Had the pleasure of being in my attic, in July in TX. It was suspiciously cooler than I anticipated.
Got near the a/c unit and I could feel a lot of cool air leaking out between the tan sealer. Is duct tape the preferred solution, or is there something better?
Use HVAC foil tape instead.
You may also consider some insulator wrap to cover the entire tube, once the leak is fixed. The unit won't have to work as hard to keep temp.
I'm no expert but I did HVAC work part time here in Texas for a few years.
First, that plenum is lined internally with insulation. An outer blanket of added insulation would be redundant. Be redundant..
Your mastic has simply cracked or failed. Run down to Lowes or similar and get a bucket of mastic and a disposable paint brush and go around and paint another liberal coat of mastic on the seams and you'll be good to go until the the unit needs to be changed out down the road sometime in the future.
I think we used to pay about 12 bucks for a bucket of mastic at the AC supply house but that's contractor pricing through the commercial account. I'd imagine the home improvement store will be about double that since you're not getting it wholesale.
I would pull off whatever loose tape is there around the area that leaks. If your attic space gets half as hot as mine and you say it's cooler than expected, then you've got a significant amount of cooling going to waste. Make sure your duct system is intact before you cover it back up. I've seen systems that use ducts made from ductboard and tape. After 10 years or so, the adhesive fails, and the duct falls apart. At least that section of yours looks like sheet metal. That's good.
There's a brand of sealing products called Hardcast that includes a foil-backed mastic tape that's good for sealing leaking joints. Will definitely be easier to use than most other options and should hold up very well.
https://www.carlislehvac.com/product.aspx?id=9
NMNA. I used to sell the stuff in a previous lifetime.
Seconding Cousin Eddie on this. Get some water based duct sealer either from a local supply house or an online vendor. We put it on with chip brushes and once dry it will seal air leaks very effectively. You'll need to wipe off any dust / dirt from the surface and remove any obviously loose tape first. Don't neglect any joints on duct pipe and fittings as those little leaks add up.
On the internally lined thing, if the duct "thumps" when you tap it more than likely has internal insulation. If it makes a more metallic "pong" sound there's no insulation and as you can imagine in an attic, not ideal. (It would also be sweating a lot if un-insulated.) More insulation is always better, we wrap any unlined duct with 1 1/2" foil faced fiberglass to meet code up here. In an attic I would line and wrap it if possible.
Also and semi-ironically, the one thing duct tape doesn't do well is seal joints on ductwork. Foil tape works OK but in my experience the water based mastic is the best.
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