If rainwater enters at this location, it will have very limited drying potential and can lead to moisture accumulation and damage within the assembly. Therefore, it is particularly important that this detail is designed and executed to be watertight.
A properly designed and correctly connected window sill should normally prevent rainwater from penetrating into this part of the construction. For that reason, I would only include a rain source here if you specifically want to assess the consequences of poor workmanship or a failure of the weatherproofing details.
But, is it realistic to add that source of rain leakage in that particular position if I suspect to have poor workmanship, a failure of the weatherproofing details or being in the worse scenario?
