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I'm planning on installing a bath fan in a first floor bathroom with one exterior wall. This ceiling has 2x8 joists running parallel with the exterior wall. This exterior wall also has a window right in the middle.

My questions are: 1) can I drill 4" holes through each joist to get to the outside? 2) How can I mount the fan perpendicular to the joists? Do I have to attached cross braces to mount the fan? 3) Is it ok to drill through the window header and vent right above the window? I guess I'll have to keep the window closed forever.

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    Commented Dec 17, 2019 at 0:29

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I have done several different things here. In several houses I have mounted the fan directly on the wall close to the roof with the exhaust going through the side. I have also mounted the fan in the first 16” and punched a 4” hole through the joist is sitting on the wall so it’s not a structural problem. The 3rd and hardest but best looking was to build a small plenum with intakes mounted horizontally at the top with the exhaust through the wall, I also incorporated some lights in the plenum so it looked very nice. The fan on the wall direct exit was the easiest but did not look that great to me. I have used this on homes we flipped where cost and speed matter.

With the fan in the ceiling close to the wall it looked ok but it was closer to the wall than I thought looked good I have used this method many times on new construction it can be done in a remodel but drilling through the sheathing and siding is a bit tougher on a remodel but can be done, if pulling some Sheetrock down it is much easier.

In my own home I have the plenum it really looks nice and is the quietest but close to 3x the cost of just a fan and a exterior jack with a damper. we used this method on ~half a dozen spec homes and I like it the best.

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In addition to Ed's good ideas, it's common to just frame and drywall a box around the duct, which you'd drop below the joists near a wall where it's out of the way. Basements across the Midwest have primary HVAC ducts boxed in this manner.

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