The indoor climate conditions of monumental buildings are very important for the conservation of these objects. Simplified models with physical meaning are desired that are capable of simulating temperature and relative humidity. In this paper we research state-space models as methodology for the inverse modeling of climate responses of unheated monumental buildings. It is concluded that this approach is very promising for obtaining physical models and parameters of indoor climate responses. Furthermore state space models can be simulated very efficiently: the simulation duration time of a 100 year hourly based period take less than a second on an ordinary computer. INTRODUCTION Effects of climate change on ecosystems and on the global economy have been researched intensively during the past decades but almost nothing is known about the influence to our cultural heritage. Although these historical monuments are exposed to extensive loads caused by stampedes of visitors, there are many other factors deteriorating World Heritage Sites. The impacts of climate change are a longterm and substantial menace to the sites. Lots of monumental buildings are used as museum or storage for paintworks, books and artefacts. The indoor climate conditions of monumental buildings are very important for the conservation of these objects (ASHRAE, 2007). The influence of the changing climate on the indoor climate of monumental buildings is unknown. It is impossible to prepare adequately for the future, by anticipating on this change and adapting the installations, because of this lack of knowledge (CIBSE, 2005). The result is that the conservation of the buildings and the collections are at risk. Furthermore, the worldwide energy problem also affects these monumental buildings. Due to the ancient building techniques used, which result in huge transmission and infiltration losses, the energy consumption of these buildings is high. Three problems can be identified with respect to the current research methods: Firstly, due to the long simulation period (hundred years with time step 1h), combined with detailed physical models, the simulation run time is long. Secondly, the detailed modeling of the buildings itself requires much effort: the monumental buildings are old and protected. Therefore, blueprints are hard to find and; destructive methods to obtain building material properties are not allowed. Thirdly, the used modeling approach does neither facilitate in an easy characterization of the building nor in an easy characterization of the energy performance. A simplified model with physical meaning is desired which is capable of simulating both temperature and relative humidity. The objective is the successful application of inverse modeling on a simplified thermal and hygric building model, in order to determine the parameters with physical meaning. The simplified model is needed for the prediction of the indoor temperature and the indoor relative humidity and for characterization of the building parameters and the energy performance. The paper is organised as follows. We start with a literature study on simplified models. The methodology of the inverse modeling development for climate responses is presented and applied for a group of four unheated monumental buildings in The Netherlands and Belgium. The approach is evaluated and conclusions are drawn.