This was a very general question, but to mention just one example: The thermal conductivity in a porous material may be substantially reduced (i.e. improved thermal insulation properties) by making the pores smaller into the nanorange by utilizing the Knudsen effect, see e.g. (a) B. P. Jelle, ”Traditional, State-of-the-Art and Future Thermal Building Insulation Materials and Solutions - Properties, Requirements and Possibilities”, Energy and Buildings, 43, 2549-2563, 2011, and, (b) B. P. Jelle, T. Gao, L. I. C. Sandberg, B. G. Tilset, M. Grandcolas and A. Gustavsen, ”Thermal Superinsulation for Building Applications - From Concepts to Experimental Investigations”, International Journal of Structural Analysis and Design, 1, 43-50, 2014.