Installing a bathroom exhaust fan and the proper way to vent it...

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thru
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Maybe a hole in your roof isn't a big deal. It is in mine. And you're absolutely correct that I won't be working on your house. I don't do business with assholes.
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OK A case in point. My parents house has 2 bathroom fans that vent into the attic, no ducting, just straight from bathroom to attic. On top of that her over the range vent ducted into the attic too. I was up there last summer doing the range vent properly through the roof because she was having a kitchen remodel done. I expected to see the buildup of 35 years of grease, I expected to see mold, I expected to see wood damage. To my surprise there was none of the above. I did vent the range properly, but left the bathroom vents. I am not saying you should do this but in there case it did not cause problems. There could be many reasons for this. It is a 2000ftsq ranch house with attached garage and attic all the way through with excellent ventilation. On my small 1000 ftsq hip roof house I'm sure it would cause a problem. I redid my bathroom in January and found that there was a complete 1" gap all around my masonry chimney from basement to attic (1953 house). This caused years of moist warm air to flow up this gap and be drawn into the attic in the winter. All around the chase the wood was rotten in the attic and needed replacement which was no problem as I had taken the masonry chase out and replaced it with a B vent.

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You keep talking about "the code". Please tell us where you live (not the street address) so we can look up your states building code on line. If there is a restriction from venting a bath fan into the outside air through a dedicated hole in the soffit I would really like to see it.
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Only you are talking about venting into the attic. No one else. Try to keep up.
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MOLD! Homes that have poor venting that just vent into the attic eventually grow MOLD. MOLD is unhealthy! People don't buy moldy houses!
I've had my best friend walk away from 3 houses because the inspections found mold in the attic due to poor ventilation. 1 house was bulldozed after inspections found it was uninhabitable.
ALWAYS vent to the outside air. It may cost a bit, but well worth it if your house is your nest egg. Nothing worse than seeing your nest egg bulldozed because of toxic mold.
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MOLD! PEOPLE PLEASE DO YOUR RESEARCH! New Jersey has some of the worse mold cases/stories because old contractors and do-it-yourselfers vented into the attic and not to the outdoors! Home inspections now often include mold inspections when the house is older; my friend has walked away from 3 homes in New Jersey because all had toxic levels of mold. 1 house was bulldozed recently because it was unihabitable due to mold. Nice home, wasn't trashy BUT THE OWNER VENTED THE BATH INTO THE ATTIC FOR 15 YEARS!
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snipped-for-privacy@optonline.net wrote:

The point is why do it that way when there are better ways?
I don't know if there are documented problems when it is properly vented out the soffit, but it is common that someone will just put the vent close to a soffit vent and consider that proper venting. That has been reported as causing problems.
I have heard of a couple of cases where someone reported that even when properly venting out through the soffit there has been problems, but I am slightly skeptical of those so I would not say it has been documented. .
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Joseph Meehan

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wrote:

You are probably referring to the ventilation of combustion appliances in which case you have a valid point. But unless you are dealing with someone with extraordinary toxic farts the problem with a bath fan vent is not a issue worth worrying about. You are making a mountain out of a molehill.
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BP wrote:

There does seem to be differing opinions on the subject. While I am inclined to believe that it is not a big problem, I would not want mine vented that way.
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Joseph Meehan

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I seem to recall, either in Fine Homebuilding or a CMHC publication that under certain circumstances in cold climates venting up through the roof can lead to thermosyphoning hot air continuously when the fan is off. Which wastes heat, and in a properly vented "cold" attic, can lead to condensation problems inside the vent line, resulting in water dripping from the fan and corrosion. In a previous home, we had exactly that problem on really cold days - sitting on the throne getting dripped on from above (no comments about below ;-)
In those cases, venting out horizontally through the wall, or down through the soffit was superior, because it won't thermosyphon.
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Chris Lewis, Una confibula non set est
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"I seem to recall, either in Fine Homebuilding or a CMHC publication that under certain circumstances in cold climates venting up through the roof can lead to thermosyphoning hot air continuously when the fan is off. "
Veeery interesting! I had never thought about that. I suspected a similar and worse effect occurs when people run a bath vent to the underside of a roof mounted fan. When the roof fan runs in summer, it will suck cool airconditioned air from the bathroom, even if the bathroom fan is off. I would think as you pointed out, that just having a straight up run from the bathroom to a regular roof vent would provide and ideal path for convection.
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As I mention, that's merely wasteful... It gets vastly worse if you have condensation dripping into the room.
I don't know whether putting a "trap" (S curve) into the vent (as others have mentioned) will be sufficient to break the thermosyphon. I would suspect not in most cases, but, at least it should prevent condensation draining _through_ the fan... But, the "trap" might get rather yucky after a few years.
Yeah, running the bathroom vent to the underside of a roof mounted fan would be bad even in the summer too if you had AC.
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On Thu, 17 Nov 2005 17:22:15 GMT, "Joseph Meehan"

When we had siding done a few years ago, I was not sure where a future exhaust fan would be mounted within the bathroom. An exhaust vent (a BuildersEdge product, special ordered from Home Depot) was installed in the soffit at that time. The vent, which has no moving parts, was centered within the span of the existing bathroom window; that bothered me for the reasons that you mentioned, Joe.
Recently had a contractor gut and redo our bathroom including the addition of a Panasonic FV-08VQL3 fan. He strongly urged me not to go through the roofline, (apparently because he was not a roofer). As it turned out, the fan was mounted within the wet area, which made it NOT inline with the existing vent. After some discussion about positioning of the flexible duct (I objected to having any bends in the layout of the duct), the existing vent was simply relocated such that there is a short and straight run of approximately four feet to the (relocated) vent in the soffit.
Going through the roof would have been agreeable with me. Not enough thickness to mount this fan within the vertical wall. The use of soffit vents is commonly used. The damper is within the fan assembly itself, not in the exhaust vent that is mounted at the soffit.
I have not been up on a ladder to test the air flow yet; but am hoping that the duct tape does not dry out and fall apart ...
FWIW.
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That is not done here and I believe it is against code. The moisture in the winter builds up ice and causes problems in the soffit and with any wood in there.

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I would want to see "the code" that says this. I think a lot of people are confusing the hack job of pointing the end of the ducting into a vented soffit, with the proper installation which is to run the duct out *through* the soffit terminating with a grill or louvered vent cap.

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" I would want to see "the code" that says this. I think a lot of people are confusing the hack job of pointing the end of the ducting into a vented soffit, with the proper installation which is to run the duct out *through* the soffit terminating with a grill or louvered vent cap. "
I agree!
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snipped-for-privacy@optonline.net wrote:

i have a 1962 built ranch in cols ohio. the above is what i did when i replaced a bath fan. put the metal pipe about 1/2" or more past the original wood soffit. it has vinyl soffit over that. i then used the vent soffit over the pipe. next vent opening 2 to 3 feet away. my only question is when i did this the old fan had no vented piping at all. just emptied into the attic. there was no damage over the last 30 plus years. so if the attic has good ventilation you may not have to worry which way you go depending on you climate?
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I used 6" flex duct myself for a fan that I recently put it. (I used the insulated stuff too)
I vented it to the soffit and put on a auto opening grill to vent the bathroom air to the outside.
Nothing like having a 200 cfm van venting the bathroom. No more smells, no fogging the mirrors nothing. Just sucks the hell out of the heat in the house :)
Didnt want to go with a through room install. My feelings are the less holes in my roof, the better off I am.
Tom
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snipped-for-privacy@gmail.com wrote:

Yes, I did the same thing. Insulated flex duct out to the soffit. Very nice setup.
TP
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This may be a little OT, but if a person was dead set for some reason on venting to the attic OR they came across such a set-up, you can simply change your habits to minimize risk of attic condensation.
For instance, don't run your fan when it gets chilly enough outside to see your breath (or thereabouts). In cold weather, heated interiors experience a drop in humidity and can absorb lots of moisture without the humidity getting high. It may be a good idea to run your forced-air furnace and keep the bathroom door cracked when you take a shower to keep an influx of low-humidity air into the bathroom. In the winter, I can take a shower without running the vent fan and the mirrors don't even begin to "steam up".
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