limit the nightly energy loss through a single exterior wall of a proposed building. The wall will consist of a 12cm layer of concrete (thermal conductivity 1.4Wm−1K−1) directly bonded to a layer of rigid foam insulation (thermal conductivity 0.030Wm−1K−1). The wall area is 40m2.
During a typical winter night the indoor air temperature will be maintained at 18∘C while the outdoor air remains at −4∘C for 10 hours. Neglect convection and radiation so that conduction dominates.
(a) Derive an expression for the effective thermal resistance of the composite wall in terms of the individual layer thicknesses.
(b) If the foam insulation layer is initially 5.0cm thick, calculate the total thermal energy that would escape through the wall during the 10-hour night.
(c) The design goal is to reduce the nightly energy loss from the value found in part (b) by 60\%. Keeping the concrete layer unchanged, determine the foam thickness required to achieve this target.
(d) Discuss two practical considerations, other than purely thermal calculations, that might limit the maximum thickness of foam insulation an architect can specify in an actual building project.