I am installing a bath fan. My house is a bungalow style. I like to vent it
out by the soffit just outside the bathroom; but I am not sure how I can get
a vent pipe thru to the soffit.
My other option is to run a vent pipe down the middle of the attic space to
the side of the house vent. This pipe will have to be about 22'. Would this
be too long of a run?
Soffit venting is not ideal because of the tendency for the warm,
moist air to get sucked back into the attic, but in your case might be
better than running the vent so far.
22' will reduce your air flow a lot, especially if you are using
insulated flexible duct. If your fan is oversized (say twice the
needed CFM) for the space it might be ok. If it gets cold where you
are you should use insulated duct to minimize condensation. Try to
pitch the vent so condensation drains out.
For such a long run, you might consider an inline fan (google fantech
inline fan) As a bonus, they are much quieter than even the best
ceiling mount fans. (but more $$ too).
I've got one and they're great, well worth the added cost. I put one of
their low-voltage light/intake assemblies in the tub/shower ceiling. An
informal test using some strips of plastic showed no significant change in
intake power between the fan ducted into just the attic space (during
construction) and then using insulated ducting to a side vent on the gable
about 16' away. But a roof vent will certainly give better flow and will
probably cost a lot less than running extra insulated ducting.
Depending on the fan 22' may not be too long a run if the pipes do not have
bends, and you can use a slightly larger diameter pipe to reduce the
As this is a bungalow, why not go right up through the roof ?
They do sell roof caps that are quite good and do not leak as long as they
are properly installed.
One of our bathrooms is right in the middle of our house. I did not want to
have another roof penetration, so I ran a pipe through the attic to the
side wall, about 20'. To minimize airflow restriction, I used solid 4" PVC
pipe for the straight run, with a short section of corrugated flex pipe at
the end to make the connection to the fan. It works fine and I haven't
noticed any reduction in airflow.
Thanks for great ideas everyone.
Some of you mentioned that I use "Insulated" flex pipe for a long run. I am
not sure if I have seen this pipe. The ones I see at the hardware store are
the regular flexible plastic pipe for bath fans. If I use a 4" PVC pipe,
would this provide enough insulation??? and why do I need to have insulated
pipe to begin with?
Most of the big box stores carry it, at least here in OH. 20 or 30
feet of it come smooshed into a box about 3 feet long. The duct
itself is like the plain flexible type you have seen, but is wrapped
with fiberglass insulation and thin plastic.
In cold climates, the attic will be cold in the winter. The warm
moist air in the duct will condense against the cold sides of the duct
and water will puddle in the duct. This collects dust and often
provides a place for mold to grow. Sloping the duct so water can
drain out will help, but because the duct is not perfectly smooth
inside, it is still not ideal.
Plain PVC pipe won't prevent the condensation, but does tend to drain
If you live in a warm climate it can work in reverse in the summer,
with warm moist attic air condensing on the outside of the duct as it
carries cooler air conditioned air. So either way, using the
insulated duct is a good idea.
I've seen the insulated stuff at our Lowes store too. Was expecting a huge
roll, and was surprised to see it in such a small box... :)
My "attic" is actually within our heated space, sort of an oversized
soffet. So, I didn't have the condensation issue to worry about. But, I
still installed the PVC with a slope towards the wall vent. Any moisture
that collects inside the pipe will drain off to the wall vent, and not back
into the ceiling fan.
If my attic were exposed to cold air, I would wrap the pipe with
insulation, or just bury it under the normal roof insulation if possible.
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