Bathroom fans

Published on July 20th, 2013 | by Henri

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Condensation from bathroom vents

Q.    We have been frustrated with water leaks that have stained our ceiling for about two years. The house is 20 years old, 2-story with finished basement. The roof was replaced about three years ago. At that same time we decided to create additional usable attic space with access from the second floor and plywood on top of the blown insulation. We vented the two upstairs bathrooms and dryer to the outside rather than into the attic as was present when we purchased the house 10 years ago.

We believe that we had initial problems with flashing but the handyman who helps us has corrected that problem. Additional leaks this winter have been traced to excessive condensation in the vent pipe rather than from the roof. A note: Several members of the family love long showers (as long as 20 to 30 minutes) and so the amount of steam from these showers is substantial.

The exhaust fans for the two bathrooms were tied together into one roof vent. Our only solution this winter has been to disconnect the vents and put them into a bucket in the attic that we empty every four to six weeks (with two to three gallons of water). With this temporary solution for the winter we have stopped additional ceiling stains.

How do we prevent this problem short of limiting shower duration? We have considered the possibility that a horizontal vent may lead to less condensation but don’t want to spend the time and effort on that solution if this will not correct our problem.

A.    From your description, the bathroom vents were terminating through the roof. So condensation would run right back into the ceiling, usually where the fan is. Tying two bath vents into one is a mistake. What often happens is that the moisture generated in one bathroom returns into the other one, especially with a near vertical installation. You have temporarily solved the problem by disconnecting the vents.

The best solution is to replace the vents with schedule 20 drain pipes (one for each bathroom and with the bell ends toward the fans). Keep the pipes as close to the attic insulation or floor  as you can but give them a slight slope to the outside by using small wood blocks of decreasing thickness.

Terminate them through individual hooded aluminum or plastic wall jacks, preferably on the south side of the house if it is the shortest run. Avoid louvered plastic jacks, as they are prone to breakage. Snug fiberglass batts on each side of the pipes and place another set of batts on top. Properly insulated, condensation will be reduced and drain to the outside.

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