Bathroom Exhaust Insulated Ducting?

I see that everyone recommends insulated ducting for bathroom exhaust that is in unconditioned spaces. Is there a building code that requires it?
A builder was stating that the code requires it to be un-insulated (pretty sure they got that wrong but can find a hard requirement either way).

Thanks in advance for your help.

Just curious, who is “everybody” that is recommending this?


If I see uninsulated bathroom vent ducting, I don’t call it out as a defect.

I do call it out if it’s not terminated to the exterior or if it’s terminated at a soffit vent.


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Good points and I agree that insulation will help with external condensation, but it may not completely contain it in certain situations and possibly cause internal condensation. Plus, as I see, it’s only a recommendation and not required unless by a local jurisdiction.

I note it as information only in my reports.,located%20in%20the%20conditioned%20space.


In MN, we have this… However, it is in our energy code, which makes me wonder if they actually intended to have it apply to bathroom fans. Nonetheless, it is enforced, at least in some areas, on bath fan ducts in the attic. Typically when the insulation is missing, there will be a water stain on the ceiling around the fan.


Depends on your area. Where are you located?
Around here (Florida), I have never once seen insulated exhaust ducts.


I rarely see condensation issues with uninsulated vents. I suspect that it may be because, the run through the attic is usually short or they are infrequently used by the inhabitants.

So where is this “condensation” everyone is talking about?

Do you require dryer vents to be insulated? Why/Why not?

Are you thinking the cold vent condenses water inside the vent?
So warm moist air passes through the vent and because the moisture is against the cold vent, it will condense? Let’s consider “Heat Transfer”;

At what temperature will the air in the vent condense? You must know this. Do you?

What is the temperature of the vent when it’s operating?
Is this temperature below the dew point of the air inside the vent? You must know this.

Hot goes to cold, so the heat and moisture (heat) heats the duct above the outdoor air temperature which it is passing through. How do we condense when we are adding heat, not removing it?

Air movement: when you blow air across your sweaty body, does it not make you cool? This is because of evaporation. How can you have condensation when evaporation is occurring?
The air movement through the vent has the ability to remove any water inside the vent at any temperature where the air is not saturated. Water vapor has a lower relative humidity (wet bulb temperature) than water, which is at 100% Rh.

Are you thinking condensation will occur on the outside of the duct?
The outdoor air in the winter (when the vent is at its coldest) always has a lower quantity of moisture per pound of air, than indoor air (even if it is at 100% Rh). When the vent is not operating, it is the same, or higher temp than the outdoor air. It can not condense at this temperature, which is above that dew point temperature we discussed earlier.

Regardless of the Building Code you’re trying to enforce, if you do not have active moisture/damage, you have nothing to report.

There are times this occurs, but is not because there is no insulation on the vent. You need to fix the cause, not cover things up with insulation.
If you have an excessive low interior building pressure (a problem), leaking outdoor air through the building envelope (another problem), condensation can occur on numerous components near the exterior of the wall, the vent being one of those. Most of the time I find leaking vent fittings blowing onto the building envelope material, which has greater mass and lower conductivity than the vent. It will then condense under the right condition. If you insulated the vent, it might leak less, but the leak is still there (still the problem). You’re insulating the building envelope, not the vent in this case.


Typically on the inside of the duct in MN. Especially on longer runs or applications where the fan damper does not seal well. It gets very, very cold in MN attics sometimes!

over the years as I saw duct and ventilation materials change i have wondered how much was for added value to the system as compared to savings for the contractor…You don’t have to pay a qualified tinner if the job can be done with a couple of rolls of flex duct and some tape…

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I have never seen a qualified tinner on a resy job. In the early 90’s I worked for an HVAC contractor. I don’t anyone he had was a qualified tinner including me. It’s all about bodies.


In colder climates the condensation can be a big problem. I remodeled a bathroom once and had the plumber run two separate sets of lines (as opposed to just a diverter that would direct water from one head to the other). Basically, the shower put out an obscene amount of hot water. So, a couple months after finishing and using my new luxury shower I was running the exhaust fan and heard some gurgling. Sure enough I went into the attic and the condensing water in the (uninsulated) duct had collected and pulled the duct to the attic floor. I put a bin on the bathroom floor and was shocked at how much water came from the duct when I lifted it up to drain back down through the fan. Easily a couple gallons. The duct had been poorly placed with a low spot or I probably wouldn’t have noticed a few drops coming out each day. Anyway, yeah… the condensation collection can be a problem.

Numerous times I’ve also found damage in bathroom ceilings right next to the fan and it’s from water leaking out where the ducting connects to the fan output. Seems like it’s often been on multi-family where eight people are living in a two bedroom apartment.

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It’s enough of an issue that they actually make a condensate collection apparatus, although I’ve never actually seen one installed in my area.



A normal shower or a “luxury” steam type shower? I could see possible bigger issues with steam type showers and ducting.

Thanks for everyone’s input.
From everyone’s comments it appears there is not a hard requirement. If the run is long an an unconditioned space in a cold climate it is recommended but not required per any building codes.

(3) Where an exhaust duct passes through or is adjacent to unheated space, the duct shall be insulated to not less than RSI 0.5.

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Uninsulated bathroom exhaust becomes a problem here when it’s installed with flex duct and not properly supported. In the winter, the low spots fill up with water and freeze solid. The same concept of a clogged P-trap.