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Community and Q&A

Weatherproofing Ductwork during Construction

nmullenonline | Posted in General Questions on

We’re building a “pretty good house” in SE Wisconsin, trying to do as much as we can on our own since the educational journey is part of the fun.  We’re using open web floor trusses so that hopefully all the ductwork, plumbing and electrical can be routed through them.  One of the basement walls is a walkout and that’s where we have access to insert long duct runs through the trusses.  My question is how do we weatherproof the ductwork so we can put those long runs through the trusses before the house above is closed in.  We would like to build that walkout wall soon but there will be an LVL board covering the access through the trusses at that point. The subfloor is there above but water of course still runs through the gaps in the subfloor panels.  Do we need to add the Alumaguard kind of product to make them ‘exterior’ ducts or can we just wrap them in plastic or something cheaper like that?

Thanks everyone for your help!  I’ve learned so much on GBA in the last few years!

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Replies

  1. walta100 | | #1

    Did you say what the duct was going to be made of?

    Galvanized steel will not be a problem in the weather and in my opinion is the best duck work money can buy but it is not the low-cost DIY friendly option .

    Walta

  2. nmullenonline | | #2

    Sorry, yes I was planning on galvanized steel but my understanding is that it's still not a good idea to have it getting wet consistently. Is that not true? Also at least the supply runs will need to be insulated - can that get wet?

  3. Expert Member
    BILL WICHERS | | #3

    It won't matter as long as it's not KEPT wet. That means if you want to keep water off the ducts, tent them, don't wrap them. Ductwork is a sort of flash galvanized sheet steel, not the same as the much more durable hot dipped galvanizing that is used on galvanized bolts (note that the "galvanised" bolts are the dull gray ones, not the shiny ones). If you're going to have the ductwork exposed for a month or two through normal weather (no snow accumulation or excessive rain), you're probably fine -- just cap the ends sufficiently to keep water from getting INSIDE the ducts.

    If you are going to close down for the season, then I'd tent things. Put some furring strips up, or scrap bits of framing lumper, so that you can keep a tarp suspended over the ducts enough to allow air to circulate. Tie things sufficiently that wind won't blow your tarps away. That's all you really need to do. You don't want to wrap the ducts in plastic, because water will find it's way in there, pool, and THAT will be a problem with rust. As long as the bulk of the water runs off of your tarp "roof", and some air can circulate enough that the ducts don't have water collecting on them and staying there, then you should be OK.

    Bill

    1. nmullenonline | | #6

      Thanks Bill, that makes sense. Does it matter if they're insulated? I would think the insulation shouldn't get wet but maybe that film that's over the insulation would protect it. And maybe if I 'tent it' as you say that would protect it enough.

      1. Expert Member
        BILL WICHERS | | #10

        The insulation makes it a bit more critical -- you don't want the insulation to get waterlogged. I'd try to avoid insulating until you're dried in if at all possible. If there is no way to avoid insulating before you're dried in, I would absolutely be sure to do a good job of tenting everything to avoid it getting rained/snowed on.

        Bill

  4. Expert Member
    Michael Maines | | #4

    If you're planning to insulate the ducts, I'd suggest not insulating them before installation, which for long runs will cost you more for installation (in time or money) than pre-insulating them.

    1. nmullenonline | | #5

      So you're saying to insulate them after they're inside the trusses? Wouldn't that be too hard to do? Or am I misunderstanding your comment? I guess I was assuming I would need to pre-insulate them. They will be in the conditioned basement but my understanding is that it's still better to insulate the supply ducts but not the return ducts.

  5. Expert Member
    Akos | | #7

    I'm in a colder climate but with pretty hot and muggy summers. Here ducting inside conditioned space is never insulated. About the only time I've seen it sweat is when it was run through a vented crawl or after a place was freshly mudded during a build.

    This makes sense if you think about, the coldest spot would be right at the cooling coil where the air would be leaving at indoor dewpoint temperature. As soon as the air is through there all the ducting downstream receives a bit of heat from the house. This is not much but it is enough to increase the temperature of the ducts a bit above dewpoint, so you don't get condensation.

    1. Expert Member
      BILL WICHERS | | #9

      I'm a similar climate to you I think. About the worst I'll see is a very (very) light mist-like coating of condensation on the ducts SOMETIMES when conditions are right. Often that's only right above the A coil. I've never had an issue with rust, and I've never seen it get bad enough to form droplets and drip off. Very minor, not something I'd worry about.

      Bill

      1. nmullenonline | | #11

        Ok thanks so I'm not going to worry about insulating the AC ducts. However I do need to insulate the ERV intake and exhaust and one of those is a long run so I guess I still need to figure out how to weatherproof the insulation for that one but one is better than multiple anyway. Maybe for that one I could treat it like it will be exterior ductwork and wrap it for that? I see something called a 'pipe insulated jacket' that might work?

        1. Expert Member
          Akos | | #12

          The outside connections to your ERV need to be insulated. They are also much smaller than your typical supply trunk so you can install them afterwards.

          If it is a straight shot, regular insulated flex duct will work pretty well.

          I have also used semi rigid aluminum (similar to dryer duct) with an insulation sleeve for tighter installs. These flow almost as well as rigid duct but much easier to run.

        2. Expert Member
          BILL WICHERS | | #14

          I've sometimes just removed the insulating sleeve from some pre-insulated flex duct and slid the sleeve over rigid duct. While I hate "wasting" the flexible duct part of the assembly, this is sometimes easier to get than the seperate insulating sleeve for rigid duct.

          Bill

  6. walta100 | | #8

    I think it is a bad idea to install ductwork outside of the conditioned space where it would need insulation. Insulating ductwork inside the conditioned space does almost nothing. Any energy losses go into the conditioned space so it is not a lost.

    Walta

  7. Malcolm_Taylor | | #13

    nmullen,

    Is the LVL a load-bearing beam, or just a rim-joist?

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