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AlleyG 10-05-2012 09:52 AM

Venting Bathroom Fans Thru Roof
I am installing bathroom fans in an older house that didnt require them by code back then. I already have two of the Broan roof vents with dampers installed in the roof, and I added a 5" long pipe stub to connect to. I also plan to remove the damper in the fan, and add a better sealing damper in the duct by each fan.

The question is about the duct itself. My first thought was to use the 4" insulated flex duct. But thinking back to dryer use, I remember it restricting airflow compared to the 4" alum flex duct (non-insulated) that I replaced it with. The clothes dryed faster, and you could just tell more air was going thru the alum flex duct, which is a true 4" diameter, not restrictive like the wavy flex duct.

The duct runs should not be longer than 6-7'. Should I not be concerned about the reduced airflow thru the insulated flex duct for such a relatively short run?

Or should I use the alum flex duct for better flow, and add insulation? I definately want flex duct VS piecing together elbows, etc.


Windows on Wash 10-05-2012 01:08 PM

The rigid lines are better because, like you said, there is less restriction and less places for water to catch an edge.

I prefer to use the rigid and over insulated them by hand.

Gary in WA 10-06-2012 09:47 PM

I agree with WoW, and I like to use the first two feet of the run sloped lower than the fan outlet which keeps any condensation from running back into the fan and dripping in the room (so it can dissipate later). Use 45's rather than 90's wherever possible, foil-tape all joints (even the individual elbow ones) to save reconnecting one later, insulate and v.b. (totally air-tight) the whole run. If you go flex, it has to be extended fully and supported well for no kinks later. For longer runs, flex needs to upped one size;


anjaleena 10-08-2012 02:52 AM

These are nice tips.

AlleyG 10-09-2012 10:30 AM


Originally Posted by GBR in WA (Post 1025519)
I agree with WoW, and I like to use the first two feet of the run sloped lower than the fan outlet which keeps any condensation from running back into the fan and dripping in the room (so it can dissipate later).

Won't the water that sits in the low spot of the pipe create a mold or mildew situation?

I have thoughts of buying an insulated flex duct, slipping off the insulated casing and trying to slip it over a semi-rigid duct. I like how you can form the semi-rigid with a large radius, plus it flows a whole lot better.

The duct length should not be over 8 feet.


drtbk4ever 10-09-2012 11:30 AM

You can buy insulation for the vent ducts that came with it's own vapor barrier. I think I got mine from home depot, but can't remember for sure.

Gary in WA 10-10-2012 08:39 PM

"Won't the water that sits in the low spot of the pipe create a mold or mildew situation?"--------

If the straight pipe is pitched down and sitting on the drywall, it will stay warmer than the rest of the pipe from the ceiling conduction. This warmer temperature will help evaporate the collected condensation, and be closer to the fan for stronger air flow helping dissipate it. Otherwise, the farther the moist air is from the fan, the less air pressure to keep it air-borne, and because the temperature in the pipe drops, cooler air is able to carry less moisture than warmer air so the pipe will get wetter. The more moisture deposited close to the fan, the less moisture in the pipe farther away. Better than having the fan drip water from the ceiling after it stops running. Remember to air-seal the fan box, insulate, and run the fan at least 20-30 minutes past the shower cycle to reduce condensation in the pipe. Just my two cents. Some good tips:


AlleyG 10-15-2012 09:37 AM

I got the Panasonic fans yesterday, they look nice. I had bought additional Aldes dampers because of the reviews stating the plastic fan dampers didnt seal tight. I already have a damper in the roof vent.

I got to thinking about what would happen if I put the Aldes damper in place of the one in the fan that doesnt seal tight. When the fan was off, the pipe would be 100% sealed off because of a tight damper at each end, and any moisture in the pipe would have no air exchange to dry it.

It made me think that the built in Panasonic damper was purposely made to not fit tight to allow a little room air help dry out the pipe. They do call for a damper at the roof, which I have. Should I just leave the fan damper in place instead of using the Aldes damper?

Also, the sealing lip on the Aldes damper is only 3" diameter, that is going to reduce the fan CFM a little. The Panasonic damper is a full 4". I will be using 4" flexible R6 insulated piping.


Gary in WA 10-15-2012 10:02 PM

I agree with WoW on the flex having twice as much surface area due to the folds not stretched out completely to stop moisture. I probably should have emphasized that before. Two dampers shouldn't be a problem as there is nothing to mold inside the pipe due to lack of organic material. With flex, the pipe will be colder with insulation between it an the ceiling. Either way should work, just giving my way of installing. I'd keep the piping the full 4" for maximum volume (think; less restriction/moisture).


allthumbsdiy 10-16-2012 08:48 PM


Do you have one or two exhaust caps for your bathroom fans?

If you have one, is that Panasonic fan supposed to be mounted just under the exhaust cap?

AlleyG 10-17-2012 07:26 AM

I have one roof exhaust cap with damper for each fan, so two total. The fan gets mounted in the ceiling, and you run hose from the fan to the roofcap.


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