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So, our bathroom fan leaks when it is cold. In the attic, the PVC pipe connected to the exhaust is uninsulated, and frosted -- shock!

So, my question is, what is the best way to fix this? Do I cut the PVC pipe, and run an insulated wrapper around it? Or would there still be issues?

Should I leave some sag in a section of the insulation I add so that any moisture will drip there, and later evaporate?

I'm kind of wondering if I am overlooking something as the other ducts are insulated. Of course, they don't leak.

Here is a picture of the situation in the attic:

enter image description here

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If you mean the big black pipe made of solid material, I don't think that's your exhaust fan. I think that's your house main vent stack. I'll bet anything that the exhaust fan connects to the insulated, flexible ductwork. –  The Evil Greebo Dec 28 '12 at 13:00
    
Thanks for the reply. I'm not sure if the black pipe or the insulated one goes to the leaky bathroom fan. They both go into the area of the leaky fan. So, above the bathroom is a decorative gap before the roof. So, to figure out which of these pipes go to the fan, I'd have to cut into it, through the vapour barrier, and drywall. The area that is above the fan looks like this: –  stubblejumper Dec 28 '12 at 20:36
    
Can you just climb up there and move the insulation aside? –  The Evil Greebo Dec 28 '12 at 20:40
    
Thanks. You are probably right about it being the vent stack, both go into the area of the leaky fan. So, above the bathroom is a decorative gap before the roof. So, to see more, I'd have to cut into it, through the vapour barrier, and drywall. The area that is above the fan looks like this: [IMG]tinyurl.com/c3b6eab[/IMG] I see moisture on top of the insulation by the grey insulated duct, and around the PVC pipe. The only moisture I see below the insulation is drips of the ice from the vent. So, what should I do? Insulate the main vent stack?? –  stubblejumper Dec 28 '12 at 20:49
    
Can you just climb up there and move the insulation aside? <- I tried. It goes down about a foot and a half, then I hit vapour barrier, and drywall, which would still be about a yard from where the fan drip is, and it is all dry except for the frost melting on that vent. –  stubblejumper Dec 28 '12 at 20:51
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1 Answer

It's a near certainty to be the insulated flex duct. For the sake of discussion, insulation will help quite a bit, but some drip back is inevitable due to the configuration. Condensation will happen. Bathroom exhaust has a very high dew point. As soon as it hits a surface colder than dew point, condensation happens.

With insulation, condensation will occur much higher up, but it will happen. Then it runs back down the duct to drip out of the fan into the room. The only solution is to not vent straight up. The duct should turn to horizontal as soon as possible after leaving the fan. The remaining duct should slope slightly away from the fan to eliminate drip back. If possible, the duct can be exhausted out a nearby sidewall. If the roof exhaust is the only option, the duct could be configured to have a type of drip loop where water can collect instead of running out of the fan. Hopefully any water collected would evaporate before much can accumulate.

You might consider placing a very small drip hole at the lowest point, and let condensation drip out of the duct into a more suitable container from which it can evaporate.

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I thought about adding a drip hole to the insulated flex tube like you mention, but read that having a gap in the pipe was a bad idea. It make sense to me though.I'll give it a try. I also insulated the main vent. –  stubblejumper Dec 29 '12 at 0:24
    
Yes, it's not the best practice. You are essentially building a leaky pipe. Sometimes it's the lesser of two evils. Keep the hole as small as possible while still serving the purpose. You'll also need to figure out how to keep the water from wicking into the insulation, for that would be rather bad. –  bcworkz Dec 30 '12 at 0:39
    
+1 Although I think you meant either "bathroom exhaust has a very low dew point" or "bathroom exhaust has a very high relative humidity". –  alx9r Jan 28 '13 at 23:34
    
@alx9r: Thank you for your support. The latter choice works, as for a given temp, high relative humidity equates to a high dew point. When dew point and air temp are equal, RH is 100%. I think of that as high dew point, but I suppose it depends on how you look at it. It's all rela... no, I can't do it, never mind:) –  bcworkz Jan 29 '13 at 23:48
    
@bcworkz :) I don't want belabor the (dew) point. But dew point actually has a concrete definition. It is defined as the temperature below which condensation occurs, and is given in degrees C or F. So the higher the RH the lower the dew point, not higher as you say in your comment and imply in your answer. (All puns intended.) –  alx9r Jan 31 '13 at 5:48
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