Cumulative installed PV wattage in Thailand since the 1980s was over 30 MW. A typical module would have a structure of glass-EVA-cell-EVA-tedlar back-sheet. During the hot season, temperature can be over 40 degrees C coupled with the humidity of over 90% in the monsoon season. Apparent physical deteriorations of modules under long term field-exposure have been observed. In this paper, we report results of visual physical deteriorations of 39 x-Si modules with more than 15 years of operation. We compare these with insulation measurement based on the IEC 61215 Standard. It is found that even with severe delamination of the top EVA sheet from cells, the insulation resistance of all degraded modules is still within a limit of the prescribed IEC standard, which is 102.67 M Omega m(2) for a module area of 0.3949 m(2). We further note that degradation of modules resulting in large decrease insulation resistance is peeling of tedlar back-sheet and edge sealant. (C) 2010 Elsevier B.V. All rights reserved.