It was just a regular shower… I just said “luxury” due to the ridiculous amount of water it put out 
Although, I sure do love steam showers. I always dream of having one on my house. We’ve stayed at a place in Vegas called Tahiti Village a couple times (on the strip but a few miles south of town near the airport) and they have them. Absolutely love the steam… especially, in the dry Vegas air.
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Reading these replies about actual water issues, each one hints or specifically states the defect causing the problem.
I hope you consider my post to be why there should not be moisture issues in the duct, and that when you do come across this problem that you look further into the actual cause, not just that it needs insulation.
On a side note about insulation; it is often perceived that insulation “prevents” things from freezing/condensing. It does not. It “delays” the event. Time is a factor in this process.
Long runs in the attic: improper design.
Unsupported duct: improper installation.
Probably didn’t use the fan, or use it correctly. Moisture will move through the duct when it is not in operation. Also, the cold unused fan housing will condense the bath air leaking through the ceiling penetration.
Not saying condensation can’t happen, just that there is more to it than no insulation.
Hot goes to cold. High moisture goes to lower moisture. Building air pressure passes through the duct slowly when not in operation, which gives us the “time factor” resulting in condensation buildup.
Ethan - just want you to realize insulation is not the only issue (code or no code). If you insulate the duct, it will get cold when not used. The insulation affects the time factor by keeping the duct colder, longer. More air/moisture passes through the duct before it gets above the dew point, resulting in more condensation. We don’t keep running the fans after we finish the bath, which is when this all happens. It used to be common to have a timer on the fan to facilitate getting all the excess moisture out before it shuts off.
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Yeap! This is why they buy crap like this instead of proper winterization, completely useless without a heat source:
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2024 International Mechanical Code IMC Chapter 6 contains protection to life and property from the hazards associated with air-moving equipment and systems. This chapter states requirements for the installation of supply, return and exhaust air systems.
2024 IMC Section 603.12 Condensation. https://codes.iccsafe.org/content/IMC2024V2.0/chapter-6-duct-systems#IMC2024V2.0_Ch06_Sec603.12
Provisions shall be made to prevent the formation of condensation on the exterior of any duct.
Condensation can form on a duct when the temperature of the air in the duct is near the dewpoint of the air around the duct. The application of insulation with a vapor barrier covering prevents the duct from “sweating” by preventing moisture vapor from penetrating the insulation and reaching the duct surface. Duct sweating (condensation formation) is typically a problem for cooling ducts that pass through unconditioned areas where the humidity is not controlled. Condensation can cause insulation damage, corrosion or duct failure, and the accumulated water can cause damage to the building. Condensation can also occur inside of ducts, such as when ducts conveying warm, moist air from a bathroom exhaust pass through an attic space or similar unconditioned area. Condensation in duct interiors can degrade the duct, promote mold and fungus growth and cause structural damage.
2024 IRC N1103.3.3 Ductwork located outside conditioned space. https://codes.iccsafe.org/content/IRC2024P2/chapter-11-re-energy-efficiency#IRC2024P2_Pt04_Ch11_SecN1103.3.3
Supply and return ductwork located outside conditioned space shall be insulated to an R-value of not less than R-8 for ducts 3 inches (76 mm) in diameter and larger and not less than R-6 for ducts smaller than 3 inches (76 mm) in diameter. This section requires that all supply and return ductwork located outside of conditioned space, regardless of size, be insulated to R-8 or R-6 depending on duct diameter. While this code section focuses on HVAC supply and return ducts, the underlying principle of preventing heat loss and condensation applies equally to mechanical exhaust ducts.
Chapter 11 is about energy efficiency. It is specifically referring to HVAC supply and return. There is no energy saving by insulating a bathroom EXHAUST duct. It will fill with cold air when not used in winter climates.
As @dandersen pointed out above insulation will simply keep the exhaust duct cold longer.
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