The problem of multi-objective optimisation of air-conditioning (AC) systems is treated in the paper in the framework of intuitionistic fvzzy (lF) set theory. The nature of the problem is multi-objective one with requirements for minimal costs (generally, life cycle costs; more specifically, energy costs) and maximal occupants' comfort (minimal discomforl). Moreover, its definition by conventional means is bounded to a number of restrictions and assumptions, which are often far from the real-life situations. Attempts have been made to formulate and solve this problem by means of the fuzzy optimisation . The present paper makes further step by exploring the innovative concept of IF sets  into definition of the trickiest issue: comfort and discomforr definition. The new approach allows to formulate more precisely the problem which compromises energy saving and thermal comfort satisfaction under given constraints. The resulting IF optimisation problem could be solved numerically or, under some assumptions, analytically Il]-t21. An example illustrates the viability of the proposed approach. A solution which significantly (with 3S%) improves comfort is found which is more energetically expensive with only 0.6Yo. This illustrates the possibility to use the approach for trade-off analysis in multi-objective optimisation of AC systems.