HI- The window is a vinyl window, and we waterproofed it as if it were the outside of a house, using waterproof tile membrane instead of the bituthane self-adhesive membrane you would use outside. The "wood" trim is actually fypon engineered lumber (mostly plastic, and unaffected by moisture)

On similar windows where water sat stagnant, we have used vinyl blinds or glass "shutters" to shed water sucessfully.