The performance of an intrusion detector depends on several factors, like its internal architecture and the algorithms it uses. Thus, distinct detectors can behave distinctly when submitted to the same inputs. The project diversity theory has been successfully used in the fault tolerance domain, and can also bring benefits to the intrusion detection area. The objective of this paper is to propose and evaluate a mathematical model, based on the fuzzy set theory, for the composition of heterogeneous intrusion detectors analyzing the same event flow. This model intends to combine the individual detectors' results into a more accurate global result. Experimental results show the usefulness of this approach.