Note I am in Canada that can get up to -15C/5F and -20C/-4F during the winter.

A few years ago, we did some major renovations and we brought the dryer and washer onto the second floor. Due to the location, the only opportunity was to vent the dryer through the attic and out the roof. Long story short, the original contractor simply connected the those through the main vent, blocking about 50% of the vent.

We found this out when we went to go add more insulation to the roof and they suggest we move it to a dedicated vent.

Fast forward to now, we installed a dedicated CT4 vent for the dryer and thought all was finally well. Come the last few weeks, we noticed the dryer taking longer and longer cycles to dry.

I ended up snaking a vent cleaning brush inside to see if there was lint blockage, and there was very little that came out. What I did notice was the snaking rods were very wet. I looked into the attic and found that parts of the insulation in the ducts were actually frozen. I tried blowing air through the vents to see if i could see anyhting come out of my vent on the roof but nothing was coming, as if there was a complete blockage.

This has led me to believe that there must be a massive ice ball in my duct somewhere. I purchased an endoscope to confirm tomorrow but i had a few questions for you guys.

Here are some images of what I found:

  1. https://imgur.com/L1Z63Gw No insulation wrapping towards the top of the vent - could this be a major contributor to the condensation freezing?

  2. https://imgur.com/xVMMvdr I could feel ice had entered the insulation and had now froze. There was a small amount of water leaking through.

Does this duct work need to get completely re-done? it was completely mangled when the roofer installed the vent and you can clearly see it was jerry rigged back together. At my wits end here and looking for some potential suggestions. I was considering re-wrapping the entire duct work in more insulation, but unsure if that will solve anything.

Any suggestions are greatly appreciated.

  • 1
    Is there any way this route could be shorter? Your pictures make it look like a very long vent, which doesn't help anything.
    – KMJ
    Commented Jan 23 at 5:46
  • @RobertChapin Perfectly normal in a cold climate where the duct is not in a warm space until it exits the wall. Always better to keep it as short as possible, though.
    – Ecnerwal
    Commented Jan 23 at 16:32

2 Answers 2


Looks like an overall sloppy job. That won't work in challenging conditions.

Putting in a dedicated vent, it should have been put at a location permitting a short, direct path. Appears it wasn't, & probably too much to expect you'll change that now.

Replace the duct (feel free to re-use undamaged parts if they are solid duct) with all solid duct & fittings (no flex at all) and continuous slope back to the dryer, since continuous slope to the exit is not possible with a vertical roof exit. Taking it out a gable end wall you can go vertical to the attic and then slope down to the gable-wall exit.

Completely seal the duct joints with good quality aluminum foil tape.

Insulate heavily. Adding insulation will help if you take the previous steps to prevent water pooling in the pipe and seeping into the insulation. It won't help much if you don't solve that part, first.

  • Thanks @Ecnerwal for the suggestions, they are much appreciated. I believe the lack of continuous slope is a major problem here that i will try to rectify first. In terms insulating the pipe, would it make sense to continue to wrap batt insulation around the current sleeve and then cover it with some type of plastic wrap/vapor barrier? or completely remove the current damaged insulation/barrier and have just one continuous set of insulation/vapor barrier? Commented Jan 23 at 20:31
  • 1
    The duct is the warm wet side of the insulation, so the sealed duct should be the vapor barrier, or any applied vapor barrier should be right up against it. Any vapor or water that leaks from it should be free to dry from the insulation around it. Putting a vapor barrier outside the insulation is flipping off fate and begging for sodden insulation...
    – Ecnerwal
    Commented Jan 23 at 21:47

The keys to preventing things from freezing are these:

  1. Keep the liquid water out
  2. Keep it warm

Clothes dryer exhaust is a high-humidity air stream and so the two are intimately linked. Keeping the walls of the duct warm allows the water to remain as vapor rather than condense to liquid. No amount of insulation alone is going to keep the duct walls warm: when the duct sits unused for hours or days it'll cool to somewhere around the average daily attic temperature.

Minimizing the thermal mass of the duct may help. There's quite a lot of heat being sent out the duct while the dryer operates. There'll be condensation in the beginning when the duct is cold, but if dryer cycles are long enough, the duct will warm, condensation will stop, and then condensate will evaporate. If you have a choice of metal gauge pick the lightest/thinnest available. Choosing the smallest diameter duct allowed by the dryer manufacturer will also help the duct warm quickly.

If that alone doesn't cure the problem then think about supplemental heat. One strategy would be to use uninsulated duct. Move the attic insulation aside so that the duct can rest directly on the drywall ceiling, then bury it generously with insulation over the top and sides, while preventing insulation from getting under the pipe. This arrangement promotes passive heating from the drywall to the duct.

An extreme-climate strategy would be to use insulated duct, but install electric pipe heating cable onto the metal duct before insulating.

Even with steps take to avoid condensation it may be a good idea to prepare the duct to handle liquid condensate. This would include attention to details such as slope, lapping joints with the slope rather than against it, sealing joints, and drainage from the low point in the system.

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