Things You'll Need
Bathroom fan exhaust kit, with hose, exterior vent with attached vent tube, and plastic or metal belt clamps
Hole-saw bit slightly larger than the diameter of the exterior vent tube (generally a 4-1/4 inch saw)
Did you ever wonder where the air from your bathroom exhaust fan goes after it gets sucked out of the bathroom and up into the ceiling? Well, sometimes, that's the only place it goes: Up into the ceiling. It's surprisingly common to find that previously installed exhaust vents are merely pulling the hot, wet air from the bathroom and into the attic above. This can cause mold and other problems up there, so you should take an hour and vent it to the outside. Roof venting is one option, but wall-venting is preferable (if possible with the layout of your home) because you don't have to worry about causing roof leaks.
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Find the mounted exhaust unit in the attic above your bathroom. Turn off the power to it.
Connect the hose from your exhaust kit to the vent tube on your exhaust unit, using the provided metal or plastic belt-clamp to hold the hose to the machine.
Take the other end of the hose to the an exterior vertical wall of the attic. Decide where you want it to go out, picking a spot between studs and away from electrical wires. Drill a 1/4-inch pilot hole at that spot.
Go outside, climb your ladder, and locate the pilot hole. Use your hole saw to put a full-sized hole there, cutting all the way into the attic. (Note: Make sure the hole-saw you're using is just a little wider than the exterior vent tube that came with your exhaust-fan kit. Generally, the vent tubes are 4 inches)
Slide the exterior vent tube into the hole, and press the attached exterior vent up against the siding of the house. Caulk around the edges of the vent to seal it to the siding. Secure the vent there with the provided screws and your screwdriver.
Back inside the attic, connect the provided vent hose to the exterior vent tube with the other belt that came with the kit. Caulk around the hole where the exterior vent tube is coming in. Turn the power back on.
If you discover that the pilot hole you've drilled will put the exterior vent plate on the uneven line between two pieces of siding, adjust the position by a few inches so it will be entirely on one flat span of siding.
Make sure the power to the exhaust fan is off before you attach the exhaust hose. Wear eye protection when using your power drill.