Once the fan is installed, the author strategically cuts a hole as close to a framing member as possible. This member will provide support for the vent pipe. Temporarily using pipe strap, he attaches a short length of 3-inch PVC pipe to the rafter next to the hole.
To further help prevent condensation, the author installs a secondary damper near the outlet, using this type of PVC backdraft damper, which is available online for around $10.
He can find the damper only in a 4-inch diameter, so he tapes it to a 3-inch PVC coupler.
The author glues the damper assembly to the short PVC pipe on the rafter, making sure that the damper is in the appropriate closed position.
After cutting the duct to length, the author tapes its interior sleeve around the damper assembly.
The insulation jacket is cut longer than the inner sleeve to allow the insulation to be pulled over the short PVC pipe, insulating it up to the roofline.
To secure the insulation, he backs out the screws on the pipe strap, pulls up the insulation, and after a final check outside to make sure the pipe is plumb and straight, he reattaches the pipe strapping permanently.