Roof leak, Exhaust vent for bathroom fan

[RedBaron]

The last two years I have had water come in due to ice damming under the roof vent from the bathroom exhaust fan. I looked all over the internet and can't seem to find anything on how to prevent this.

Huge lump of Ice builds up under the exhaust and any warm days snow will start to melt and find it's way into the bathroom. If I clean the snow out around the ice chunk water immediately stops coming in.

Southern Ontario Home attic has blown in insulation (recommended amount for area) R50 I think, no other trouble anywhere else, Shingles are also in worse condition under this exhaust then the rest of the roof

How would one go about fixing this, and why don't other people seem to have damming problems at their exhaust, what would different about mine?

 

[Wuzzat?]

You couid attach a thermostatically controlled heating element under or on the roof area where the ice normally builds but sizing this heater can be tricky - but not as tricky as figuring out why this is happening to this area of your house.

The continuing cost you will pay for electricity is sort of a premium on an insurance policy that protects your bathroom from damage.

 

[oldognewtrick]

RB, is the ice forming around the pipe itself? If so, could be a loose seal where the duct attach's to the exhaust flange, is the exhaust pipe insulated? Warm moist air from the bath can condense of the cold exhaust fitting. Is the ice forming on the wood decking? If so, it could be that the warm air from the fan melts the snow load on the roof. Is there ice and water shield under the shingles sealing the base of the roof top vent?

An option you may pursue is running the insulated exhaust duct to a gable wall and venting out the vertical surface avoiding the snow load on the roof. I would not recommend venting into the soffit.

 

[RedBaron]

Ice forms about 6" below the vent on top of the shingles. Pipe is about as short as you can get and fan is slightly closer to the centre of the house. Vent is at a very low part of the attic so the blown in insulation is covering the pipe almost all the way from ceiling to the roof, you can just see it exit. Water actually comes in through a pot light not far from the vent, probably the closest opening in the poly. (Note there is no ice build up over the pot lights which are about 2' away from fan) Water stops dripping into the bathroom as soon as I clear a path around the ice chunk through the snow. I can't see any significant moisture anywhere, plywood seems pretty dry considering, so I am guessing the water is running down the outside of pipe.

In the rest of the attic there is frost on the nails but no other signs of water

Only have water when it gets warm enough outside to start melting the snow on the roof. After the first time it happened this year I stopped using the fan in that bathroom but had the same problem a few weeks ago. Damper inside the fan body is present and moves freely. I haven't checked the damper in the vent.

I have to redo the roof this spring so I'm not so worried about what's there but don't want to have the same problem going forward. Closest spot on the gable end is probably a minimum of 16 ft away. I was thinking soffit vent but seem to find alot of negative comments about that.

 

[Wuzzat?]

I'm wondering if the warm, moist air from your vent necessarily forms the ice and the only way to stop this is to heat the air from the vent because hot air must hold less moisture than warm air.

Your sequence of events can be traced out on one of these
[ame]http://www.google.com/search?client=safari&rls=en&q=psychrometric&ie=UTF-8&oe=UTF-8#q=%22psychrometric+chart%22&rls=en[/ame]
and this chart could tell you how many grains of moisture you need to remove from the bathroom air to stop the process.
But, this method is beyond my skill level.

Maybe you could rent a small dehumidifier for only the bathroom to test the idea or allow more house air to enter the bathroom.
http://www.way-technovation.com/Dehumidifier/calculat.htm

Can you aim the airflow in the opposite direction and maybe modify the vent opening to prevent rainfall from entering the now upward-sloping vent?
Can you disperse the outgoing air with homemade baffles?

BTW, if you have this problem once/week or once/month or once per 'period', whatever method you choose would have to work 3 consecutive weeks/months/periods for you to be reasonably assured that it works.