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?