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Community and Q&A

Best Way to Connect Ducts to Wall Caps?

jamesboris | Posted in Mechanicals on

Short: Best way to connect/seal wall cap to duct?

Long: I’m ducting a 450 SF guest house in Zone 2A: a) Dryer (8” straight shot out of ext wall), b) bath fan (5’ straight shot out of ext wall), c) Whispercomfort ERV (~9’ down through a vertical service chase on an exterior wall, adjustable elbow out of wall. I plan to use Imperial wall caps with built-in dampers, which get good reviews.

Manufacturers often recommend sealing the cap to the ductwork with sealant either inside the duct like shown here, or around the wall cap’s tailpiece (3rd image here). I *think* this’d require ending the duct flush with the exterior wall (a trim block, in my case). But if I do this, I may never be able to remove it, violating this idea from Malcolm: “Vent terminations of all kinds should be detailed so the they can be easily switched out and still maintain good air-sealing to the duct.”

What are the alternatives / best practices? One might be to end the duct slightly proud of the exterior wall (per Hammer & Hand). Remove the tailpiece from the Imperial cap, then slide it over the end of the duct. Problem is, it wouldn’t be sealed to the duct, so the damper won’t work as well.

Bonus question… what if I include cape dampers *in addition to* the wall caps? i.e. in the duct, just inside the exterior wall. Belt & suspenders? Overkill? Risk of worsening the performance (due to too many “blockages” in the ducting)?

Thank you as always!

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  1. Expert Member
    Akos | | #1

    Unless you are chasing passive house air sealing, I would not worry about it. Small gaps like that don't leak all that much air when you do a blower test. I've never done anything other than slide the wallcap into/over the duct sticking through the wall.

    Be careful with the grate on that cap, it will clog with dryer lint fairly quickly.

    1. jamesboris | | #2

      My concern wasn't just air leakage from the house, but air leakage from the duct itself, messing with static pressure and making the damper less likely to function correctly. But I'm guessing you'll also tell me I'm over-worrying about that too, and you're probably right ;)

      I like this wall cap because the grate and damper are both removable, so it can be used for dryer (damper, no grate), ERV intake (grate, no damper), and bath/ERV exhaust (grate, damper).

  2. Expert Member


    It's a connection that is ripe for re-thinking. The Hammer and Hand detail deals with air and water sealing, but is vague on how the termination is connected. The standard detail presupposed the walls are open in the interior and make s replacement difficult. Ideally there would be be an expansion coupling you could pull out several inches, attach the termination to and push back into the wall.

    What I do is bring the duct out slightly proud of the outside of the trim block, then surround that hole with a gasket. I then cut off the attached duct from the termination, and mount it to the block.

    1. jamesboris | | #6

      Thanks Malcolm! That sounds like a great solution. Think it'd work just as well to put the gasket on the back of the wall cap? Kinda like what's reocmmended (with sealant) in the attached? Seems like it'd be easier to attach the gasket there. And how much do you usually end up cutting back?

      Also, what kind of gasket do you use? I have some leftover EPDM plate gasket material from Conservation Technologies... they told me "No problem with sun exposure -- we use the same compound to make glazing gaskets and have installations where the edges of those gaskets have been exposed for 25+ years."

  3. AndyKosick | | #4

    Glad your thinking about this. Sadly the industry hasn’t thought this through very well. The tail provided with Imperial style wall caps is useless in most applications because HVAC doesn’t set the wall cap unless working with blocks, and many builders and not.

    I wish Tamarack Technologies would sell wall termination hood attached to they’re cape damper so it could just slide inside of a standard duct size with that seal. Done. Maybe we need to bug them about it.

    1. Expert Member
      MALCOLM TAYLOR | | #5

      User ...628,

      That would be great.

      I can think of a few ways that would work well, but most are prohibitively expensive. Lee Valley sells a magnetic connection for dryers that could be re-purposed for the termination.

      1. jamesboris | | #7

        I've been thinking of using this connection, but I have a gas dryer, so I can't risk any CO-ridden exhaust leaking out... I wonder if the force of the dryer exhausting would handle it?

        As for cape dampers, I'd avoid them on range hoods and dryers... but what do you guys think of my idea of using em *in addition to* dampered wall caps on the ERV and bath exhausts?

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