Many studies are nowadays focused on the application of energy conservation measures (ECMs) to building sector because of the reductions of both greenhouse gases and energy consumption set by the European Union. Heating, Ventilating and Air Conditioning (HVAC) systems are the main causes of most energy consumption in the existing buildings. One of the methods to reduce energy consumption is the modification of setpoint temperatures adapted to external climate conditions but maintaining acceptable comfort levels. The adaptive comfort model of ASHRAE 55–2017 uses the prevailing mean outdoor temperature, which records temperatures of the previous days weighted by a constant α which depends on weather changes. This paper analyses the possibilities of optimizing building energy saving with setpoint temperatures according to the α-value and the number of previous days considered. A total of 390 simulations were conducted by considering three representative climate zones of Spain. The results showed that the α-values between 0.4 and 0.6 had the lowest energy consumption. The use of a low number of previous days also achieved greater reductions in heating energy consumption, whereas a larger number of days was applied to cooling energy consumption. This study makes progress in using adaptive setpoint temperatures optimally to reduce the energy consumption in the building sector. © 2019 Elsevier Ltd