Can a bathroom vent exhaust duct be TOO short?

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itsttl

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I am installing a recessed bathroom fan/light combo in my bathroom and the only place it can be put (due to joists and spacing) makes the vent for the blower about a 16 inches away from the plywood of the roof. I have been trying to google over and over but I always find people who are making ducts too LONG. In this scenerio could it be too short? There is currently no hole and it has been vented into the attic for the past 47 years (the fan it is replacing is from 1970). The roof is vented at the top of it and at the soffits so I cannot go out the soffit, correct? In a perfect world I would like 0 cold air blowing into the bathroom and no condensation falling on me while I am on the toilet :) I live in Michigan so the winters get cold. If it is any help it is a North facing roof at what looks like a 26 degree pitch. Thanks.
 

Jadnashua

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It's never a good idea to vent hot, humid air into an attic. Is it done, certainly. Is it ideal, no. It can cause snow to melt on your roof, condensation under the roof, and in the insulation that can lead to issues. We average (only) about 60" of snow a year...mine vents out the roof since there isn't an accessible wall or peak (a row townhouse). Been there 16-years now, hasn't leaked, hasn't been a problem. The fan assembly has a damper, and the roof jack also has one, which helps along with the fact that the ducting is insulated so there's not as much chance of the moisture condensing and dribbling back into the room. Mine is on the east slope of the roof with prevailing westerly winds, so the damper isn't subject to banging or leaking as much as one that might be closer to the prevailing winds.
 

itsttl

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Thanks for the reply. Luckily the attic doesn't "look" moldy yet but I am still wearing a good mask while working up there. I have noticed ice dams above the bathroom every year since we moved in (that I remove with a roof shovel) and I hope this will help with that. So the issue of dripping has to do with insulation only? I have no problem spending the big bucks and insulating the 12 inches of exhaust pipe haha. Can you recommend the best damper for the winter climate to keep the heat bills down? Thanks again.
 

Jadnashua

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Assuming you intend to use the vent all winter, you need to consider how deep your winter snow load can get on your roof over where it must exit the roof and put the outlet above that point, or the hot air going out will just end up melting snow, and, if the snow was very dense, the air may mostly be blocked. I haven't looked lately, so am not up on what the options really are today. Personally, I don't use mine as much in the winter, but do use it regularly in the summer, so am not as worried about the snow depth. Mine is somewhat like this one. You probably want one out of aluminum or SS (probably not plastic or steel since those make break or rust). There are some other styles, and some of them are taller. This one has a damper built in, so the combination of one in the fan/light assembly plus the one at the roof should help keep convection and drafts down if you insulate the ductwork going to it. Being that it's such a short distance, it may not be that big a deal, but everything that can help.
Backdraft roof hood.jpg
 

Dana

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Side venting it through the wall, below the soffits can work, but some of the humid air will be drawn into a nearby soffit vent. Even if some of the vented exhuast gets pulled into a soffit vent it's still vastly better than venting it directly into the attic- most will stay outside. At least side-vented it doesn't provide a convection path that pulls air out of the house into the attic 24/7 all winter. Air sealing the holes in the ceiling and around the fan is important for preventing the ice-dam starting heat leak.

Venting it through the roof is only slightly better than into the attic from an ice-damming point of view, but vastly better from an attic-mold perspective.

Extending a wall termination an inch or two beyond the siding (not right at the siding, as in the picture below) or using a termination designed for a bit of "throw" rather than directed downward is somewhat better, since it keeps the drips from forming icicles and ice sheets along the siding.

Bathroom%20exhaust%20fan%20illustration%20-%20FHB.jpg
 

itsttl

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Alrighty just to update: After I installed the fan the distance from the vent opening to the roof truss was actually 3 inches. The distance to the top plate was 8 inches. It was way too close. I decided to buy another fan with the exhaust port facing the opposite direction so I could give the vent a proper 2 foot "flat" section before the 90 elbow takes it to the roof. This will also help prevent ice damming as it is higher on the roof and further away from the heat of the bathroom underneath. Plus I can cram a lot more insulation above the IC rated fan while keeping the soffit airflow uninhibited. Venting through the wall was not possible as it is a ranch. I'm not excited to drill the hole in my roof in the spring but it must be done. Maybe I will learn something about roofing in that time as well because my 30 year roof is at 25 years :( Thanks for your time :)
 

Jadnashua

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Just to throw another wrench in the works...when I did some remodeling on my bathroom, because it is in the middle of the house (it's a townhouse), the room did not have any windows. To bring some light into the room and fix the venting which the builder just terminated in the attic! I installed a tubular skylight with their light/fan kit. I chose www.solatube.com because it had the best color temperature (most of them tend to block some colors - this one was the closest to natural light). It's been in there now about 15-years, and has worked like a champ. It's surprising how much light the thing brings in, and with the fan remote, it's nearly impossible to hear it run. The fan kit on mine is different...they changed the design a few years after I got mine and this one has been in use for quite awhile now. It even lights the room up at nighttime from reflections. The make optional tunnels to raise the dome further if you wish, but that also needs extra tubing (which is not inexpensive!).
Vent_Kit_Large_0.png
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itsttl

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I originally had the Broan 744 but now have the Panasonic WhisperRecessed. Even the Panasonic is expensive to me I cannot imagine what a skylight costs :p
 
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