Walter Cohen wrote:
Due to logistical reasons in my attic I need to route the new bathroom
fan exhaust duct up through the roof instead of out the side wall. It's
also a shorter run if I go vertically instead of horizontally.
I'll be buying 4" aluminum ducting so when I buy a hole saw to cut the
roof hole do I buy one that's 4" round, 4 1/4", or 4 1/2"?

Anything else I should do or watch out for?

Thanks.
Walter

Make sure you insulate the duct. If you don't, the moist air will condense
in the duct (because of the cooler air in the attic) and run down the sides
and drip in the fan.

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