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Insulating bathroom and laundry room exhaust ducts; also PEX return loop sizing

Cougar86 | Posted in General Questions on

Marine zone 4c, 1965 Ranch , no A/C, Gas furnace and hot water in garage.
Bathroom/laundry room remodel. Have 2 whisper green fans with separate 6 inch metal exhaust ducts, longest run under 10 ‘, low pitch roof puts most of exhaust run under 14 inches of blown cellulose. 20 inches total depth.
Do I
1. insulate duct from fan housing to roof vent?
2. only insulate exhaust above cellulose?
Next question
Air sealing Panasonic fan housing.
1. EPS foam surround? Or Sheetrock?
2. Direct contact to metal duct ok with all Borate Cellulose, should I use something different?
Existing 10 inch insulated return air flex duct also, plan is to hang above insulation and insulate sheet metal housing. Should I rework this to 10 inch metal duct?
Remaining 16 inch all metal return duct to be sealed, should it be insulated also as additional insulation will almost bury it.

New pex plumbing to all fixtures,  60 +ft insulated return loop to hot water tank sizing question, 1/2 or 3/4? From bath containing soaking tub and walk in shower.

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  1. Expert Member
    BILL WICHERS | | #1

    Code requires insulated duct for exhaust fans. From a functional standpoint, you’d only need to insulate the exposed duct to the vent, and probably a little ways into the cellulose too to be safe. The cellulose would serve to insulate the buried portion of the run. No idea if that will pass inspection though, probably depends on the inspector.

    Air sealing between the fan housing and drywall can be caulk or canned foam, but details depend on your ceiling construction too.

    I’m not aware of a problem with the cellulose contacting ductwork. I’d rework the flex duct to be metal duct for less restriction to airflow. Smooth metal duct is much better than any kind of flex duct, but it’s more difficult to install. Best is to move all ductwork out of the attic if at all possible. Make sure to seal everything very well if it has to stay in the attic.

    If the PEX return line is for a hot water recirculating loop, 1/2” is fine, and will often work better than a larger line. The return flow is very small even with a pump running. Make sure you have a check valve on the return line at the water heater too, you want flow to only go towards the water heater from that recirculating line. You don’t want to draw off the bottom of the hot water tank.


  2. GBA Editor
    Martin Holladay | | #2

    You can bury your bath fan and bath fan ductwork in insulation in your climate, as long as the parts of the exhaust ducts that are exposed above the insulation are insulated ducts.

    The return air ducts that are part of your forced-air heating and cooling system can't be bare metal ducts under buried insulation, however. These ducts need to be wrapped with duct insulation that includes a polyethylene jacket to prevent condensation problems in the summer. For more details on code requirements for buried ducts, see "Burying Ducts in Attic Insulation."

    1. Expert Member
      Dana Dorsett | | #3

      >These ducts need to be wrapped with duct insulation that includes a polyethylene jacket to prevent condensation problems in the summer.

      Not really.

      In marine zone 4C the outdoor summertime dew points average in the 40s F, not high enough to cause condensation problems even on buried SUPPLY ducts, let alone RETURN ducts. (There are parts of the Gobi desert with higher summertime dew points than climate zone 4C.)

      If the ducts are heating-only that's a complete non-issue.

      1. GBA Editor
        Martin Holladay | | #5

        It's not a simple issue, but I was stating the requirements of the newest version of the International codes (the 2018 IECC). Of course, most jurisdications have not yet adopted the most recent version of the IECC. The code requirements apply to return ducts as well as supply ducts. See the requirements of section R403.3.6 in the image below.

  3. Jon_Lawrence | | #4


    Is your hot water going to be on-demand or continuous recirc?

  4. walta100 | | #6

    If you are going to make changes to your ductwork getting the furnace out of the garage and into the house would be a huge safety and a good efficiency improvement.


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