Increasing attention is being paid to the application of uncertainty and sensitivity analysis methods to model validation and thermal systems simulation. The idea is to let users to apply uncertainty bands to their model input data. These bands are then propagated through the model to determine the uncertainty bands of the simulation results. One of the main difficulties the practitioner finds when trying to apply these techniques is the lack of information on the uncertainty that affects to typical input variables. This paper is a contribution to fill this gap. It quantifies the uncertainty that can be expected in the thermal conductivity of insulation materials in the lack of specific experimental measurements. Results are presented for specific materials and for groups of materials, in order to consider situations in which the material under consideration is not well defined. The proposed conductivities are derived from a large set of measurements that was compiled in a previous European project headed by the BRE Scottish Laboratory. To illustrate how the uncertainty bands can be used in practice, an example is discussed on the validation of the mathematical model of a solar thermal collector. (C) 2010 Elsevier B.V. All rights reserved.