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Daviddasdj 03-23-2011 08:44 PM

Bathroom Exhaust Vent
I wanted to know what is the best option to terminate the exhaust vent for a ceiling mounted bathroom exhaust fan, would it be exiting at the roof (vertically) or along the ceiling at exiting at the soffit (horizontally)? I've had experiece that when installed at the roof, wind opens the vent lid and snow enters and water eventually drips down the vent... :( But I've also heard that exiting at the soffit can cause vapour to leak back into the attic... any advice welcome, thanks!!!

algored2deth 03-23-2011 09:15 PM

Welcome to the forum!!!

The vent should exit at the roof. Use rigid pipe with insulation (a fiberglass wrap most likely) and seal up all seams at the two end connections with caulk. Also seal the exhaust fan box to the ceiling drywall with spray foam or caulk (use best judgment here). Venting at the soffit provides a chance for moist air to recirculate back into the house. Use a good quality vent that is sturdy. Moisture can come back if your vent pipe is not insulated because you will have warm bathroom air meeting with cold attic air.

jklingel 03-24-2011 12:58 AM

You can also spray foam the pipe till it gets outside. I put 2-90 elbows (ie, 180) on my 3" ABS vent pipes just for the water issue.

Just Bill 03-24-2011 06:27 AM

Rain or snow should not be an issue. Roof caps have a flapper that only opens one way, and wind will actually hold it closed tighter. Condensation inside the vent pipe is sometimes a problem. As suggested above, insulating the pipe up to the roof exit will reduce this problem.

Daviddasdj 03-24-2011 08:33 AM

Thanks for your replies. The vent that I installed does open with the wind...I heard it the other night flapping away, when we had strong winds and snowfall the next morning a found a small wet patch on my bathroom rug... so are there different kinds of vents or better quality vents?

algored2deth 03-24-2011 08:54 AM

Yes, you can go to HD, Lowes, etc and check out different versions. Braun makes them (as an example). They should have a flap that shuts when the indoor fan is not running. Build quality varies in terms of the sheet metal that the vent is made from. Also check with bldg supply stores as they may have something made with a heavier gauge of metal. Plumbing supply stores might have something...

Mike Mike 04-26-2011 10:26 AM

I've just gone through the roof vs. soffit battle at my place, so wanted to add my 2 cents for anyone researching this issue. It may save you my pain.

I tried it through the roof. I live in Ontario where it can get very cold. I used flexible pipe up through the roof with one of those small Broan vents with the flapper. I insulated it to prevent condensation. It did not work. We got condensation dripping back through the fan.

The only downside that i see to venting through the soffit is the potential for moist air to re-enter the attic. However, venting through the roof has many disadvantages. Extra hole in roof means potential for leak. Condensation can only drip downwards and back into fan. Insulation can be difficult to apply. Noise of the flapper. Snow buildup on the roof can plug the vent.

I have now changed it entirely. I am using rigid pipe out through the soffit. I angle the pipe towards the soffit so that any condensation that may develop will drip outside and not back in through the fan. I insulate it all around and cover it with some of the attic insulation also.

Will have to wait for winter to see if this is works better, but I suspect it will.

Also, if you do decide to vent through the roof, don't buy the crappy vent with flapper from Broan that they sell at Home Depot. There are other kinds that extend up higher to get above the snow and have a wind guard to prevent the flapping. A roofing specialty store should have one.

Mike Mike 01-17-2012 02:49 PM

Further to the bathroom exhaust saga ....

As I mentioned, I now vent the exhaust through the soffit. I think I'm still getting some frost inside the vent pipe, but this is kind of inevitable. The section where it passes through the soffit is impossible to insulate.

Overall, I'd say it is working pretty well. My only additional advice is to use smooth-walled aluminum instead of galvanized. The galvanized will probably rust eventually.

mrgins 01-17-2012 09:39 PM

Soffit for me. Easier to pitch the pipe and insulate

jklingel 01-17-2012 10:07 PM

I built in '80 in Frb, and ran ABS pipe out through the roof. As mentioned above, I installed a 180 degree elbow at the end, and have never had water dripping back into either bath vent. A winged ghost appears around the pipe after long cold spells, but it has never closed in. (It may have once back in the early '80's, when we had some real nasty temps. ???) The DWV pipe does close once a year or so, as there is no fast, warm air to melt and blow and ice/condensation away (I gather). Run a fan with adequate CFM, and avoid long, steamy showers (saves on fuel, too). Be very careful venting through the soffit, as the likelihood of moisture getting back into the roof area is there.

Minus08 01-17-2012 10:36 PM


Originally Posted by Daviddasdj (Post 615949)
Thanks for your replies. The vent that I installed does open with the wind...I heard it the other night flapping away, when we had strong winds

It flaps because of the low pressure area created on the outside of the damper by the high winds. The same way a plane's wing creates lift.

In my home I have 2 bathrooms upstairs and each exhaust fan was vented out the soffits. I questioned our home inspector about that and he said, The termination point for your fans is typical for the age of the dwelling. Since 2003 the building code changed to require that bath fans terminate directly to the exterior with a hood and back draft damper similar to that like a dryer vent.

As soon as we took possession, that was the first thing I changed. Installed 2 roof vents and vented them out that way.

I believe This Old House had a demonstration once where the fan was vented out the soffit, and showed how the warm moist exhaust air would go right back into the continuous soffit vent.

If you decide to do soffit venting, just monitor the area for any signs of moisture collecting on underside of roof sheating

gcan 01-18-2012 06:09 AM

Does anybody have a picture of the soffit venting?

My house is 1-1/2 story and the 3 bathrooms are spread through the house and I really don't want to put 3 holes in the roof. I haven't seen a soffit vent that I felt would perform very well.

Thanks in advance

mae-ling 01-18-2012 08:21 AM

MY HVAC guy says To put the bath fan on the top of an inside wall. then duct it down the wall and out the floor joists. Insulate where it goes out the flor joists.

4just1don 01-18-2012 09:57 AM

mae-ling., so your mounting it on the vertical wall towards the top of exterior wall?? why exit so low then? seems it would reduce exterior wall insulation

If your using ceiling mount how do you get vent down thru top double plate to get to stud cavity and seems same question on why down to floor joists?

Minus08 01-18-2012 11:36 AM

4 Attachment(s)

Originally Posted by gcan (Post 826523)
Does anybody have a picture of the soffit venting?

Two types of soffit venting. Continuous soffit and "indiviual soffit". First 2 photos are individual soffit vents and the last ywo are continuous type

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