A system for providing adaptable control of an HVAC system which regulates environmental conditions within an enclosed area. In a preferred embodiment, the apparatus and method of the present invention measures selected environmental variables in the enclosed area, calculates a value of a comfort index which is a function of the values of the selected environmental variables and a plurality of parameters that predicts a thermal sensation rating of an occupant. The system receives an actual thermal sensation rating from the occupant and compares the predicted thermal sensation rating with the actual thermal sensation rating to determine a difference between the predicted thermal sensation rating and the actual thermal sensation rating. The system uses a parameter estimation process that estimates the value of at least one parameter in the plurality of parameters to reduce the difference between the predicted thermal sensation rating and the actual thermal sensation rating and then changes the value of at least one parameter, if necessary or desired. In one embodiment, the system repeats these steps until the thermal sensation rating provided by the occupant is substantially equal to neutral thermal sensation. In a preferred embodiment, the comfort index is an explicit, linearly parameterized function which represents the non-linear behavior of environmental variables.