In order to improve the sensitivity of an interferometric humidity sensor based on a photonic crystal fiberwith a sputtered SnO2nanocoating, a study of the effect of its thickness on the sensitivity is presented inthis paper. Sensors with coatings of different thickness were performed by applying distinct sputteringtimes in order to obtain an optimal thickness: the resulting nanofilms ranged from 470 to 1800 nm.Sensors were tested increasing the relative humidity from 20% to 90%, finding that the thickness wasa key parameter which has to be optimized at nanometer scale to get the best sensitivity. The studypoints that there is an optimal thickness and higher or lower thicknesses worsen the sensitivity of thesensor. The optimal sensor showed a humidity resolution of 0.067%HR, a wavelength shift of 67 nm anda negligible sensitivity with the temperature.