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In order to vent a kitchen cooktop exhaust hood I am running rigid 6" round metal duct from my ceiling to a roof vent. The cooktop is on the front wall of my house. For aesthetic reasons I wish to avoid venting on the front wall or front roof. Instead, the duct will run 2 ft vertical into the ceiling, through a 70 deg bend, then 16 ft straight on a diagonal to reach a vent that I put in near the ridge line on the rear roof.

Where the duct penetrates the kitchen ceiling is close to the eave and there is very little space in that part of the attic to work. I'm struggling with how best to secure the vertical, bend, and diagonal pieces near the eave. I would like to leave a stub coming down from the ceiling to which I can attach the vertical duct as I install the vent hood. The diagonal run needs to be insulated to avoid condensation I'm using 5' lengths of foil-faced fiberglass insulation packaged as a tube sized for 6' round duct. The adds some bulk and weight and makes it very difficult to see what I'm doing in the attic near the eave.

My plan is to use the bend piece as the stub that penetrates down from the ceiling. In the comfort of my living room I will attach the bend to a 5' length of duct (3 nails plus foil tape). Then secure one end of the insulation as far down the bend as practical (flex tie around the fiberglass, cover with foil and wrap with tape). I will take this piece up into the attic and position it so that the input end of the bend pokes out from the hole in the kitchen ceiling. I have a roll of metal hanger strap for holding the diagonal duct in place between rafters.

This is where I get unsure. I think it would be best to secure the bend to the ceiling and not just to the vertical duct that will fit into it. To do this I could cut 3 tabs into the bend with tin snips, bend these down against the drywall around the hole, and secure with screws. I would still secure the vertical duct to the bend with 3 screws and wrap with foil tape.

Is this a good idea?

The ceiling hole was cut with a 6" hole saw so it's quite tight. The duct probably can't rattle in the hole. Maybe I'm overthinking this and should simply attach the vertical duct to the bend and call it a day?

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I think you are being too concerned about support of your pipe down where the working space is limited. Attach the hood fan itself rigidly. Secure your round vent damper rigidly to the hood fan. Then secure your vent pipe rigidly to the vent damper. (Through away the 3-1/4" back draft damper.) This will easily support four or five feet of 6" 30-guage round pipe with an elbow and insulation. As soon as it is practical start using your plumbers tape at regular intervals. Insulate the pipe well. The entire length must be well insulated. Leave the insulation light and fluffy. Don't strap it down tight. This kills the insulation value. Use another elbow as you pass through the roof. Try to keep it tight to the roof line. Cut some dove tails in the upper elbow on the top side of the roof. screw them down to the top side of the roof. Install your roof vent cap over it. Now fry fish to your hearts content. P.

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