Bathroom Fan Vent Insulation

[Brady]

Just purchased a new house and have 2 bathrooms without bathroom vents. Both are on the upper floor and I have access to the attic. I purchased a Panasonic 80 CFM fan for one bathroom (45 sq ft.). I plan on venting through the roof. The gable wall is over 20' away which is why I plan on going up. Panasonic recommends 2'-3' of straight run before going up. I plan on using 4" rigid metal duct. The straight run will be under insulation. I live in Colorado so the attic will get cold. Which brings me to my questions.
1. What type of insulation do I need to wrap the vertical section? R value?
2. The run will be less than 10' total. Is it worth using a 6" duct? Fan only has a 4" outlet but says it can do 4" or 6"
3. Is it worth spending the extra money on the vent cap to get one with a starting collar already attached? Probably not that important but figured I would ask anyway.

Thank you all for the help.

 

[Dana]

A 4" duct would deliver a higher velocity, has lower surface are, and would heat up the duct faster when operating, all of which adds up to less duct condensation/frost in winter.

R4 duct wrap is plenty.

If the fan doesn't come with one, be sure to install a butterfly type backflow preventer. That will limit the amount of parasitic stack effect air flows that pull air 24/7. But that stack effect draw will still be there, and it won't seal perfectly, so you could still have some condensation issues even when the fan isn't running.

You say that the total run is about 10', but it sounds like there will be at least one elbow, which for 4" duct adds about 5 "equivalent feet" if using the typical adjustable ell:

So from a friction/velocity point of view it's more like 15'. With a single tight ell and 10' of straight pipe you're looking at about 15 "equivalent feet", which at 80cfm results in 0.05 water inches of pressure drop. The rated cfm for bath fans is at 0.1", so even with 4" it will deliver at LEAST the rated cfm, thus no need for bumping up to 6" for lower static pressure.

Think about running it to the gable. That would allow you to build some slope into the long run through the attic so that any of the parasitic stack effect condensation would run out the side termination, which is better than letting it drip into the ell at the bottom of the vertical run and rust it all out (or leak, staining the ceiling.) Play around with this online static pressure calculator. Count each ell as 5', and add it to the straight lengths to come up with a realistic equivalent length number.

Using that tool, if you ran it say 25' to the gable with say two ells (a 10' total adder) for 35' equivalent length, at 80cfm and a 4" duct you'd see about 0.15" pressure drop. According to the spec for this this Panasonic 80cfm fan , at 0.25" static pressure it would cut the flow from 80cfm to 62cfm, so if it's only 0.15" at 80 cfm you would still be getting better than 70 cfm out of the fan, which is pretty good. (To get to the specified 0.25" @ 62cfm in the fan spec takes over 90 equivalent-feet.)

With a horizontal run you'd still want to insulate portion of duct that's above the attic insulation (or bury the duct in more attic insulation) so that even when it's 0F in the attic you won't get continuous & copious condensation when the fan is running.

 

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[Brady]

Dana Thank You Very Much. That was a lot of great info. I think I am still going to go through the roof. Mainly because the side of the house is over 30 feet high and the ground is sloped. But Ill look into it to make sure.