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Venting a bathroom exhaust fan

Posted by rlirwin (My Page) on
Fri, Feb 21, 14 at 11:04

We are remodeling our bathroom, and we are installing an exhaust fan for the first time. The contractor is planning on venting the fan by laying behind the screen for one of the eave vents (we live in northern California, and our home doesn�t have soffits). That seems like it may be an issue for moisture even where we live, although he said he does it all the time and has never had a problem. The other alterative he suggested was putting a dryer vent through the 2x4 under the eave. That seems better, but it will still exit under the eave and relatively close to eave vents. The bathroom is approximately 20 feet from the nearest gable wall. It seems like the best alternative is a roof vent, but the contractor says putting more holes in the roof is not a good ideal. Thouhgts?

Follow-Up Postings:

RE: Venting a bathroom exhaust fan

The termination of the vent ducting needs to be on the outside of the building as a general rule. There is a good chance of the moisture being vented winding up in the attic/vent space during windy/rainy/snowy conditions due to reduced air flow.

IMHO, a properly sealed through the roof vent is preferrable to side of the building vents.

RE: Venting a bathroom exhaust fan

"but the contractor says putting more holes in the roof is not a good ideal."

Does he think it is better to have warm, moist air creating mold in your attic?

I agree with handymac's suggestion that 'the vent ducting needs to be on the outside of the building'

RE: Venting a bathroom exhaust fan

Maybe HE doesn't know how to do a roof penetration properly where it won't leak. So, hire someone that does. It's honestly not a big deal when done correctly.

RE: Venting a bathroom exhaust fan

Roof venting is they better way to go if possible. How far up is the roof line? If it is a long distance and the pipe goes straight up the moist air can cool down and moisture can trickle back down the venting pipe, a properly sloped venting pipe to the side of the building is sometimes the better option.

RE: Venting a bathroom exhaust fan

If you can go through the roof, that is best. They make exhaust fans in different hp sizes to accommodate for the length of the run. Make sure whatever exhaust fan is installed is rated for the distance of the run.

Here is a link that might be useful: Remodeling and Home Repair

RE: Venting a bathroom exhaust fan

DH and I installed a bathroom fan, and vented it through the roof. If we can seal the hole so it didn't leak, I would think a reputable contractor would be able to do the same. Any exhaust fan should be vented to the outside - to a space where the exhaust isn't drawn back into the house.

We used an in-line fan, which is very powerful, but because it is mounted to the rafters and not the ceiling, one can hardly hear it from the bathroom.

In our case, it would have been better to go out the gable end (even though it's close to 20') than the roof, because we are in North Dakota, and the roof vent on the north side of the house can get snowed under and/or freeze up in the winter. I never thought of going out the gable end, until our neighbors had one installed that way.

RE: Venting a bathroom exhaust fan

I agree about not cutting a hole in the roof for a bath fan.
most bath fans don't have enough cfm's to push up & out..
esp if roof pitch is high.
and why create another leakage site in will leak
as all flashings fail eventually.

better to vent to eave/soffit.

best of luck.

RE: Venting a bathroom exhaust fan

energy_rater, if you vent the humid air from a bathroom to the soffit, what's going to keep that moist air from going right into the attic?

RE: Venting a bathroom exhaust fan

"energy_rater, if you vent the humid air from a bathroom to the soffit, what's going to keep that moist air from going right into the attic?"

back draft damper attached to bath fan housing.
venting attached to back draft damper.
venting exits soffit to exterior.
fan being on will push air out of venting, venting
to exterior of attic will keep moisture out of attic.

also, if you ever take off the cover of the bath fan from
inside the house..there is an oversized cut in the ceiling
that is covered by the cover of the bath fan.
sealing this cut is recommended to keep attic air/temps/
insulation particles out of houses.
I use Hardcast brand mastic tape #1402.
I also use this tape to attach back draft damper to
housing of bath fan for air tight seal, and to attach
venting to back draft damper.

best of luck.

RE: Venting a bathroom exhaust fan

Check with local code about venting into a soffit. In this area, it is against code.

Energy-rater, I don't know where you get the idea all flashings fail. I have yet to see a correct flashing fail. It's always poor workmanship in the first place, where the problem is.

RE: Venting a bathroom exhaust fan

flashings fail, it just takes time.
some flashings are incorrect to start with, but
even correctly installed materials don't last forever.
I"ve seen too many materials that have just worn
out to not believe differently.

code here says out of attic, doesn't specify through
roof or soffit just that it must have appropriate termination.

RE: Venting a bathroom exhaust fan

energy_rater- Should the exhaust exit through a roof vent, the roofing material will fail before the vent with attached flashing will fail. Whenever the roof material is replaced, it is common to replace all flashing and flashing materials. Although some will use the old flashing, and of course it will fail.

The flashing should never fail. I am curious of one instance where flashing has failed, using the appropriate flashing material.

I spent nearly 40 years installing custom bent flashing, on jobs where either inappropriate material was used, or the installer thought they knew what they were doing, and didn't. This is in an area in the Midwest where weather extremes are common.

RE: Venting a bathroom exhaust fan

I think that there might be some confusion here. Venting through a soffit vent, the soffit vent referring to the screen or mesh that is being used to vent the attic is certainly a bad idea. Venting through a soffit vent that is designed for that purpose, often with integral backdraft butterfly dampers can be a good idea. It should probably be placed away from any attic intake vents.

energy_rater lives in a relatively high-wind area where roof penetrations are feared and avoided by many.

RE: Venting a bathroom exhaust fan


You said you were 20 feet from the end gable? I'd run the exhaust straight up into your attic say 2 feet and elbow to a slight down pitched run to the end gable and out thru a dryer exhaust door/flap or similar.

I did that for a 20 foot run three years ago... I did insulate the metal piping.

This post was edited by saltidawg on Sun, Mar 16, 14 at 17:40

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