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Old 12-30-2017, 09:38 AM   #1
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Insulating bathroom fan to cold exterior


I'm venting a bathroom fan to the exterior. Pictures show the outside hood I currently have and the duct layout. This metal hood conducts the cold so much that the next piece of 4" duct also gets really cold. I'm concerned about the most air condensing inside the pipe. It's difficult to get any slope on the pipe, although I could put in elbows at the fan to get the duct higher there. Insulating the pipe would just make the problem worse. Switching to a plastic hood might help with less cold transfer but wouldn't last as long.

The other problem is standard HVAC duct (crimped end) is too big to fit inside the entry of this hood. Maybe this hood is designed for flexible piping?

So, looking for ideas to get this warm moist air outside without condensing in duct.
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Old 12-30-2017, 10:02 AM   #2
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Re: Insulating bathroom fan to cold exterior


Does the hood have a damper, how about the fan? At least one should but both would be better. The dampers stop flow of warm humid inside air from flowing out due to stack effect or cold from pushing back in due to wind or being drawn in when things like the dryer or furnace runs. Without air flowing in or out, the air in the duct will be still. Your always going to get a little bit cold at the end, metal or plastic would both to conduct some cold.

I doubt insulating the duct will make it worse, should make it better. It would keep the cold inside the pipe and keep the outside of the pipe from sweating. May also allow the duct to warm up quicker once the fan is kicked on. When the pipe warms up then humidity will stay with the air and get out without condensing.

Vent fans almost always have a built in damper, you should make sure it's clean and closing fully. They get coated with moisture and then dust sticks, over time it can build up and prevent the damper from closing tightly.
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Old 12-30-2017, 11:06 AM   #3
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Re: Insulating bathroom fan to cold exterior


A pair of the cheap crimp pliers for the "not fitting so well duct/hood transition" and some foil back insulation from Lowes helped me, do my through the wall exhaust fan vent project for our bathroom.
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Old 12-30-2017, 11:32 AM   #4
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Re: Insulating bathroom fan to cold exterior


A plastic hood would definitely transmit less heat/cold, but I understand if you don't want to switch it out. If you want to cut the heat transfer (yeah, cold but technically only heat moves) from the vent hood, then you need some kind of thermal break that doesn't transmit heat. A foam gasket maybe? Insulating tape between the duct and the hood at the connection?
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Old 12-30-2017, 02:48 PM   #5
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Re: Insulating bathroom fan to cold exterior


Be sure to run the duct is such a way that it naturally drains any condensation to the vent and outside. Make the exhaust port of the fan higher at the fan end and let it run down to the exterior. That will also stop any tendency for cold air to run back to the inside.
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Old 12-30-2017, 03:01 PM   #6
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Re: Insulating bathroom fan to cold exterior


As a rule, first 5 feet of exhaust duct should be insulated.

And if applicable, the entire run of an intake (ERV / HRV) should be insulated.

Speaking as a sheet metal foreman working hi-rise and ICI.

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