Al2O3 thin films deposited by atomic layer deposition on Pt nanoparticle-based strain sensors were studied as humidity barrier coatings for sensor protection. The effect of two deposition parameters-film thickness and growth temperature-is discussed in relation to an Al2O3 coating's ability to isolate the nanoparticle surface and protect strain sensitivity from humidity variations. It is shown that transmission electron microscopy images cannot confirm the effective protection of a nanoparticle surface, thus x-ray photoelectron spectroscopy and electrical measurements have been employed. The existence of a critical thickness of the Al2O3 protective film above which resistance and gauge factor variations are suppressed during humidity change was observed at different deposition temperatures. This ability is linked with the existence of incorporated pinholes and intrinsic hydroxyl groups in the Al2O3 thin film, which are responsible for humidity transport through the oxide.