Expert: Daniel Humphrey - 4/1/2010

Question
Mr. Humphrey I moved into a home that has in the bathroom a fan to draw odors out built into the ceiling that is about 8" in diameter with a small light mounted in the center of it. This unit is in a metal case mounted in the ceiling, It is not in the roof at all. What is happening is the metal I think is creating condensation and dripping down the side on the metal and soaking in the drywall and staining it. I also have a cathedral ceiling with some lights built into the ceiling of that, the same about 8" in diameter in a metal case. I think the heat from the light there is also creating condensation because I am seeing the same kind of staining on that drywall around the light. Just to confirm these are not in the roof and the roof don't leak in anyway.
How can I stop this staining around my lights and fan? Thanks

Answer
Dear Bill,

The moisture condensing on both the bath fan and the recessed lights is coming from bathing and cooking. When the warm, moist interior air contacts a surface cooler than the dew point, it condenses. This is a common problem in Alaska.

There are two ways to combat this:

1. Prevent interior air from leaving the room, and
2. Insulate the metal parts so that they remain above room temperature.

The bath fan needs to exhaust to the outside--not into the attic space, which obviously would cause condensation on the underside of roof decking and show up many places, appearing as roof leaks. Check to confirm that the air is indeed leaving the building. Seal all joints in the ducting and use insulating duct wrap to keep the ducts warm.

Caulk or foam around the light cans a the ceiling to prevent air from leaving that way. You should strive for a seal that would keep all interior air inside the room. One remedy is to cover the lights with 5-gallon buckets in the attic, setting them into a puddle of sheetrock compound. The wires passing through the bucket can be caulked, and insulation can then wrap the bucket. This keeps the air from encountering a cool surface.

You can also look around for some better fixtures, such as insulated bath fans and ducting, and airtight recessed light cans.  But I would work with what you have, if they are otherwise acceptable. Your house may not be getting enough ventilation, and you may consider increasing the capacity of your fan and perhaps putting the fan on a humidistat or a timer.

Write back if you need more detailed info on anything I've mentioned.

Daniel