Fibre optics sensors have been identified as very good candidates for environmental monitoring inside the silicon detectors operated at CERN's Large Hadron Collider. The objective of this dissertation is the development of a Relative Humidity (RH) distributed fibre optic sensor, based on coherent Rayleigh scattering, for long-distance applications using phase-sensitive optical time-domain reflectometry technique, which is a technique that is gaining the attention of industry and academy due to its ultra-high sensitivity. Optical fibres are known to be intrinsically sensitive to temperature and strain; however the influence of humidity on coatings induces a secondary mechanical strain due to expansion/contraction when absorbing/desorbing water. Turning an optical fibre into a thermo-hygrometer requires a detailed investigation of the best coatings to measure RH and mitigate the cross-sensitivity with temperature. Several coatings were studied in order to find a fibre with an approximately constant humidity sensitivity at large range of temperature, as well as a purely temperature-sensitive optical fibre. Polyimide-coated fibres are the best candidates for RH sensing over a temperature range from -20°C to 50°C, while the standard acrylate-coated fibre have a surprisingly non-negligible humidity sensitivity but a completely different behaviour at different temperatures. Additionally, two families of humidity insensitive coated fibres were identified as excellent candidates for a pure temperature reference. The first is made of a multi-functional acrylate coating (Desolate) that provides a purely temperature-dependent measurement above15°, with a negligible response at lower temperatures, while the second one uses a silicone composite that secures a complete immunity to humidity from -20°C to 50°C.After identifying the best pair of coated optical fibres, an in-field application case was devised and validated at CERN. The new sensor makes use of a pair composed by a polyimide-and a desolite-coated optical fibres to monitor temperature and RH inside a block of concrete since the earliest moment of its curing process. The proposed solution represents a breakthrough improvement in the civil engineering monitoring systems.