This is for a kitchen hood exhaust, rather than for an HVAC system, but I see your point. Mastic was not used, nor flex duct. A silver tape was used, but it didn't look like classic duct tape. It was shiny, like aluminum foil.

Bummer. We don't have a downward duct pitch. :(

It's not a downdraft exhaust, it's a range hood with 10" ductwork going up through the cabinetry and across the top of the cabinets (in a newly built soffit) and out through the newly built chase going across the garage ceiling.

If corrective action is impractical, then at a minimum the hood should be run for several minutes when cold to warm up before having it exhaust greasy cooking effluent. Otherwise, as noted, and depending on air velocity in the duct, grease can condense on the ducting. And failure to pitch it down to the outside is just asking for clean-out costs and possibly fire risk. I don't know the historical risk, but consideration of a halon injection point may not be totally beyond the pale.

So, the only hazard is rust? Drips?

Responding to Stax:

2017 NFPA 96 Standard for Ventilation Control and Fire Protection of Commercial Cooking Operations par. 7.1.4 calls for a 2% slope for horizontal ducts up to 75 ft. long. Given the National Fire Protection Association's scope of responsibility, I think it most probable that their requirement is based on a grease fire hazard perspective.

I am unable to find a slope requirement in the International Residential Code, so I am curious where your information is taken from.

Bathroom exhaust ducts that run through an unconditioned attic are insulated in order to avoid the creation of condensation. They are be run vertically or horizontally. If horizontal I would think you want it the an upward pitch to allow the warm air can rise and escape outside.

In this case if you are boiling four large pots of water, then a significant amount of warm air would be going through the duct. If its running through an unheated chase then there will be condensation. The vent should be insulated to prevent this. I also think the duct should have an upward pitch.

If you are worried about grease collecting in the duct then have a bigger worry of grease collecting in your range hood and exhaust fan. Commercial kitchens have to periodically degrease their range hoods. If they don't it is a potential fire hazard.

Wouldn't want that warm air to lose heat to the garage before it reaches the outside?

lol

My exhaust hood duct isn't in yet, but it's going about 3' horizontally inside the house and up the external wall and out of the first story roof (the exhaust exit on the roof is already built/installed, and is the same one we used before the reno. Can you tell me what insulation to use and where? Should the entire length be insulated?

The entire length should be insulated. Standard duct insulation, secured with metal tape.

https://m.grainger.com/mobile/product/6ZKK3?gclid

Thanks LWO