Optimum insulation thickness for building walls in a hot-dry climate

The optimum thickness of an insulation layer in a typical building wall is determined under steady periodic conditions using the climatic data of Riyadh. A finite-volume implicit procedure, which has been previously validated, is used to compute the yearly heat transmission loads for various insulation thicknesses. These loads are input to an economic model, based on the present worth method, in order to minimise the total cost of insulation and energy consumption over the lifetime of the building. Cooling and heating loads are integrated separately over the year and treated with different costs in the economic analysis. The wall yearly transmission loads, yearly-averaged dynamic R-value, time lag and decrement factor are presented versus insulation thickness and compared for different wall orientations. A parametric study is performed to establish the sensitivity of the results to changes in economic parameters. The optimum insulation thickness is found to increase with the cost of electricity, building lifetime and inflation rate; and decrease with increasing cost of insulation material, coefficient of performance of air-conditioning equipment and discount rate. The results also show that the wall orientation has a significant effect on the thermal behaviour but a relatively smaller effect on the total cost and consequently the optimum insulation thickness. The south-facing wall is the most favourite orientation since it gives about 12% lower yearly transmission load and 5% lower total cost compared to the least favourite orientation which is the west-facing wall. Among the insulation materials investigated, molded polystyrene is found to be the most economical type with an optimum thickness of 9.3 cm.