An electrochromic switchable mirror can be switched between reflective and transparent states by applying voltage. This device can potentially be applied in new energy-saving windows because its reflective state can effectively control incoming solar radiation. We previously evaluated the effects of environmental factors such as temperature and humidity on the optical switching properties of the device using a thermostat/humidistat bath to realize its practical applications. In this work, we focused on the effect of relative humidity in the atmosphere on the properties of the device. When the device was stored at 50 degrees C and 80% relative humidity (RH) for 15 days, its surface became much rougher (R-a = 23.2 nm) as a result of the degradation of the Mg4Ni thin film on the surface. The degraded device showed no optical switching properties. On the other hand, when the device was stored at 50 degrees C and 30% RH for 15 days, its surface was barely degraded and the device showed clear optical switching properties. These results suggest that not high temperature but rather high relative humidity in the atmosphere strongly affect the properties of the device, resulting in surface degradation. A protective coating layer using a fluorocarbon polymer was also developed to prevent the degradation of the device in the environment. (C) 2011 Elsevier B.V. All rights reserved.