Urban Heat Island (UHI) identifies the phenomenon that occurs in built up areas which are hotter than nearby rural areas. One of the main strategies to mitigate UHI is the use of cool materials which increase surfaces reflectivity and reduce the amount of radiation that is absorbed by urban surfaces. In this paper the effect of cool materials applied on building envelopes in urban canyons are investigated. The experimental facility is composed of two different arrays resembling urban canyons with H/W ratio equal to 1, 0.5 and 2. A white cool diffusive material and a grey diffusive material were applied on the vertical surfaces of the two arrays. The results of a monthly monitoring campaign were analyzed with the focus on the temperature trends inside the canyons with same geometry but different envelope materials. The cooling properties of white vertical surfaces in urban canyons with different geometries were assessed. Finally, a thermal simulation was completed in order to extend the results obtained in the canyon physical model to a full-scale urban canyon.