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Thread: Insulating square duct inside soffit

  1. #1

    Default Insulating square duct inside soffit

    I have a 1960s split-level with an HVAC supply trunk to the upstairs bedrooms housed inside a soffit on the garage ceiling. There is a (poorly plugged) outlet into the garage in the duct floor and near it, the start of some mold growth on the gypsum. I'm guessing the root problem is a combination of air leakage condensing inside the drywall and simple condensation from having a very cold AC system in the humid garage and organic gypsum.

    I plan on pulling down the soffit, sealing the outlet with a suitable galvanized sheet, etc. but I want to insulate/vapor seal the duct properly so the condensation and mold won't occur again. The garage is pretty poorly sealed at the door, so the cavity temperature and humidity variation is pretty large.

    I'm thinking of using either polystyrene or polyisocyanurate (foil-faced foam) board to insulate and vapor-seal around the duct, then frame out the soffit with 2x2 and 2x4 and fill the remaining problem areas (joints and around seams) with spray foam.

    Is this a reasonable plan or should I use something else to seal/insulate the duct? Would duct-board be better or worse than the XPS/RMAX sheets?

    Dan

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    First thing to do is to seal any air leaks in the ducts. They make a mastic for that, but the foil tape works, too, and isn't as messy to install. The duct needs to be dry and clean to do either of those. As to the best insulation, I'll leave that to others.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3

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    Quote Originally Posted by jadnashua View Post
    First thing to do is to seal any air leaks in the ducts. They make a mastic for that, but the foil tape works, too, and isn't as messy to install. The duct needs to be dry and clean to do either of those. As to the best insulation, I'll leave that to others.
    Check. Foil tape and mastic are already planned for the work. The original ducts were made well, but done so in 1964, when it was okay to let things "breathe" a little. Every time I expose duct, it gets the silver and white treatment The insulation, vapor retarder, and soffit composition are the real questions.

  4. #4
    Retired prof. engr. gator37's Avatar
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    For duct you could use duct board with stapled and taped joints or metal duct wrapped on internally insulated. If you are not used to working metal or do not have a metal duct fabrication company in your area I would recommend duct board. Typically there are a couple of cutting tools for duct board one for the corners and one for the lap joint although you could use a knife but it is a little tricky but easy to cut. When cutting you never cut the foil backing only the insulation. Basically it is a "V" cut in the corners and a lap joint in at the end you are joining. If you look on the internet for duct board manufacturers, you should be able to find a pamplet that explains how it is cut.
    If I remember right polyisocyanurate "can be" a fire hazard dependant on how it is used or installed like in exterior roof insulation.

  5. #5

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    I'm not planning on doing any new work with the metal duct itself except plugging a 4"x10" hole that some (wonderful person) put in it. It's been cleaned and sealed from the inside and airflow is satisfactory so I'm just going to expose the metal, clean it, and foil/mastic all the seams before I close it all back up. I'm trying to figure out what is the best to cover/fill the rectangular gaps in the soffit frame before I put the densarmor fiberglass wallboard up. I guess tyvek tape over whichever insulating board I use and just lapped over the wood structure is still leaps and bounds better than what I have now (no insulation, mold, air leakage, and gypsum drywall).

    Dan

  6. #6
    Retired prof. engr. gator37's Avatar
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    For the hole I would use a flat piece of metal from another piece of duct and attach it with 3/8" self taping metal screws. I would recommend using a mastic since over time the foil back tape may come deattached from what ever surface it is on.
    If you cannot attach the insulation directly to the duct you may want to consider batt insulation or duct wrap w/ vapor barrier it you can get it around the duct.
    If I am understanding the problem, I think you could apply a vapor barrier to the face of the joist than add batt insulation between the duct and joist and also on the bottom of the duct then install a layer of vapor barrier which you could attach from on joist to another which would be on the top side of the replace ceiling.

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