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Old 10-20-2008, 09:20 AM   #1
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excess moisture in bathroom with new vent


hi all,

i just bought my first home and have been lurking on this site for some time now. i just registered as a member today and i wanted to say how insightful and helpful you all are on this site. keep up the good work!

i have a bit of a problem with the excess moisture in the bathroom that perhaps you all could shed some light on. i recently installed the nutone model 684nt with 80 cfm hoping that would suck up all the moisture. however, everytime the shower is used, the entire ceiling and all the walls are completely soaked. i never had this issue with my old apartment before. also, the moisture seems to be accumulating around the vent's grille and actually dripping water after turning it on for a while. i assume this is NOT normal.

so how can i remove the excess moisture? is the simple answer, buy a new vent with higher cfm?

a bit of background, the vent exhausts right out to the exterior wall which is about 8 ft of travel total. there are a few bends in the piping but nothing crimped that would restrict ventilation. the louver flaps on the outside open correctly. i can feel alot of exhaust from that opening, almost feels like a small fan blowing, so it seems like the vent is working properly. the bathroom is small, i dont know the exact dimensions or square footage but i can definitely say it is on the small side for a full size bath. so would 80 cfm be enough? or should i upgrade to 110 or 150 cfm?

perhaps there is something else that i should try before buying another vent...

thanks all!
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Old 10-20-2008, 11:35 AM   #2
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excess moisture in bathroom with new vent


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so how can i remove the excess moisture? is the simple answer, buy a new vent with higher cfm?
Possibly. It depends upon how big your bathroom is, in cubic ft.

I think that the recommended vent sizing is one such that it turns over the air in your bathroom 8-10 times/hour.

So... if you had a 10x5x8 ft. bathroom (400 ft.^3), you would want a fan that could move 8 times that volume / hour. So 8 * 400 ft.^3 = 3200 CFH (cu. ft. per hour). Since fans are rated in CFM, 3200 / 60 = 53 CFM.

So plug your bathroom into this: Length x Width x Height * 8 / 60 = Fan rating you need in CFM (minimum).

Although it can't hurt to go higher, you don't want to go too high as you'll be sucking lots of conditioned air out of your house.

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also, the moisture seems to be accumulating around the vent's grille and actually dripping water after turning it on for a while. i assume this is NOT normal.
Not sure where you're located, but if it's cold out (well, colder than in your bathroom when showering) and your vent fan and duct are uninsulated, condensation may be forming in the fan housing and/or in the duct.. then dripping down. That's bad. Even if it's not cold now and this is not your problem, if it's going to get cold in your area this winter you should insulate!

You could also be getting a lot of condensation on the walls if you're in a cold climate and your house is poorly insulated (i.e. the walls are cold). Even if the bath is not on an exterior wall, there could be cold air infiltration into the wall cavities from a poorly insulated basement/attic/crawlspace.

If you're in Arizona or some such, it could be that your A/C has cooled down the walls so much that you're seeing a lot of condensation..

All that being said, I'm thinking you may indeed need a fan with a higher CFM as you suspected.
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Old 10-20-2008, 11:41 AM   #3
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a bit of background, the vent exhausts right out to the exterior wall which is about 8 ft of travel total. there are a few bends in the piping but nothing crimped that would restrict ventilation. the louver flaps on the outside open correctly. i can feel alot of exhaust from that opening, almost feels like a small fan blowing, so it seems like the vent is working properly.
I should have mentioned that you hit the nail on the head thinking that it might be poor airflow.. Remember, even if you can feel a good breeze at the exhaust, that doesn't mean you're getting the full CFM out of the fan.

Are the bends you mentioned 90*? Those are restrictive, and each one cuts down airflow a little bit.

Also, the piping/ducting, after rising out of the fan unit, should be pitched away from the unit towards outside when running horizontal -- otherwise you can get condensate running back into the fan. (If there are low spots in the ducting, water can also collect and stagnate there and restrict airflow. This is especially a concern w/flexible duct).
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Old 10-20-2008, 12:04 PM   #4
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excess moisture in bathroom with new vent


whoa, thanks for the quick and thorough response.

i will try that formula that you suggested. i dont know the exact dimensions of that room right now so i will need to measure when i get home from work. thanks for that info!

i live in boston, ma. so yes, it is a cold climate and we are already seeing 40* weather. as you predicted, the crawl space above the bathroom ceiling is not insulated. it is just blank space up there surrounded by supporting wood beams/joists. so do you think insulating that area would help?

there are two 90* bends in the flex duct but its not crimped and it doesnt seem too "restricted". but i am no expert so perhaps you are right.

thanks again for all the help!
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Old 10-20-2008, 03:29 PM   #5
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Did the old fan work when you first moved in. If so, did it have the same problem.

Was its duct insulated.

Does the bathroom door have enough under cut to allow enough air to come into the bathroom.

Did you seal around the blower housing so that the fan only pulls air from the bathromm, and not from the joist space.
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Old 10-20-2008, 03:38 PM   #6
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i live in boston, ma. so yes, it is a cold climate and we are already seeing 40* weather.
Yep, gettin to be that time.. It was probably around 40* here last night (Long Island).

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the crawl space above the bathroom ceiling is not insulated. it is just blank space up there surrounded by supporting wood beams/joists. so do you think insulating that area would help?
Woah.. is there insulation underneath the roof, or is there nothing between the roof and bathroom ceiling????? If the underside of the roof is insulated, then no problem if the bathroom ceiling is uninsulated (in fact then the condensation in the duct wouldn't be much of an issue b/c the crawlspace wouldn't get cold).

But if there's not a stitch of insulation up there, it's 100% worthwhile to insulate the entire "floor" of the crawlspace between the joists. (Completely aside from this bathroom fan issue).

Also beenthere makes good points to check out.. Esp. about the airspace beneath the door -- if no air can get in, then naturally no air can be removed.. Or "less air".. I'm sure there's some gap.
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Old 10-20-2008, 04:02 PM   #7
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beenthere, i will check the gap underneath the door when i get home. it didnt occur to me until now but when i checked the exhaust from the louver flaps outside of the house, the bathroom door was wide open. and naturally, when i was showering, the door was closed. that might be it...

about sealing the area around the housing, how could this be done? right now, the hole in the ceiling is fairly snug up to the sides of the housing. probably only a quarter inch gap around the whole thing. it doesnt seem like much air from the joist space could be sucked up but i dont know for sure. how should i seal it? apply joint compound until all the gaps are covered???

there was no "old fan". the bathroom didnt have any vent in there before. so i installed a fan for the first time in there. and prior to that, i never took a shower in this bathroom.

scottr, the duct is not insulated. its just a flex duct that is about 8 feet long and goes straight out to the exterior. the joist space is also not insulated. this space is between the 1st floor ceiling and the 2nd floor floor... so its not the roof. does it still need to be insulated?

again, thanks for all the info guys! i really appreciate you two taking the time to educate a home owner noobie like myself!
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Old 10-20-2008, 04:08 PM   #8
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Caulk.
Its easy to remove if you ever need to pull the housing.

After you seal the housing.
Close the bathroom door. Let the fan run for 10 minutes, and see if its blowing out close to the same amount of air as before.
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Old 10-20-2008, 04:23 PM   #9
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beenthere, i will try that and report back to you.

thanks for the suggestion!
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Old 10-20-2008, 10:53 PM   #10
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scottr, the duct is not insulated. its just a flex duct that is about 8 feet long and goes straight out to the exterior. the joist space is also not insulated. this space is between the 1st floor ceiling and the 2nd floor floor... so its not the roof. does it still need to be insulated?
Ohhh.. I get it

Disregard what I said.. I was thinking attic crawlspace. Between floors you may not need to insulate the duct either. Stick your head up there when it's cold out (e.g. tonight ) -- if it's room temp in the crawlspace than insulating the duct is not so much of an issue.

Just occurs to me that if the walls of the crawlspace aren't insulated, it might get cold up there, and it might help ur heating bills to insulate the walls. Otherwise your heat is going right through your ceiling, into the crawl, and out the uninsulated walls.
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Old 10-20-2008, 11:04 PM   #11
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What was the flex duct made of? If you used HVAC insulated duct you got a problem.
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Old 10-20-2008, 11:16 PM   #12
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What was the flex duct made of? If you used HVAC insulated duct you got a problem.
I think he said it's uninsulated..

But do you mean insulated duct with plain insulation on the inside is bad? Because then I'm 100% with you..

You're not talking about this, are you?



B/c I was under the impression that was the correct type to use (in the pic, I mean).
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Old 10-20-2008, 11:25 PM   #13
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Originally Posted by ScottR View Post
I think he said it's uninsulated..

But do you mean insulated duct with plain insulation on the inside is bad? Because then I'm 100% with you..

You're not talking about this, are you?



B/c I was under the impression that was the correct type to use (in the pic, I mean).

Might get some differing opinions from other pros. That stuff is not rated for bath room fans if it is regular HVAC flex duct.What did the package say it was rated to be used as?

Unless that stuff is pulled taut the ridges inside are going to create a lot of resistance to air flow.

Even under cutting the door wont help that.
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Old 10-20-2008, 11:28 PM   #14
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Might get some differing opinions from other pros. That stuff is not rated for bath room fans if it is regular HVAC flex duct.What did the package say it was rated to be used as?
I meant similar in appearance, anyway -- plastic / insulation / foil. Not sure of the rating, I was just going by looks for the hypothetical.

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Unless that stuff is pulled taut the ridges inside are going to create a lot of resistance to air flow.
Good point.. guess that's why I've mostly seen it used vertically for roof venting.
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Old 10-20-2008, 11:39 PM   #15
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Do your self a favor and hard pipe that with aluminium 4" pipe and a few elbows.

You can keep the out side termination cause its made for that application.

Buy a length of insulation to keep condensation off. The aluminium will resist rust if not eliminate it completely.
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