This work presents an optical humidity sensing technique based on the combination of a whispering gallery mode microtoroidal cavity sensor and a nm-scale thick humidity-responsive polymer coating deposited via the initiated chemical vapor deposition process. As a result of the conformational change by the polymer in response to humidity, the sensitivity is increased by nearly two orders of magnitude in comparison to conventional refractometric sensing. Additionally, the dependence of the device performance on the film thickness is studied. Specifically, the thinner film enabled a faster response rate, yet a slower recovery rate, as compared to the thicker films.