Dryer vent seal by roof

Discussion in 'HVAC Heating & Cooling' started by DanMc, Jul 30, 2013.

  1. DanMc

    DanMc Engineer

    My washer and dryer are on the 2nd floor of my 2 story house. The dryer vent goes into the wall and then straight up into the attic. Once in the attic it tips to maybe 30 degrees from vertical and continues up to a roof vent. I can stand in the attic but it is one which is accessed via a pull down trap door and the "floor" is just some bits of plywood here and there. My problem is where the solid duct meets the roof vent. Basically it is just shoved up in there and there is not an air tight seal. The result is that probably half or more of the moist hot air from the dryer ends up in the attic and not out the roof. Surely this can't be good.

    My initial thought had been to take some of that foam insulation that you can buy at walmart intended to be shoved around a window air conditioner. Then I started wondering if maybe that isn't a good idea around a duct which will have some amount of heat being a dryer duct.

    It would seem like something which permanently seals off the air may be less than desirable because it will make it harder to periodically clean the duct.

    Any suggestions on a good approach here? I'd rather avoid anything which requires being on the roof. The only roof I will go on would be much flatter and closer to the ground so that right away adds the cost of hiring professional help. If it is the only way, I can do that but we already have enough other projects around here that are needing pros....

  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    New England
    Does the roof vent have a collar or something? Could you not put an elbow on the pipe and then turn it to make the connection? A picture might help. I've seen round collars that you might be able to use - the flange around the collar would be attached to the roof deck, and it has a fitting to then slide the end of the duct over. Some duct mastic to seal things up and maybe some aluminum tape, and it should solve that issue if there's no collar on the bottom of the roof jack.
  3. Dana

    Dana In the trades

    Can-foam is flammable, and using it to seal a dryer duct would be a code violation.

    Some duct mastic would have similar flammability issues, but others not- you need to read the specs. If you have a bunch of ducts & duct-boots in need of sealing it's worth buying a bucket of the stuff (and using it), but if it's just for the dryer vent a caulking-tube's-worth would do it.

    The stuff sticks to most things pretty well, but it doesn't take a jackhammer to break it back out when you need to clean out the duct.

    BTW: Air sealing air leakage between the attic floor and the conditioned space is always a good idea, which includes electrical & plumbing penetrations as well as any AC/heating ducts penetrate that boundary. Sealing where the dryer duct passes through would also need to be done with a non-flammable goop &/or metal, as would any flue penetrations, but other electrical & plumbing penetrations can be sealed with can-foam. Under current GA code new homes need to be blower door tested and be under 7 air changes per hour @ 50 pascals pressure, which is pretty easy to hit if the builders are paying any attention to it, but homes older than 2011(?) did not have that requirement, and many if not most would fail. While this has been promoted as an energy saving measure (which it is), the affect on limiting mold risk in the attic and limiting risk of attic insulation fibers from getting drawn into conditioned space air is significant. Leaks into the attic create allow stack effect infiltration to go on 24/365, and independent of the ACH/50 number it hits under blower door testing, leaks at the attic floor plane and at the foundation sill & basement are far more significant for both energy and indoor air quality issues than the leaks in-between. (If you block both the bottom & top of the stack, that air doesn't move, even if it leaks a bit under wind pressures.)
    Last edited: Jul 30, 2013
  4. DanMc

    DanMc Engineer

    Here is a sketch of what I have currently. The duct is 3" solid duct. The hole in the roof is probably 8-10" diameter although I didn't measure.

    On top of the hole in the roof is a metal thing which is somewhat cone shaped. It necks down to a 3" diameter opening. A second metal piece slides over that to stop rain but let hot moist air escape. That second piece is secured with a couple of sheet metal screws to the cone shaped piece. I imagine that the roofers who did my roof about 8 years ago (and are no longer in business) put that piece up. On the inside of the attic, the vent duct is just stuck part way up there. Since there is no seal to speak of, a huge amount of the air that blows up there simply blows back down to the attic. See the "BIG LEAK" label. I also suspect that contributes to lint collecting up there instead of being blown out.

    Here is a picture on top of the roof.


    You can see all the lint up there. I stuck my hand up (being careful of those nasty sheet metal screws) from the inside and cleaned out a big wad of the stuff and pushed more out. Not going to clean from the outside because the roof is way too high for me.

    And finally, here is a not so great picture inside the attic. Had troubles with the flash reflecting when I tried to get one more up inside the hole.


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