In this paper the results of a research project to establish a fundamental understanding of the physical relationships between the material structure and solar optical properties of polymeric transparent insulation (TI) materials are described. First, a systematic characterization of potential materials for TI applications in respect to their relevant physical properties in the solar radiation range was done. Second, based on the investigation of polymer films with different thicknesses model functions for the solar optical properties were established. Third, the solar optical properties were interpreted on base of polymerphysical relations. The characterization in the solar range and the calculation of solar optical film properties have shown, that the solar extinction is dominated by scattering occurring mainly at the surface. A linear relationship between the solar optical thickness and the film thickness was obtained for film thicknesses ranging from 12 to 150 mu m. For various amorphous and semi-crystalline films the surface roughness correlated well with the solar optical thickness. (c) 2005 Elsevier Ltd. All rights reserved.