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Will air supply ducts in a poorly insulated wall present a potential moisture problem?

tstan42 | Posted in Mechanicals on

These are warm air supply ducts (3.25 x 10) in a 2×4 wall with just 3.5″ fg batt insulation. The ducts are against the interior face of the exterior board sheathing. This house has not been occupied in the winters (coastal maine) but is being renovated to be year-round. Prior to demolition we had planned to foam the wall cavities where interior finishes were stripped but did not know the walls were only 2×4. Will warm air traveling thru these ducts condense its moisture on the inside of the duct or cause condensation at the sheathing plane? In the dead of winter, will that air even be very wamr when it gets to the second floor?

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  1. GBA Editor
    Martin Holladay | | #1

    It sounds like you have forced-air supply ducts in your exterior walls. That is never a good idea.

    The main reason this is a bad idea is the energy waste. During the winter, these ducts are very hot, and your stud bays are very cold.

    If you ever install air conditioning, that's when you have to worry about condensation and mold. During the summer, the ducts will be cold, and any humid exterior air leaking into the wall cavity will contribute to condensation.

    The solution is to remove all ducts from exterior walls, and fill the stud bays entirely with insulation. Relocate the ducts to new chases that are framed on the inside of your house. And be sure that your chases are air-sealed, and your duct seams are also sealed.

  2. davidmeiland | | #2

    Todd, what is the heating fuel and appliance you are using? Any chance you can get rid of ducts entirely and switch to a ductless heat pump? Or, if you're going to continue with ducts, get them entirely inside the house? That means no exterior wall stacks, nothing in an unheated attic, etc.

  3. tstan42 | | #3

    Thanks for the info Martin and David, and I would love to remove the ductwork but the budget may prevent it from happening. So, in the winter condensation will not form on the inside of that ductwork because the air is moving too much?

    David, the furnace is actually only about 2 years old, and is oil-fired, hence the owner is resistant to make a major change to the system. They definitely before the cart before the horse on this one. I may be able to get them to move the ductwork inside if we can find space to accommodate it all.

  4. stuccofirst | | #4

    Ducts can never be completely sealed, even after intensive sealing of the seams. Condensation will most likely occur wherever there is a temperature difference between hot and cold air.
    Ducts on the outside wall is a very bad idea.

  5. dickrussell | | #5

    Todd, you said: "The ducts are against the interior face of the exterior board sheathing." Whether there could be any condensation within the duct in winter depends somewhat in the ductwork configuration. Here is a scenario I envision. Say there is a return register somewhere in the house and not feeding another duct inside the exterior wall, so that the "warm" air comes up through the supply ducts into the rooms, then returns to the furnace via the rooms themselves. When the heating system is not running, there could be reverse convective looping, with room air settling down through the registers, passed the blower and furnace and into the rooms again. The air inside the house, dry as it may be in winter, almost certainly will be humid enough so that some condensation could conceivably occur at that cold outer duct surface next to the sheathing. Of course, if there are motorized dampers, as there would be with a zoned system, then the convective flow wouldn't occur. Edit: Actually, at least some systems involving motorized dampers leave all the zones open when the blower is not running, then close all but ones calling for heat when the blower runs. In that case, perhaps some reverse convective looping could occur.

    Even so, the others are right, you need to get the ductwork out of the exterior walls. The savings in heat cost will pay for the relocation in short order.

  6. wjrobinson | | #6

    Todd, you can add depth to the wall at duct locations or whole sections of the walls. Then move ducts inward and add ridged foam behind. Thousands of homes haves similar ducting. No it is not the best idea but all building are full of best compromises.

    Some of the above posts are not understanding your need to work within a budget and the needs you have.

  7. cjeffers101 | | #7

    I like 'aj builder, Upstate NY Zone 6a' answer, but I was left with a framed-in a shell with 2x6 walls exterior wall connected to a 2x10 cathedral ceiling joists as my return channel, what do I do? Please help!

  8. GBA Editor
    Martin Holladay | | #8

    Chris Jeffers,
    You should build a chase.

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