So, you have hot, humid outside air coming in contact with the cold surface of your A/C ducts. Having all ducts run inside the climate-controled envelope is the preferred solution. Having all of the ducts properly insulated (wrapped and sealed) to the appropriate R value for your area is, perhaps, the second best option.

This is a general answer as I'm not picturing the specifics of the structure.

JM$.02W

You have the stop the warm humid air from entering the space between the two floors. It is going to require insulation and sealing air infiltration.

I appreciate the response. You are correct, there is hot humid air making contact with the ducts. I could stop it one of two ways, I'm not sure which is best. Isolate the carport entirely from the envelope or blocking off the gable and ridge vents.

Condition the air in all the spaces the ducts are in.

Wouldn't the conditioned air just blow out the ridge vent and gable vent between my floors? Is venting the area between floors to the outside something that's ok?

I know I need to stop the air from coming into the envelope. The question is, should I do it by isolating the carport or just sealing off the gable and ridge vent in the carport.

Read Joseph Lstiburek at Building Science on the ways that conditioned space is different than unconditioned space. https://www.nrcs.usda.gov/wps/PA_NRCSConsumption/download/?cid=nrcseprd1559517&ext=pdf

Merely closing vents will not do what you want. The space you want to be conditioned must be enclosed in a vapor barrier and insulation. Unconditioned space must have proper ventilation in order to avoid problems such as ice dams in the winter and mold in the summer.

You need to call a builder who can inspect and remedy the deficient design/construction of your second floor addition.

I would think sealing off the house from carport would be the easiest solution. Sealing means insulating any openings and then use what is necessary to stop air infiltration. Insulating and sealing with a closed cell foam insulation may be a good choice.

You should seal off your vents separating your house envelope from your carport. Do you have insulation in the rim joist area between your first floor ceiling and second floor floor? If not you could spray foam the entire area including vents from the attic of your carport.

Oh, forgot to add if your FAU is in your carport attic you should treat your carport attic as part of your house envelope but hopefully it’s in your house attic otherwise it will be a lot more work.

Thank you everyone. I have decided to cut an access to the carport attic and get up in there and seal off the area between the house and carport.

I thought fire resistant drywall is 5/8 inch thick. If a 1/2 inch fire resistant drywall exists, it might be a special type and possibly more expensive than the 5/8 inch version.

Technically any thickness gypsum board has fire resistance. You're thinking of the 5/8" Type X for fire separation like between garage and rooms above. Fireblocking and draftstopping is a different situation and 1/2" standard gyp (not Type C) is all that is required to prevent free flow of air and spread, not stand up to a fire for a certain time period.

I was thinking of using super tuff-r, tape and foam. It's not an area that will ever be seen by anyone. I'm more worried about the air at this point and I'm concerned that drywall will be a much bigger project than I want to take on at this time. I remember that the contractor closed the floor prior to inspection and paid a fine for it. I think that's why this glaring issue was missed. The worst part is, I have had several contractors come out to try to figure out why the ducts are sweating and even spent a bunch of money on air sealing but nobody caught this because it's so antithetical to how houses are normally constructed.

When you added the second floor, why did you not remove the attic?

I didn't personally add the second floor. I paid a contractor to add the second floor. They cut the roof off all the way to the point where the house meets the carport and just left the old roof in place. They ran the new trusses(floor joists) right next to the bottom of the old roof trusses and then built the second floor on that. They should have sealed off that attic first or right after the trusses were installed and before the floor boards went down.

Also, through the whole renovation, the first floor ceiling was never disturbed and that's another reason I think it was missed.

That's why I chose the words "life safety." Tape and foam ain't gonna do it. Go through the carport ceiling (access panel or make one) and stuff insulation into the edge truss. Cut the 1/2" drywall into smaller sections, whatever size manuevers and piece together to cover the joist + any open lower 2nd floor wall, and screw them to it. Caulk/can spray foam any gaps/pipe penetrations. Done.

Technically to code you can just install mineral wool or fiberglass batts and be acceptable for fireblocking. But they have to be securely retained, so drywall without insulation is easier. The insulation in your case is to help the sweating, but if you can keep it from falling or blowing out then leave off the drywall.

Is the carport enclosed with a door? If it is then wouldn't that make it a garage and the code for fire blocking apply?

Thank you. I'm going to take your advice and use drywall. The carport is open on the front and side so it's definitely not considered a garage. I've contacted the original contractor and I'm hoping he'll help me out. I'm starting to think this is a big enough deal to raise a stink about if he doesn't. I know it's taken me 5 years to figure it out, but who would think any reputable contractor would do something so obviously wrong.?

I remember that the contractor closed the floor prior to inspection and paid a fine for it. I think that's why this glaring issue was missed.

I suggest your refresh the contractor's memory as to what happened and ask him to help you fix this properly. He is probably not going to do it for free, but I feel he should at least provide the labor and you pay for the materials.

5 years, good luck getting him back. 1/2" drywall is $8/sheet. 1 1/4" coarse screws $5/lb. Insulation $10-$20. Foam can $4, cheap caulk $3 as needed. Handyman for $25/hour?

As noted by 3onthetree the construction doesn't meet code requirements. Your contractor should fix it so it does. Period.