The paper develops a non-linear finite difference model for the simulation for capillary water absorption in porous building materials based on Richard’s equation. Subsequent to its validation for two distinct materials – fired clay brick and OPC-lime-sand mortar, the model has been used to analyse the sensitivity of moisture intrusion characteristics, namely, (i) the moisture content and (ii) the moisture penetration depth, to the parameters Do and n of the hydraulic diffusivity function, D(θn) = Do exp (n θn). For the considered range of n (-4 to +4 % of the actual) and Do (-10 to +10 % of the actual) values, the shape of the moisture profile remained unaffected. The difference between the simulated and actual values of moisture content and penetration depth have been observed to increase or decrease linearly with a corresponding increase or decrease in the values of Do and n. Numerical experimentation also reveals that for a given material, the moisture intrusion profiles are more sensitive to perturbations in the magnitude of the parameter n in comparison to Do. The reported results highlight the need for further studies towards the quantification of errors induced in the estimates of hydraulic diffusivity function in relation to different modelling techniques.