Install the new 4 inch insulated duct in the bathroom ceiling by pulling with fish tape.

This project is continued from How to Replace a Bathroom Exhaust Fan and Ductwork – Part 3.

Fishing Flexible Duct through the Drywall Ceiling

To install the new run of 4 inch insulated duct for the bathroom exhaust fan, I needed to enlarge the ceiling opening where the ductwork will enter the interior soffit in the garage. Clearly the architect and home builder had failed to plan for the bathroom fan vent duct. I would have used a 4-1/4 inch hole saw to cut a nice round hole for the 4 inch duct but this was not possible because ceiling opening is boxed by floor joists and it would have been a major error to cut the bottom of a floor joist. Cutting the top or bottom of a joist greatly weakens it where the load carrying stresses are highest. The best I could do was open up the ceiling drywall by another 3/4 inch against the face of the floor joist and notch the soffit corner framing:

I now have a ceiling opening for the duct that is 5-3/4 inches by 2-3/4 inches for a total area of 15.8 square inches. The area required by the 4 inch duct is 12.5 square inches. A rectangular hole for a round duct is not ideal but it’s the best that can be done given the situation:

Fish Vent Duct in the Drywall Ceiling

The new 4 inch insulated air duct will be installed in two sections:

1. 17 feet of flexible duct from the bathroom to the soffit in the garage.
2. 7 feet of flexible duct inside the soffit to the exterior wall.

To install the new 4 inch flex duct in the bathroom ceiling, I pushed a dozen feet of nylon fish tape from the garage soffit into the ceiling space between the wood joists. The fish tape mostly just curled up at the far end of the joist bay. I duct taped a metal hook to the end of my telescoping painters stick to grasp the fish tape and pull it to the fan mounting hole in the drywall ceiling of the master bathroom, being careful to pull the fish tape OVER the electrical cables and copper water pipes:

I collapsed the telescoping painters pole to shorten it and pull the nylon fish tape through the bathroom vent fan mounting hole in the ceiling:

4 inch insulated duct will be installed for the new Panasonic WhisperCeiling™ FV-11VQ5 vent fan:

The reason I chose insulated duct is to prevent condensation as the warm humid air from the master bathroom runs through the interior soffit in the unheated garage. If water vapor were to condense in the duct in the cold garage, it could form puddles of water that might leak over time or became stale and allow mold/mildew to grow. The insulated duct will also minimize noise from fan vibrations although this shouldn’t be a problem with the super quiet Panasonic vent fan.

I pulled the new 4 inch vent duct through the bathroom ceiling to the garage by the following steps:

1. Cut back about 12 inches of insulation to expose the inner duct.
   Only the inner duct will make it through the ceiling hole in the garage.
2. Duct taped the insulation tight around the neck of the inner duct so it wouldn’t get hung up on nails, pipes and wires inside the ceiling.
3. Duct taped the nylon fish tape to the end of the inner duct.
4. While a helper in the garage pulled the nylon fish tape, I fed the flexible duct into the ceiling hole.
5. When the duct reached the hole in the garage ceiling, I traded places with my helper in the garage, then reached in the hole and guided the inner duct into the soffit.

The flexible duct after pulling with fish tape to the garage interior soffit. The procedure when surprising well with no hangups:

Here’s the new 4 inch insulated flexible duct in the joist bay as seen from the bathroom fan mounting hole:

Back in the master bathroom, I cut the flexible duct from the main roll and trimmed back about 10 inches of insulation to expose the inner duct. This end will be connected to the Panasonic FV-11VQ5 exhaust fan.

This project is continued if How to Replace a Bathroom Exhaust Fan and Ductwork – Part 5.

Take care,

Bob Jackson