A thin mesoporous film of alternate layers of Poly(allylamine hydrochloride) (PAH) and silica (SiO2) nanoparticles (bilayers), was deposited using the layer-by-layer (LbL) method upon the cleaved tip of an optical fibre coupler with the aim of developing a sensor probe for humidity measurements. The sensing film behaves as a Fabry-Perot cavity of low-finesse where the absorption of water vapour changes the optical thickness and produces a change in reflection that can be used for humidity monitoring. The sensor was calibrated in a bench model against a commercially available capacitive sensor. The sensitivity and response time of the fibre-optic sensor were assessed in the humidity range from 5 % relative humidity (RH) to 95 %RH for different numbers of layers up to a maximum of nine. The sensitivity of the optical fibre humidity sensor increases according to the number of layers deposited; a sensitivity of 2.28 mV/%RH was obtained for nine bilayers. The humidity response time constant of the sensor (obtained by measuring the 1/e point as the device transitions from 95.1 %RH to 37.5 %RH) was 1.13 s ± 0.30 s which was faster than the commercial device (typically 30 s). After calibration, the optical fibre humidity sensor was utilised in a bench top study employing a mechanical ventilator. The fast response time enabled changes in humidity in individual breaths to be resolved providing a potential means to measure breathing rate.