Bath fan exhaust line

trekka | Posted in General Discussion on November 3, 2009 11:06am

In kitsap county (near Seattle), most of the builders are using 4″ flexible insulated duct to run from the cieling mounted bath fans to the exterior vent hood (sidewall mounted in my case). This duct is very similar to the flex duct used for forced air heating. I wondered about moisture pooling inside as the interior is not smooth and being flexible it can sag. It will run about 6 feet. Any one use rigid pipe? What about galvanized vs aluminum or PVC? Of course these would need to be insulated seperately, (a code requirement here). Also the exterior vent caps that mount through the wall siding/ sheathing, is there anything better than the square plastic three louvered things they sell at Home Depot? Thanks.

Replies

YesMaam27577 | Nov 03, 2009 04:54pm | #1
I was about to post the same questions, since I will soon be adding vents to two of my own bathrooms.

One of the thoughts that has occurred to me is that I might want to use the solid pipe to go up into the attic sppace about one foot. Then add an angle that creates a downward slope to the sidewall vent.

That, in combination with some wrapped insulation, will hopefully keep condensed moisture to a minimum, and send it outside. Of course, I'll need to be careful about caulking and such, to avoid problems with rotted sheathing and siding.

I won't be laughing at the lies when I'm gone,
And I can't question how or when or why when I'm gone;
I can't live proud enough to die when I'm gone,
So I guess I'll have to do it while I'm here. (Phil Ochs)
rdesigns | Nov 03, 2009 11:26pm | #2
Whatever code-approved duct you use (the IRC requires that flex duct meet UL 181, which the type used in your area probably does if it's used for duct systems), it is good to bury it in the attic insulation as an extra protection against condensation.

Smooth metal duct that you would insulate would have the advantage of far less friction that any flex duct--generally about 20% better airflow. In your particular case, the short, 6' run will make it entirely practical to run the flex, if you want, without losing much airflow to friction.

Whatever type of termination hood you use, it will still be a good idea to check it every year to ensure free operation of the louvers--mine has been crippled twice in the past by wasp nests.
barmil | Nov 04, 2009 01:48am | #3
I'm no expert. In one situation (a rental), there was an existing attic roof vent for the bathroom virtually above the fan. In that case, I used flexible aluminum ducting, as it was a short, nearly vertical run. In another (my house), the run is into the attic and then about four feet to a side vent, one el. For this I used PVC which I insulated with that foil stuff. No problems in my house in the last thirteen years. I would never consider that plastic wire coil stuff some use for dryers. My house is too valuable to me to skimp in this area. Also a very good fan in both cases -- Panasonic.

Check WW Grainger on line for quality wall vents -- where I got mine.

Edited 11/3/2009 5:50 pm ET by BARMIL
davidmeiland | Nov 04, 2009 04:26am | #4
I have used insulated flex a few times without problems, as well as rigid galv pipe with a separate insulation wrap. In all cases slope the pipe down towards the outlet if the run is thru unconditioned space. Sometimes this means going up first and then sweeping gently back down to run downhill to the outlet. Don't let it sag anywhere if using flex. Good wall caps are available from Broan http://www.broan.com/display/router.asp?ProductID=100265 and better ones from Seiho http://www.seiho.com/product/index6.html (call Air Commodities in Seattle for those).
RedfordHenry | Nov 04, 2009 05:56am | #5
Have used galvy, alum, and PVC on various projects over the years with similar success. Galvy is the most difficult to work with, mainly because it takes some effort with to cut and crimp. Also, with alum and galvy, all seams (including all joints on elbows) need to be sealed (I use aluminum tape for sealing). The longitudinal seam in each section should be placed on the top. PVC may be a little easier to install, though there may be some concern about the buildup of static electricity. I understand the theory behind static buildup, but have yet to see anything bad ever happen from static electricy building up in a PVC pipe. I suppose if you were using it to vent flammable vapors static elec. could be an issue, but not in a bathroom situation.

As you describe, pitch your run slightly so that any condensate can drain to the outside.

Yes, there are other options available than those POS louvered ones they sell at HD. I used aluminum vent hoods with a flapper door that has a light spring to keep it closed when not in use. I get them at a local appliance store. Have also seen ads for stainless steel versions though I haven't used them.

Edited 11/3/2009 9:57 pm ET by RedfordHenry