The possibility of using different types of fly ash as a major component of pastes used for the manufacture of thermal insulating elements was studied. Fly ashes from the combustion of pulverized coal in a power plant and from the combustion of biomass were tested. The insulating capacity and compressive strength of the cured pastes were measured. The effect of some binders and additives were also studied. A good linear correlation between the insulating capacity of the mortars and the energy absorbed by them in the Differential Scanning Calorimetry test was observed. This relationship allows us to draw conclusions about the insulating capacity of different ashes, and makes it easier to choose those ashes that present better behavior. Consequently, there is no need to prepare such a large number of test pieces and the work involved in the experiment is simplified. Results show that some of the products prepared in this experiment containing a high proportion of ash, have promising thermal and mechanical properties, and show a potential use as new fireproof products. (c) 2004 Elsevier Ltd. All rights reserved.