Bathroom Fan Condensation Issue

last year

I've had this issue and thought it was fixed.. but apparently it's not. I have a lot of condensation building up around the small bathroom fans on our upper level. Both fans are 50cfm fans with flexible ducting that dumps out of the soffit. Both rum less than 5 feet before they exhaust to the outside.

I noticed it getting worse and checked the attic to see what was going on or how bad it was. The attic is spray foam on the roof and was about 78 degrees and not humid (90 degrees and 75% humidity outside).

What what I've read, the hot humid air from outside is making it's way back to the bathroom fan, and condensing when the hot air contacts the cooler metal.

I've also read that I can add a baffle as close to the outside similar to the one below:

https://www.amazon.com/Galvanized-Back-Draft-Damper-Rubber-Seal/dp/B08KGDF3JV/ref=sxin_14_pa_sp_search_thematic_sspa?content-id=amzn1.sym.5e9743e9-8872-49f4-ab55-e93006373a6c%3Aamzn1.sym.5e9743e9-8872-49f4-ab55-e93006373a6c&cv_ct_cx=air%2Bduct%2Bdamper&keywords=air%2Bduct%2Bdamper&pd_rd_i=B08KGDKBGF&pd_rd_r=34129ab5-c406-48f0-b6ef-ab80646b4658&pd_rd_w=gYnG2&pd_rd_wg=5i4RJ&pf_rd_p=5e9743e9-8872-49f4-ab55-e93006373a6c&pf_rd_r=5QBZ150G21X9P8AGSQ4R&qid=1655660392&sr=1-1-a73d1c8c-2fd2-4f19-aa41-2df022bcb241-spons&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUFZWUtVTUpRUEtGMFgmZW5jcnlwdGVkSWQ9QTAzMDc4NDYyMkczMzJZMUVYTkpPJmVuY3J5cHRlZEFkSWQ9QTA1MTIzNjIyUDE1N0FMN09ERUJLJndpZGdldE5hbWU9c3Bfc2VhcmNoX3RoZW1hdGljJmFjdGlvbj1jbGlja1JlZGlyZWN0JmRvTm90TG9nQ2xpY2s9dHJ1ZQ&th=1

And I have also read that I need to insulate the ducting as well as the fan box. Rigid ducting could also help as well?

On another note, we have 2 of the same fans on the first living level that exhaust out the side of the house and the ducting runs between the floor and ceiling with no issues. What is the correct way to fix this?

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Comments (6)

3onthetree
last year
I think any questions were answered in your other thread of 51 comments:

https://www.houzz.com/discussions/6173634/bathroom-fans-causing-moisture-stains
wdccruise
last year
last modified: last year
When copying an Amazon URL, just copy the text up to and including the code to the right of "dp/". Example:

https://www.amazon.com/Galvanized-Back-Draft-Damper-Rubber-Seal/dp/B08KGDF3JV

You can also omit the text between "amazon.com/"; and "/dp". Example:

https://www.amazon.com/dp/B08KGDF3JV
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HU-72499350
Original Author
last year
Yea I could not find my old thread on my other account. I also thought I had solved the issue.. Additionally the thread started off with some good help and ended with a couple of internet warriors.
PRO
Charles Ross Homes
last year
What did you implement--which didn't work--to solve the problem?

You indicated you have a 50 cfm bath vent fan. That's the type of low-cost/low-performance vent fan typically installed in powder rooms in entry-level townhomes. It's not a good option for venting moisture in a bathroom of any size.

The backdraft damper you are considering will most likely introduce too much pressure loss for your 50 cfm bath fan to overcome. In that case you'll be preventing backflow, but preventing venting of moisture as well.

I recommend replacing the 50 cfm vent fan with a vent fan appropriately sized for the application and running it during and for 20 minutes after a shower. If you need to add a backdraft damper, you might consider this one which was recommended in response to your original post:

https://s3.amazonaws.com/s3.supplyhouse.com/product_files/TTI-CBD4-product.pdf
HU-72499350
Original Author
last year
Any reason for that damper over a C Infinity 4" Backdraft Damper? Right now my plan is the infinity damper and replacing the 50cfm fan with Panasonic FV-0511VQ1 WhisperCeiling DC Ventilation Fan. I will use IMPERIAL 4-in x 300-in Insulated Polyester Flexible Duct and have to reduce it to 3" about 2" from the soffit because the rest of the ducting is in the soffit and spray foam. Not sure on insulating the fan box yet.. if it doesn't hurt I will do it..
PRO
Charles Ross Homes
last year
Your problem is the back drafting of hot, humid air into a conditioned living space where moisture can condense on cool surfaces like the drywall ceiling which is also a great food source for mold and mildew. Your post indicates you have a conditioned attic, so the lack of insulation of the fan housing or duct isn't an issue.

Bath vent fans including the Panasonic model you are considering come with integral backdraft dampers--although they're usually cheap plastic ones and don't perform all that well. Installing an additional damper is redundant and it will add pressure drop so I recommend you decide on one or the other. The one I recommended has a particularly low pressure drop. Whether you remove the damper supplied with the fan and install a different one or not, dust and lint will accumulate on the damper over time and reduce its ability to seal properly. Make sure whichever one you choose is cleaned and maintained periodically or you'll be revisiting the issue you are having.

Best practice is to run the vent fan while showering and for 20-30 minutes afterwards to reduce the relatively humidity in the bathroom. I have mine on an old-fashioned electro-mechanical timer, but you can install a more sophisticated one if you want it to be able to turn it on/off from wherever you happen to be in the world, which is not a bad idea if you have teenagers.

Bathroom Fan Condensation Issue

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3onthetree

wdccruise

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3onthetree

wdccruise

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