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cbay 12-05-2010 08:18 PM

Washroom Exhaust Fan Through a Flat Roof
My wife and I purchaced a house built in 1970 a couple years ago. It has a low slope roof (2 in 12) with no traditional attic, but instead the Ontario Building Code Minimum of 50mm inside of an 8" joist. Insulation is limited to R20.
The original exhaust fan didn't move much, if any, air from the washroom. Heavy condensation would always occur on the ceiling in the winter following a shower.
I purchaced a new exhaust fan which certainly moves more air, but has created new problems. Condensation is forming within the airspace now. I have made three attempts to solve this problem.
My first attempt was the worst attempt where I just connected the new fan to the outlet of the old, which only vented into the soffit. Of course I had probelms from this. Fans should not exhaust into the soffit.
The second attempt I installed a proper soffit exhaust vent. My theory is that the air just returned through the vented soffit within the airflow to the ridge vent and then condensed inside the vent space.
This last spring as part of another reno, I had the contractor install a roof vent and connect the fan to it, abandoning the soffit vent once and for all. Now we had to wait until cooler weather to see if this worked. It didn't!
The temperature is now below freezing, and guess what? Water is once again coming through the exhaust fan.
During the second attempt I had the entire ceiling down, the sheathing and insulation were soaked, all within 2 joist spaces to either side of the fan. Dry beyond. I am very certain this is washroom humidity causing this problem, not a roof leak. Its dry all summer.
I am out of ideas. Please help!

GBR 12-05-2010 08:26 PM

Bathroom fan moisture a source of worry - Winnipeg Free Press Homes


cbay 12-05-2010 08:45 PM

Thanks GBR. That article is very typical of what I keep running into - written assuming a full attic; though the thought that air is leaking through the fan housing is something I have thought of but haven't come up with a solution that satisfy's my desire to not add a bulkhead or drop an already low ceiling justto seal and insulate on 5 sides of the fan, and maintain the 50mm clear airspace. Is there any potential issues with through-wall venting of a bathroom? Would that work in a 4" wall?

handyguys 12-06-2010 11:46 AM

cbay - Thats exactly what I was thinking. Do a through the wall vent fan. The outside termination is less prone to leakage too. I don't like vent penetrations on low slope roofs, i'm afraid the will leak eventually.

So, what I would do.
Add a decent fan to the wall and vent outside.
Remove the old fan and properly insulate the space where the fan originally was
Remove the roof vent and patch the roof

Down the road, consider upgrading your roof insulation. Probably from top bu building an attic on top of your original roof. This would require some engineering and carpentry skills but would give you the insulation and ventilation it sounds like you desperately need.

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