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Bathroom Exhaust Fan Condensation:

If you could help me with a problem, it would be appreciated. There are 2 bathroom exhaust fans in the house. When it gets below freezing, on one of the fans it appears that the humid exhaust air freezes in the exhaust pipe and when it warms up, melts and gets into the ceiling wall board and puddles in the exhaust fan cover. 

The installation of the fan is as follows: The 4" exhaust fan pipe is attached to the fan and extends upward through about 8" of insulation goes about another 5" and then angles away from the roof for another 5." The other exhaust fan has not problem and is vented straight up.

Are there any techniques to avoid humid air from freezing in an exhaust pipe?


Here in Northern Michigan most houses have bath fan exhaust ducts that run horizontally to the outdoors. This way the duct is kept warm so the humid air never has a chance to condense (or freeze) until it exits the building.

Therefore, I believe the best solution to your problem would be to wrap insulation around the fan ducts in your attic. There are long strips of fiberglass insulation sold just for this purpose. The insulation is wrapped around the duct and secured with tape. I have also seen duct insulation that is a sleeve that slips over the duct. This product has a plastic sheet over the outside. However, this is intended to be installed during assembly of the duct work... you could use this product if you disconnected your duct in one location.

Actually your question pointed out a potential problem in one of our projects. We recently installed two bath fans and the ducts travel through an un-insulated attic. We will need to go back and add insulation to prevent the problem you have experienced.

Bruce W. Maki, Editor.



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Copyright 2001

Compiled February 2, 2001