Thoughts on difficult bath exhaust install

Hi all – I recently replaced the exhaust fan in our upstairs bathroom, which was set up to vent out of the eaves beneath our gutter. The eaves are not vented, so, while not ideal construction for a modern roof (this is a 1940s-built home), there is at least no concern for moist air reabsorption into the attic from the bath exhaust

In making the installation, I used R6 insulated 4" flex duct. There is about a 1-1.5 foot run from the fan’s exhaust port to the outermost joist along the back of our house, after which the duct runs about 3 feet dowhill to the soffit after running over a small opening behind the joist.

In trying to get the duct down the small 3 foot run, I ended up tearing it several times on nails that were protruding through the roof shingles down into the space where the duct ran. To give some idea, I basically destroyed 20 of my 25 feet of flex duct in trying to get this done.

I was ultimately able to run the flex duct down without major tears, but I think there may be some smaller punctures in the ducting from the nails. The section of the duct behind the outermost roof joist is also uninsulated, as I was unable to squeeze insulation in this space.

Given the short length of the horizontal run from the fan, as well as the downhill run after that first foot or so, I am not all that concerned about condensation, as any condensation would likely occur after the downhill section starts, and would run out of the onto the roof below. I could wrap whatever I end up using in the flex duct insulation.

I AM concerned about the possibility of condensation to the extent that there may be small punctures or tears in the flex duct, which would then allow that moisture to leak into the area running down to the soffit, which is both wooden and filled with insulation, so I’d rather not have moisture in there.

After much thought, it seems that I will need some sort of material that is:
a) 3.5-4" wide (I figure I can fashion some sort of reducer somewhere in the line to match with the 4" fan exhaust port.
b) Flexible enough to make the roughly ~135 degree turn over the outer joist and down towards the soffit.
c) Durable enough that it can be pulled past roofing nails without tearing.

Please let me know if you have any suggestions. I was considering PVC with angled couplings, but I think it may be too difficult to maneuver over the joist.

I am also considering trying to cut down the roofing nails by duct taping a cordless oscillating multi tool onto a broom handle of some sort to open up some more width to pull the duct through. How much nail do I need to leave before I compromise the shingle’s ability to stay in place?

Would something like aluminum dryer ducting work? Any other suggestions?

Thanks all

I wouldn’t bother oscillating the nails, you’d go through as many blades as nails and would take forever. Yes an aluminum dryer duct would be perfect.

Thanks – without cutting off the nails, I think I might have to reduce the duct diameter to 3" to get it to clear the joist and to run below the nails.

Any thoughts on whether it’d be better to fit the reducer right to the fan exhaust port, or run a foot or two of 4" and reduce to 3" as it turns over the joist?

Could you take some rigid aluminum 4" and squish it oval to fit?

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That’s not a bad idea^^^

I don’t think that would work – I’ve got very limited hand space to maneuver in there, and I don’t know how I’d either cut a hole in the soffit of the right size and at the right angle to accept the squeezed rigid duct. The flexibility is really essential as I basically have to shove it as far as I can from above and then pull down the rest from below. Good idea though.

I was assuming you had some sort of vent plate widget with a short bit of flex hose bridging the gap between the plate and whatever was inside.

I realized that I’m going to have to solve a similar problem at my mom’s house since it is brick and I can’t run the pipe to an exterior wall. I suspect I’m going to have to add a penetration to the roof for the vents.

I considered doing the roof vent but declined because I’m not too familiar with roofing work and I wasn’t sure I’d be able to get a good seal as my roof is asphalt over cedar shake shingles (grandfathered in under code).

In retrospect, I think it would have been worth a shot. This job was a nightmare.

Prototype – I’m not sure how your mom’s house is set up, but conventional wisdom seems to be you can have a pretty long exhaust run so long as it is insulated and slopes down after a few feet so any condensation will dry out as the fan runs and won’t drip back into the bathroom. A gable vent seems like the best way to run this, unless you’re talking about a run longer than 25 feet or so.

Yeah… the gable ends are brick too…

I was looking at roof vents today and I found one that looks really nice from Duraflo. Unfortunately, I can’t seem to find anywhere that sells the stupid thing.