Abstract A critical point for a new Thermal Barrier technique of indirect heating and cooling of buildings under construction is to maintain a constant temperature during the entire year. Such an effect can be obtained with the use of the proposed Gain Scheduling Control (GSC) system which implements a novel Fuzzy-Mixing Gain-Scheduling (FMGS) strategy that is based on the idea of fuzzy mixing (weighting) of local (modal) values of certain (automatically designed) control parameters. An important advantage of this approach is that the same scheme can be used both for scheduling controller gains and for fuzzy mixing the supply fluids in a temperature optimization procedure. Experiments show both excellent performance and effectiveness of the proposed HVAC control system.