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Community and Q&A

Sealing/insulating ducts at wall penetrations

tannerc | Posted in Mechanicals on

Hey folks,

I have three duct penetrations to make through exterior 2×4 walls and a 3/8″ rainscreen.  Planning to use rigid 26 ga round galvanized duct for all of the 5″ long (or shorter) “stubs” that will pass through the wall and cladding. For diameters I have one 4″ for bath exhaust, one 3″ for woodstove air intake, and one 6″ for range hood exhaust. 

1. Anything special about sealing the longitudinal seams where the duct goes through the wall? I guess I wonder about mastic rubbing off on the sheathing ( which I gather should be cut pretty tight to the duct) as well as the siding mounting block. Would foil tape or foil/butyl tape over mastic be beneficial protection?  First time working with ducts…

2. Should the ducts be wrapped in a duct insulation in the 2×4 cavity or will the rockwool I plan to insulate the wall with suffice?


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  1. Balazs_F | | #1

    Hi Tanner,

    1. Using a glass fibre tape is definitely a good idea. it will provide additional air seal should the sealant over time harden and crack. make sure the glue is compatible with the sealant. 3M, Bakor, Ductmate have quality products.
    2. That depends on the type of termination used. if you connect to a wallbox, insulate the duct up to the wallbox and wrap the box or cover it with foam insulation. in addition, if you are in a colder climate, a continuous vapour barrier to prevent potential condensation on the duct is essential. Insulate the air intake duct in its full length, the exhaust ducts up to the fan discharge or min. 2 feet upstream the backdraft damper if there is one installed. use min. 1" fibreglass duct wrap or 3/4" thick closed cell insulation (such as Armaflex). This may seem an overkill but it will prevent potential mold issues on long term.

  2. tannerc | | #2

    Great info thanks! I have attached a rough sketch of the termination. All duct runs will be very short with the bath fan mounting in the 2x4 cavity and going directly out the wall and the wood stove intake and range hood exhaust running only a few feet before going through the wall. As I am about to put up siding I am planning on just installing and flashing (Flexwrap onto tyvek as per Dupont specs) short sections of rigid duct for now and connecting to them later. Excuse my ignorance but I have a few more questions.
    1. Fiber tape goes down first with mastic over top? What is the distinction between glue and sealant here?
    2. Not sure what a wall box is despite googling.
    3. Overkill is my style and I am definitely concerned about condensation. Are ductwrap or Armaflex vapor barriers as well as insulation? Should this insulation extend from the wall cavity through the sheathing and rainscreen gap out to the trim block?
    4. Should the duct somehow be supported inside the wall cavity perhaps with a pipe strap to a stud?

    1. Balazs_F | | #3

      Hi Tanner,
      1. no, mastic (sealant) first, then the tape while the mastic is still wet. The glue on the tape is what you have to check.
      2. please see below, standard exhaust vent termination, can come in many shapes. you selected Seiho which is a good product.
      3. armaflex is a vapour barrier. Ductwrap has to include an aluminum foil (FSK) or PP fibreglass reinforced scrim (PSK) to be a vapour retarder (most ductwraps do). the seams must be taped using the same tape material as the scrim. I would extend the insulation through the outside wall to the termination or to the sheathing to minimize thermal bridging. the penetration through the drywall on the inside becomes a bit tricky - add flexible caulking around the hole (between the outside of the insulation and the drywall edge), add a paintable PVC or metal trim/collar to make it look pretty.
      4. definitely a good idea to support the duct, any movement overtime will compromise the air and vapour barriers. use plastic strap (even a thicker zip-tie) in lieu of metal - less thermal bridging

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