We had our roof replaced this summer, and the roofers installed a new vent for the bathroom exhaust fan. It's a cathedral ceiling with a PVC roof, rigid foam and batting in the joist cavities. DH and I finally peeled back the insulation to install a new fan. The roof vent does not extend all the way through the roof sheathing--it stops about 1/4" shy of the bottom of the plywood. How do we attach and seal our fan ducting to this vent?