Other than the shoddy vent boot installation and the over use of the expanding foam at the connection, is using PVC for bathroom ventilation ducts an issue?
Since it is not insulated, it exhaust will condense in the attic and drip back down into the fan compartment.
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Flow rate may be reduced. the cfm rating on the fan is tested using a standard 4" duct, not the smaller PVC pipe used here.
The reduced performance can result in more humidity in the bathroom unless the fan is left on for a longer time. This also uses more electricity.
And that flashing… Ha Ha.
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Agreed, thank you
Thanks Bert. I was thinking the same thing. Specifically, I didn’t know if using PVC is an issue.
I don’t see any issue with the material choice.
Maybe paint the pvc to protect from the UV rays so it doesn’t degrade too quickly
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Agree, material is a non-issue, but… SIZE MATTERS! 
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I think it is too small. I think 4" is the minimum. I’ll check on that.
Yes, wrong boot for the job, plus it must have a damper.
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The need for a 3D printer if I ever saw one!!
I believe the damper is in the black box area before the reduced the vent,
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The fact that it doesn’t fit should have been the 1st clue to the installer.
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It is. I am slightly concerned that the spray foam got inside the damper and sealed the damper open or closed. I didn’t think to verify air movement at the vent exterior while I was on site. But I am going to recommend, as Brian and Jeffrey suggested, that the vent diameter was too small and to have it evaluated and corrected. Thank you for the input.
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Yes, I could post this to the WTF thread.
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Yes, but my interpretation is that the damper must be at the exterior. Or maybe I assume this because nearly all bath vents have exterior dampers. I call them out when the damper is missing, damaged or clogged.
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Yes, now that I think back on it, there is a damper at both locations on my bath vent fan.
Thanks for thinking this thru with me. Now that JJ has posted this illustration, I do see these on occasion. There is obviously no damper at the exterior. So, I suspect the builder either chooses interior damper in the fan, exterior damper or both.
So, moving forward. If I see a vent hood with damper, I will report on the condition of that damper. Otherwise, I would have no way of knowing.
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My pleasure, Brian. 
Here is a visual of what Larry has said.
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I know that you’re not supposed to use PVC for dryer exhausts due to static electricity build up. I don’t know if that would also apply to a bath exhaust. I think the lint is the issue for the dryer vent, but dust and other debris could accumulate over time in a bath vent too.
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