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Makeup air through framing cavities

nj_homeowner | Posted in General Questions on

Hi!

Is there any reason not to use stud space as a “duct” for makeup air coming into our house?

We are constructing our house now in northern NJ.  We already have five 14 x 3.25″ penetrations through the walls of the first floor.  The air needs to get to the basement.  The walls are 2×6.

Our plan was to run five 14 x 3.25″ metal ducts down the walls, through the floor, and into the basement, where they join each other at a rectangular sheet metal plenum.

But we are finding that our framing will make it extremely difficult to get the metal ducts down the wall, through the floor, and to the plenum.

It has occurred to me that we could just use the framing cavities.  I will mastic all of the wood joints between the sheathing and the framing.  I can even mastic the entire back of the sheathing surface, though I’m not sure it’s necessary.

I can use plywood or sheet metal to make the side of the ducts on the “inside” side of the house, leaving plenty of room in the wall cavity for the spray foam insulation we are going to apply.

So the “framing ducts” would be insulated, and their interiors would be raw wood, mastic, and/or sheet metal.

Is this a bad idea?  If so, why?

Thank you in advance!

Brian

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Replies

    1. nj_homeowner | | #2

      Thank you. I am aware of the conventional advice against this practice, which is why I posted here.

      This is for makeup air, not heating or cooling. I would carefully seal air seal the cavity with mastic, and it would be insulated with spray foam. Each of the runs would be less than three feet long.

      So I am not proposing a crappy panned-joist over crawl space HVAC return nightmare.

      Doing it this way would be considerably easier and less expensive. It would also provide a larger area for the air to move through, reducing the flow velocity.

      Can you or anyone else explain the problem with this arrangement?

      1. user-2310254 | | #7

        Brian,

        Apologies. When I read that you were planning five duct runs, I assumed it was an HVAC application. Out of curiosity, what are you putting into the basement that requires that much make-up air?

        1. nj_homeowner | | #8

          No problem! The makeup air going into the basement is for all of the exhaust appliances throughout the house, including a 575+ cfm (max) range hood, bathrooms, dryer, wood fireplace, etc.

          The house is not huge but if you add up all of the outgoing air, if everything is operating at max, it comes to more than 1000 cfm!

          I didn't really start to understand this stuff until our framing was done, and we don't really have a place for a 13-14" round inlet, so I just divided into five 14x3.25" smaller intakes.

          I will connect them two variable speed fans and, walla, makeup air.

          But the path from the inlets to the makeup equipment has become a huge unanticipated pain in the butt!

  1. nhbean | | #3

    Brian,

    If you have the patience to shape sheet metal to completely line your joist space and both mechanically attach sheets and seal them with mastic, you're effectively creating a custom duct. Wall stack is preferred for its ease and speed of installation, but there was a time where much ductwork was site-fabricated.

    Things to be careful of:

    Be sure to mechanically tie the edges of the sheet metal. Prefab ducts use cleats, which work very well. If you're good enough at bending, you might be able to use cleats here. Pop rivets should work well, as would screws if they are biting into your studs.

    Seal every joint with a liberal application of mastic

    Use as large of sheets as you can lay hands on - larger sheets means fewer seams, smoother duct walls, and better air flow.

    I would avoid leaving any exposed raw wood, or even raw wood just covered in mastic. Makeup air can contain significant humidity, which can help support mold and other microbe growth, especially on wood and paper drywall facing - this is one of the big reasons to avoid joisted returns.

    Air flow in ducts depends on both cross-section and surface area. That's why round ducts are best - they minimize surface area and maximize cross-section. Very narrow ducts have a lot more resistance than rectangular ducts closer to square in shape. This is why wall stack is limited in its CFM.

    Finally, as your air supply is outdoor air, insulating the duct is essential to avoid condensation issues. 1" of polyiso board would be R6 and would leave you 3.5" of duct space, and would seem to be easier to manage than spray foam. This is assuming you're using exterior continuous insulation. If you are using cavity insulation, then ducts located in exterior walls are going to be a significant heat sink - you might consider thickening that section of wall.

  2. JC72 | | #4

    IMO you don't want to run these returns down through the exterior walls. I'm not even sure you can meet the required R-value with 3" of spray foam within the bay in which you want to run the return.

  3. MattJF | | #5

    Why do you need makeup air into the basement and where is it coming from? More common would be the other way where makeup air is supplied from the a duct in the basement into a kitchen with a vent hood. How many CFM are you planning?

    Is this an interior or exterior wall?

    Don't run ducts in the exterior wall if you can avoid it at all: https://www.energyvanguard.com/blog/third-worst-place-put-duct

  4. nj_homeowner | | #6

    Thank you everyone for the thoughts.

    I think I'm just going to line the framing cavities with sheet metal, section by section. It will be tedious but I don't see how else to accomplish the task.

    If we fabricate a section of duct in the proper shape, which was the original plan, we won't be able to insert it because there are too many turns and obstructions. And if we do sections of complete duct, we won't be able to connect or seal them where they meet in the middle.

    I'm not worried about condensation because there will be at least R6 insulation, and mostly R12+ insulation, between the duct area and any conditioned space.

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