Randy
Two thoughts come to mind....seal the joint with caulking...where the cheeks of the tenon contact the beam or at the housing/beam interface.
Or
drain the mortice by drilling a 1/2 inch hole diagonally up and into the mortice from the far side of the beam
or
I know this is three, but how about slapping on a piece of ice and water shield cut to the tenon end dimensions before inserting the tenon.
As for sealing up the "wall" GREAT STUFF" Foam! ;-p
Bob
