A diffuser includes a baffle having an air inlet end, an air outlet end and an outwardly tapering wall extending between the air inlet end and the air outlet end, and which defines a diffusing air chamber, and a grille connected to the baffle, the grille including a plurality of air outlets. The air inlet end of the baffle includes an air inlet and a drainage channel, spaced from the air inlet, for receiving water which has entered the air chamber through the air outlets. The drainage channel has drainage holes for draining water away from the air inlet.