I recently have been attempting to replace my downstairs (basement) bathroom fan. What I thought would be a relatively easy fix has now resulted in me discovering that the old fan was vented to nowhere (just the space in the ceiling between the joists). There is some old 3" metal ducting in there, but it's not connected to anything. It is currently being run through one of the joists (a 3" hold in the joist, see the attached picture). I obviously want to vent this fan to the outside of the house, but is it wise to make a 3-4" hole in the exterior-adjacent joist to do this? I really don't have a lot of other options for moving the hole up or down. Would running this through the exterior joist cause any structural integrity? Is it better to try to find 3" ducting to
reduce the hole size? Or should I just try to come up with some other completely different solution.

 

That is clearly not a good situation.

​​​​​​​Are you referring to the homes exterior rim joist?

 

Bath fans usually run parallel to the joists so that there is no drilling through anything structural.

 

Technically that piece of duct should not be thru the floor joist.

It looks like a block wall to the right (red arrow). Is that outside ?
That is the direction you'd need to go.
You don't want to drill any more large holes in floor joists.

That floor above looks wet. Is there a leak up there ?

 

Thanks for the responses.

The blue arrow is actually the exterior wall, and the red arrow goes into the garage (maybe that would be an option, but would require drilling through another joist).
Now that I"m looking at it more closely (with better lighting) I'm realizing that the ducting was run parallel between the joists and the above picture is showing the old metal ducting going through what I believe is a "block". This threw me off because the attic joists are perpendicular to this direction. I also see that there is another 3" hole in the next "block" or exterior rim joist (?) (above the blue arrow) that leads to the exterior. It looks like at one point the ducting was vented out, but no longer is. I'm guessing I should try to work with those two 3" holes since the house is still standing okay? This would require some kind of step down 4" > 3" duct converter as the fan I bought is a 4" exhaust (although I've read these are a bad idea). Or should I just expand those holes to accommodate a 4" flexible duct.

(On a side note, for a bathroom fan, it's fine to use flexible ducting advertised for dryers, correct? I can't find anything that specifically says it's for bathroom fans)

Thank you for adding the arrows. That helps a lot. And that staining of the subfloor looks to be old as there are no other signs of water damage (although this is directly below the main floor bathroom so it might have been an issue previously (we've only had this 1940 house for a year)).

 

If it's just blocking the hole is ok.
Put the fan on the outside wall side of the blocking.

What is the outside wall construction ?
Maybe a thru the wall exhaust fan.

 

It's my understanding that running a vent pipe through the joist is not that uncommon. But also, not preferable.
Yes, you can use the vinyl ducting for use with an exhaust fan. But I advise against it. If you must use a flexible pipe, get the semi-flex aluminum, not that junky foil stuff.

 

Flex duct can NOT be used with 3" pipe, per table 1504.2. (The op currently has 3" duct)

Flex duct may only be used if your duct is 4", the fan size is 50 CFM or less and the run is no longer than 56 ft. If the fan is 80 CFM, the maximum flex duct length is 4 ft.

Smooth wall duct that conforms to table 1504.2 is preferred, maximum lengths for 3" / 50 CFM fans is 5'. 4" / 50 CFM is 114 ft. 4" / 80 CFM is 31'.

 

The outside wall is wood, but there isn't a lot of room to work with, as it's a lower ceiling and that wall has a shower stall.

Sounds like my best option is to run 3" metal ducting through the existing holes in the blocking, with a step down from 4" ducting between the fan exhaust and the first hole?

 

If your fan exhaust size is 4", the pipe needs to be 4".