Organic coatings for building exterior walls are attracting a lot of attention because of the requirement for long-term usage of buildings and other constructs. Polyurethane (PU) has been commonly applied on building exterior walls. UV irradiation tests were carried out in a custom made apparatus at low relative humidity (＜ 10%) and several surface temperatures. For irradiated samples, IR and Raman spectra were measured to study change of chemical structure. Irradiated samples did not yellow on exposure and we could not observe an increase of double bond groups (C=C, C=O). In terms of temperature dependent degradation reactions, the Photo-Fries reaction seemingly does not depend on temperature, whereas Norrish type reactions are promoted by higher temperature. Moreover, we investigated the relationship between aggregate-state and change of elastic modulus, in early stages, it shows a better correlation between them than in later stages.