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Condensation from leaking bath fan exhaust duct

Z_Schuster | Posted in General Questions on

Last year I added insulation to my attic (Twin Cities, MN) and replaced the bathroom exhaust fans and put in insulated hoses. I also air sealed all of the wall top plates and sealed up all of the attic bypasses.

Last week we had some extreme cold temps so I took a peek in the attic and found a fair amount of frost everywhere and I’m pretty sure the bathroom vents were the culprit.

This weekend I got back up there and redid the termination of the exhaust hoses and I feel like I sealed them up about as good as I can.

It’s not as cold now but still cold enough that frost is forming. The attached photo is from this morning after some of the kids got showers in their bathroom. Am I expecting too much by thinking I can completely seal it? Is there an acceptable amount of leakage?

I did the original work about a year ago so the previous vent leakage, being worse, still caused no mold to form as far as I can tell on closer inspection.

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Replies

  1. cs55 | | #1

    did you wrap tape around both the duct and the insulation or only the insulation?

    have you tried turning a shower on max temp with the fan running and going into the attic to feel around for any leaks?

    1. Z_Schuster | | #3

      Foil tape applied around the flexible duct to the vent sleeve, followed by a worm collar to tighten it more, and a self-tapping screw to make it extra-captive. Then taped over the screw. Pulled the insulation up and taped it up to make it tight around the duct and sleeve.

      I did previously get up there with the shower running at max temp, but have not since taking this photo. I wanted some feedback to see if it would be worth it.

  2. Sinaritt | | #2

    It does look like a fair amount of leakage. It may not be coming from the termination point. Any leak along the length of the duct would likely push warm air to one end of the insulation jacket or the other.

    1. Z_Schuster | | #4

      That's a good point. I have a Flir camera for my iPhone. It worked great for my before and after surveys when I added the insulation originally, but I haven't had much luck finding holes with it, but I guess if I take a piece of plywood or something I can place it near the duct and see if there are thermal changes on the surface as I move it around the duct.

      Regardless, it sounds like it's a big enough issue that I need to get back up there. I just wish it wasn't so close to the outer wall. It's a pain to get at, but that's one of the reasons I went with 4-foot batts instead of blown in, in case I needed to get I could easily pull a section away and put it back later.

  3. krackadile | | #5

    Is the flexible ductwork the same size as the air terminal? It should be a pretty snug fit on its own. Typically I see the big zip ties used to connect the flex duct to the metal duct and then tape that and the insulation after you pull the insulation up to cover the connection.. Is it possible your exhaust terminal/vent sleeve is clogged up and pushing the air back in or that the terminal/sleeve is leaking the air? Is it possible snow or ice is blocking the discharge?

    1. Z_Schuster | | #6

      All great questions. Duct and sleeve are the same size with a snug fit. In place of a zip tie I used a worm gear collar. It is possible I suppose that the vent had an ice obstruction last week but it was clear when I re-taped it prior to the photo that I shared. I have a theory on what’s going on but I just haven’t had a chance to get back up there again to look.

  4. Z_Schuster | | #7

    We’ve finally had another cold snap. I re-taped every inch or the exhaust port on the roof deck to seal up any holes and it looks like it’s in better shape now. I’m seeing very little frost in the attic this morning.

    It does look like a little is forming near the soffit baffles. I wonder if the bath exhaust moves down the roof then gets sucked back up in to the soffit as it falls off the edge of the roof? I don’t see this occurring near any other baffles.

    Does this small amount seem normal?

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