Would anyone like to weigh in on where I should vent a bathroom fan? (Not
the attic--I know that!) I've found that some home repair sites say it's OK
or preferred to vent through the soffit; others say this is nearly as bad as
venting into the attic and that one should vent through the roof; and yet
others say to avoid the roof and go through the soffit or gable end of the
Does this all depend on where you live? For instance, in areas with heavy
snow, the roof vent could get blocked and therefore venting through the
soffit would be preferred, but everywhere else the roof is the way to go?
Is it really bad to vent through the soffit?
yeah, the gable end is preferred. the trouble with going through the
roof is that you can get condensation that will drip back into the room
no matter how well you insulate (been there). run it out the gable,
insulate the pipe, and give it a slope to the outside.
Gable end good.
Roof vent good
Like any hole in the roof it is a possible leak source. Seldom
happens in my experience.
Soffit not so good
Soffits let in air. Warm most air from one soffit is likely to be
drawn into the next, or maybe even the one you are venting through
Attic Very Bad.
Good advice from other posters. One point I'll add: Keep the duct
length reasonable. The shorter the better. If you have to run 30' to
get to the gable, but can go out the roof with only 5, you are
probably better off going out the roof. Even if you upsize the fan to
allow for a longer duct run, it's difficult to avoid low spots in a
long run of duct, especially flex. Low spots collect condensation,
which attracts dust and may foster mold. And longer ducts are more
prone to condensation because the warm air has more time to cool while
in the duct.
That's fine because your attic is actually outside. The advice should
be to vent on the outside of your insulation layer. This way if you
dump a cubic foot of air at 100% relative humidity into the outside, the
RH of that cubic foot will instantly drop to very close to the outside
value. For condensation to form, the RH must be 100% on a colder surface.
Sorry, but that's incorrect.
Dumping high RH air into a cooler area, will INCREASE its RH, with the
rise depending on the attic temp.
Compounding the problem is reliance on passive gable vents, which by
themselves will have no airflow unless there's wind from one side or
the other. So, running bath vent will just displace attic air with much
higher absolute humidity air, which on chilling will see rising RH, as
mentioned above. (Outside air in winter is typically of much lower
absolute humidity than indoor.)
If you vent through the soffit, you want to make sure that
there aren't any soffit intake-vents within a fathom of
the out-vent. Otherwise all the moist air that you've just
carefully vented to the outside will just get sucked back
into to attic.
I'm not so sure it's all that bad. Some of the ejected air may wind up
going back in through adjacent soffit vents, but I doubt it will amount
to all that much. The air coming out of the vent is going to be moving
fairly rapidly, while the air intake of adjacent soffit vents is moving
very slowly, even on a hot day. Soffit venting is very common, a
large percentage of those have continuous soffit vents, and if it were
that big of an issue I think there would be widespread reports of
problems. The ones in my house here in NJ are all done that way and I
have no problems.
The waste stack for the toilet/sink in the attic is the best.
Do not use the accordion style vent pipe as the accordion will allow moisture to condense in it.
Use something like abs or pvc pipe of an inside diameter sufficient to allow coupling to the fan vent.
Run the pipe from the fan outlet to the toilet vent stack and join the two pipes via a mechanical sleeve.
If the bath has an outside wall and you cannot get to the attic, a direct vent on the outside wall works.
| Would anyone like to weigh in on where I should vent a bathroom fan? (Not
| the attic--I know that!) I've found that some home repair sites say it's OK
| or preferred to vent through the soffit; others say this is nearly as bad as
| venting into the attic and that one should vent through the roof; and yet
| others say to avoid the roof and go through the soffit or gable end of the
| Does this all depend on where you live? For instance, in areas with heavy
| snow, the roof vent could get blocked and therefore venting through the
| soffit would be preferred, but everywhere else the roof is the way to go?
| Is it really bad to vent through the soffit?
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