Transient experimental validation and investigation of the effect of diffusivity of porous layers on the dynamic water vapor partial pressure profiles of a proton exchange membrane fuel cell (PEMFC) during load change is presented. A three dimensional, isothermal, transient, single-phase computational fluid dynamics based model is developed to validate with the water partial pressure profiles experimentally measured during start-up conditions earlier in a 50 cm2 PEMFC having a single serpentine flow path in counter-flow configuration. The fluid flow within the serpentine channel geometry is simulated using a straight channel fuel cell model with total channel length equivalent to the stretched length of the entire serpentine path incorporating the same amount of pressure drop from inlet to outlet. The model equations are solved using a multi-domain approach incorporating water transport through membrane and multi-component species transport through porous diffusion layer. The transient model predictions of water partial pressure profiles of anode and cathode channels are found to be in good agreement within the error bounds of the experimental results. This validation is also indicative of the two different time scales i.e. initial anode dip due to electro-osmotic drag and recovery due to back diffusion from cathode to anode. Steady state model predictions are compared to check for accuracy simultaneously. The model also delineates the significance of effective diffusivity of porous Gas Diffusion Layers (GDL) and Catalyst Layers (CL) on transient characteristics. In order to come up with best parameters to validate with experimental data, a sensitivity analysis with parametric variations of effective porosity of GDL and CL is performed with a single experimental data set and then applied to the remaining sets. Results show that the CL diffusivity has a more pronounced effect on water accumulation as well as on temporal water transport than GDL diffusivity. The numerical simulation thus provides a validated set of quantitative model parameters along with an insight to the underlying physics of water transport phenomena in a PEMFC.