This adaptive environment control system provides an HVAC control system that adapts to the continually-changing thermal characteristics of the building in which it operates. The adaptive environment control system periodically estimates the thermal characteristics of the building and uses these estimates to control the operation of the HVAC system. The adaptive environment control system also periodically measures the performance characteristics of the HVAC system to obtain data with which to update these thermal characteristic estimates. These thermal characteristic estimates enable the adaptive environment control system to determine both the length of time it takes the building to heat up and cool down when the HVAC system idles, as well as the amount of time it takes the HVAC system to heat or cool the building. These measurements and estimates are taken on a dynamic basis so that the adaptive environment control system responds to any changes in the environment, either within or without the building, including degradation of the operation of the HVAC system. The adaptive environment control system can calculate, through the estimates and measurements, the thermal performance of both the building and the HVAC system, so that the HVAC system is operational sufficiently early to maintain the building interior temperature within a predetermined range of the set-point threshold level of the thermostat.