Left Image

Vent Fan Condensation Cold Rain

Q. We have a house built in 1969 that has a vent fan in the bathroom connected to a round metal vent to the roof. During really cold winter time, water sometimes drains out of this fan onto the bathroom floor. It has also stained the ceiling drywall around the vent fan. Our house is a ranch style, and the vent pipe goes through the highest part of the roof.

We will soon be repainting the bathroom walls and ceiling. Before repainting the ceiling, I need to try to modify whatever is causing the water draining so it will not continue to stain the ceiling and eventually damage the ceiling drywall. It is obviously a nuisance for the water to drain onto anyone that happens to be at that spot when it starts draining - as well as the wet floor itself being a nuisance.

We have lived in this house for 25 years. Unfortunately, we can not remember if it has always done this occasionally in the winter, or if it has started sometime later. We did have to replace the vent fan 10-12 years ago, but again can not remember if the nuisance started at that time versus earlier. It certainly has been doing it for enough years that we have used Kilz in the past to cover the stain and repaint the area.

It happens so seldom (a very few times each winter) that we have not determined any specific pattern. It does not always occur after the vent fan has been operating, but I would think that humid air hitting the COLD metal vent pipe results in condensation that occasionally condenses enough to run back down and drain onto bathroom floor. (When it occurs, it is a steady stream of water draining for some seconds -- maybe an ounce or so of water.) Any suggestion you have regarding how to stop this would be appreciated. (I tried to remove the vent fan recently to get a good look at the area and up the vent pipe, but the vent fan seems to be 'wedged in' somehow, and I am afraid I will damage something removing it. However, I will get it out somehow if necessary to do whatever modification(s) you suggest.)

A. Congratulations! You have successfully duplicated a terrarium micro-environment in your bath vent system. I fully understand this situation when the vent duct rises up through the roof. During really cold weather I suspect this situation occurs more often.

This is what is happening: The warm, moist air rises and is forced by the fan up the vent duct toward the roof exhaust. The moisture laden air is cooled as it leaves the warm, insulated house cavity. As the temperature drops past the dew-point, the moisture condenses back into a liquid and "rains" back down inside the vent duct causing a dripping or rain inside.

The goal to avoid this situation is to keep the vent-fan exhaust air warm until it exits the house.

At Mosby Building Arts, when we encounter a vent fan that must be exhausted through the roof (as opposed to through a side wall and the duct can be kept covered by the attic insulation, we over-insulate it.

Typically we seal the ductwork joints, and then cover the metal vent duct with a 4" flex-duct insulation, which is a round sock-like cover, all the way to the underside of the roof sheathing or even through a side gable wall. We work very hard to over-insulate the vent duct when the length of the vent duct exceeds 6 feet in length. Our preference is to keep the vent duct horizontal with a slight slope toward the exterior, insulate it heavily, and keep it covered by the attic insulation too. All our efforts are designed to delay the condensation of the moisture vapor back into water form until the water vapor is outside the house. We use a simple dryer vent to terminate the side wall exit.

Good Luck. The concept is simple but sometimes the options for venting make it difficult to attain the goal.

Go Back


Accessible Design
Bathroom Remodeling
Custom Home Building
Deck & Patio
Design & Remodeling
Exterior Remodeling
Garage & Driveway
Kitchen Remodeling
Repair & Maintenance
Water & Moisture Repair
Join the Mosby E-Newsletter to get monthly home tips & advice.    
Follow Me on Pinterest     houzz interior design ideas
Consultation & Evaluation Design & Remodeling Update My Kitchen Update My Bathroom Scott Mosby Answers Questions Exterior Remodel
Copyright © 2014 Mosby Building Arts, Ltd.   314-909-1800 • 645 Leffingwell Ave • St. Louis, MO 63122 | Sitemap