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Insulation on ducts within conditioned space, condensation?

Nplus | Posted in Mechanicals on

Hi, I am in the planning phase of a remodel of a 2 story house in the San Francisco Bay Area, Zone 3c. The ducted multi-split system’s duct work will be located within conditioned space, and I plan on a tight install mostly with rigid ducting except for the final flex duct runs to the registers. The issue is in the 1st floor where I am attempting to run most of the ducts within ceiling joist bays (2 x 8″ joists, so 7-1/4″ actual) so I don’t lose head room. The joists also sit on a few beams that have to be crossed, so there we are also constrained by the same 2 x 8″ joist bay height.

The HVAC designer who lives in another climate zone has made a recommendation to insulate these ducts due to concerns of condensation even though they are within conditioned space. The thinnest duct sleeve insulation I can find is R-4.2 at 1-1/4″ per side for a total of 2-1/2″. This limits ducts to 4-3/4″ in height which won’t get the job done.

If we ensure the joist bays where the ducts run are well air sealed, particularly at the rim joists, will condensation be a real problem? Asked another way, can I get away without duct insulation? If insulation is important, are there any other solutions out there?

Thank you for a fantastic website.

Wilson

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Wilson,
    My judgment is that in the San Francisco Bay area, uninsulated ducts inside the home's conditioned space will not cause condensation problems.

    I'm interested in the opinions of GBA readers on this issue.

  2. Expert Member
    Dana Dorsett | | #2

    Even the average outdoor dew points in the Bay Area are pretty low, exceeding 55F no more than about half of the time even in mid-summer:

    https://weatherspark.com/m/541/8/Average-Weather-in-August-in-Oakland-California-United-States#Sections-Humidity

    https://weatherspark.com/m/1098/8/Average-Weather-in-August-in-San-Jose-California-United-States#Sections-Humidity

    https://weatherspark.com/m/560/8/Average-Weather-in-August-in-San-Mateo-California-United-States#Sections-Humidity

    https://weatherspark.com/m/566/8/Average-Weather-in-August-in-Sausalito-California-United-States#Sections-Humidity

    With sufficient air flow the duct temps would never be cooler than 55F, and copious condensation wouldn't happen unless it was more than 5F cooler than the indoor air's dew point. With the air conditioning running the indoor air's dew point will drop below the outdoor air's dew point.

    Bottom line- you'd have to go out of the way to design a system so poorly as to get condensation on ducts inside of conditioned space in that climate. Only if the sensible load is and ventilation rates are ultra low would the indoor dew points be high enough to create the problem. Or maybe if you take 12 showers a day without running the bath fan and and always keep an uncovered pot of pasta going on the stove without the kitchen fan you could make that happen.

  3. walta100 | | #3

    I have seen condensation on duct work, but you need just the right conditions and then it was limited to small section of the ductwork where the cold air enters the ductwork and the duct changes direction and the ductwork is located in a room with higher than normal humidity. All ductwork lives on the very edge of condensation because the supply air is always delivered at or slightly below the dew point of the rooms return.

    I do not think it is necessary to insulate ductwork inside a conditioned space.

  4. Nplus | | #4

    Thank you everyone for your help. Your expert input is greatly appreciated.

  5. Jon_R | | #5

    If your indoor temp is 76F and internal sources push %RH to 60%, then the dew point will be 61F (enough to cause condensation). But you can prevent this with dehumidification to ~50%.

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