The influence of moisture supply on the crosslinking rate in room temperature vulcanizing poly(dimethyl siloxane) is investigated using spatially resolved 1H nuclear magnetic resonance relaxometry. Using this technique, we observe heterogeneous crosslinking manifested in a front moving into the material from the exposed surface, which separates the crosslinked and uncrosslinked parts of the material. The presence of a sharp boundary indicates that moisture transport towards the front is the limiting factor in the crosslinking process. As a result, the moisture supply controls the crosslinking front velocity, which we capture in a diffusion-reaction model. Reasonable agreement is found between the front diffusivity retrieved directly from nuclear magnetic resonance experiments and calculated values using independently measured properties of the crosslinked silicone rubber. This result demonstrates the direct relationship between exposure moisture content, moisture diffusion, and crosslink density on the crosslinking front dynamics in room temperature vulcanizing poly(dimethyl siloxane).