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We have a bathroom in a finished basement. While removing the old bathroom fan I discovered that the duct pipe leading outside is not insulated.

Should the bathroom fan duct be insulated traveling inside the joist space? What are the risks if it is not?

From what I have researched, bathroom fan ducts should be insulated when "traveling through an un-conditioned space", like an attic. Does the space between the joists in the basement count as un-conditioned?

  • The duct runs about 20 feet to the side of the house
  • There is a strange vertical "S"-shaped bend in the middle.
  • The run appears to be roughly 10 feet on each side, with the S in the middle.

The duct also sits right next to a hot air vent. Is this all the more reason to insulate it? Is there a risk that the hot air duct gets hot enough to melt e.g. a vinyl wrapping around the insulation sleeve?

If I insulate both ends, am I just creating more problems for myself by making the condensation form in the middle of the "S", where I can't reach it? Would it be better to leave the entire thing, and have the condensation happening at the fan?

  • 2
    Somewhat off-topic: When the fan is running can you actually feel air blowing at the outside vent? 20 feet is a long way to go and that S-bend in the middle only adds resistance. You might need a higher power fan to get enough airflow to be truly effective. To the point of your question, since there's a bathroom in the basement, it's likely that the basement is inside the HVAC envelope, therefore it's running in conditioned space and your researched caution isn't of concern to you.
    – FreeMan
    Feb 8, 2022 at 15:54
  • 2
    Actually, the hot air duct next to the bathroom vent duct will help prevent condensation, at least along that stretch - it will keep the duct from getting cold enough (when the heat is blowing) for condensation to form there.
    – FreeMan
    Feb 8, 2022 at 16:02
  • the basement is inside the HVAC envelope - Hello, thank you. Yes, the basement is finished and (hopefully!) inside the HVAC envelope. I have edited the question to clarify that the basement is a finished space.
    – culix
    Feb 9, 2022 at 3:41
  • the hot air duct next to the bathroom vent duct will help - As I see the joist space should be considered a conditioned space, yes this makes sense.
    – culix
    Feb 9, 2022 at 3:43
  • When the fan is running can you actually feel air blowing at the outside vent? - This is indeed exactly the question I want to answer. Once I get the new fan installed that will be the test - to see if air actually makes it outside. I am definitely open to increasing the CFM if needed.
    – culix
    Feb 9, 2022 at 3:44

1 Answer 1


If the basement is conditioned and the floor above is conditioned, the joist space is conditioned.

Having a basement bathroom implies that the basement is conditioned.

  • Hello, thank you. Yes - the basement is indeed finished and conditioned. I will also edit this into my question.
    – culix
    Feb 9, 2022 at 3:38
  • I suppose now I have the inverse question - if I discover an insulated duct line in a conditioned space, should I remove the insulation? i.e. if I discovered this same bathroom duct was wrapped in insulation, would it have been better to remove it?
    – culix
    Feb 9, 2022 at 3:39
  • @culix then post that as a new question.
    – FreeMan
    Feb 9, 2022 at 12:28
  • 1
    Ergo, you do not need insulation because the moist air, traveling through the duct, will not hit any cold metal it could condense on. Feb 9, 2022 at 16:33

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