A new technique for observing the phase change of water vapor in an insulation material is presented. It combines neutron radiography with gravimetric analysis in order to obtain quantitative two-dimensional insight in a transient process due to a transient temperature boundary condition at one side of the insulation. The advantage of this approach is, that the neutron radiography provides high-resolution pictures while the process is slow enough to allow exposure times of nearly a minute. The article describes the experimental setup and the analysis method and shows results which will allow a future validation of transient mathematical models. (C) 2010 Elsevier Ltd. All rights reserved.