How to Insulate Bath vent duct in attic
Hi all
I’m putting an exhaust fan in a bathroom with cathedral-type ceiling; outside wall is 8′ and inside wall is 11′ , the ceiling is basically the underside of the roof. The exhasut fan has to go up high so that means it wil be high on the 11′ wall. The other side of that wall is the attic. The exhaust ductwork will go thru the attic and out the gable wall. See picture attached.
The exhaust duct will be about 3 feet off of the attic “floor” with nothing to keep it warm. Usually a bath exhaust fan is in a horizontal ceiling, and the duct is very close to the attic floor. Thus it can be covered under/amongst the attic insulation, and be kinda warmed by being close to the living area.
I am concerned that even if I wrap this duct in fibreglass insulation it will still be so cold in winter that the warm moist bathroom air will immediately condense in the duct and possibly buildup ice which will of course melt at some point. I will pitch the duct downward towards the exit out the gable end wall of the attic.
Anything else I should do?
Walk Good,
Phil
Edited 4/2/2004 9:34 pm ET by Patchogue Phil
Replies
The set-up in my house is almost exactly like that. I ran 4" thin-wall PVC up and over to a roof vent on the back side of the house. In winter, water does condense and drip into the attic. (I didn't cement the PVC joints. If I had the water would probably run all the way back to the fan.) "Some day" I'll change to insulated flexible duct.
The good news is, I don't use the fan that much in winter. The house is dry and could use the moisture inside. Plus, I don't want to be pumping all that valuable heat outside.
Al Mollitor, Sharon MA
Al Mollitor, Sharon MA
Thanks for the reply.
Well, your situation does confirm my suspicions that the duct needs to be insulated and pitched downward towards the outside. And I'll make sure that the duct pieces are glued & water tight.
Walk Good
Phil
I had the same problem in the humid south, so I bought some insulted flex duct for HVAC at lowes and slip it over the PVC, no more problems.
BrownBag
That insulated flexduct sounds more like a plan than what I was thinking - wrapping miraflex around the tubes. The insulated flex duct would go on easier and stay on too.
Walk Good
Phil
Well, you could actually box in the duct -- effectively build another ceiling above it.
Also, it's not clear that there would be anything wrong with routing the duct down, after it leaves the bathroom, so that it could run along the ceiling of the other area. Of course this depends on where you can make it exit.
Getting the duct back down closer to the warm ceiling under the insulation would be a good idea. Except that when mounting a bath exhaust fan on the vertical wall, the instructions say that the exhaust port must face upwards. That means a big U-turn down to the attic floor and then a 90* turn towards the gable. A lot of restrictive turns.
I will go the pitched duct, sleeved with insulated flexduct.
Unless someone has some more ideas... :-)
Walk Good
Phil