Influence of degree days calculation methods on the optimum thermal insulation thickness in life-cycle cost analysis for building envelopes in Mediterranean and Semi-Arid climates

The life-cycle cost analysis (LCCA) is one of the most widely used and validated methods to identify the optimum insulation thickness of the building envelope. This method depends on two parameters—heating and cooling degree days (HDD, and CDD). There are various methods for HDD and CDD calculation, the results of which vary significantly depending on the climatic zone. Therefore, this study aimed to analyse the optimum expanded polystyrene insulation thicknesses of a typical wall in Andalusia, Spain, obtained by the LCCA method to demonstrate variations and uncertainties of the results of the LCCA method based on HDD and CDD calculated by the “hourly”, UKMO, and ASHRAE methods, and with different base temperatures in the conditions of Mediterranean and Semi-Arid climates. On average, the HDD values calculated by using the ASHRAE method were lower, by 12.5%, than those calculated using UKMO and the “hourly” method. The CDD values calculated using the ASHRAE method were lower, by 29%, than those obtained using UKMO and the “hourly” method. Consequently, the optimum insulation thicknesses determined based on CDD and HDD calculated using the ASHRAE method were, on average, 14% underestimated compared with the thicknesses obtained using the UKMO and “hourly” methods. The results obtained showed that with an increase in the continentality of the climate, it becomes crucial to correctly use one or another method for calculating degree days to determine the optimum insulation of the building envelope using the LCCA method, which, in turn, requires the development of validation methods.