The main trunk line is the main air conditioning supply system and will require adhesive tape for the smaller duct lines that branch out from it. Sto manufactures two products, a mesh tape and a sticky material, designed to seal air leaks at the joints of the coatings. Closure systems used with flexible air ducts and flexible air connectors must comply with UL 181B and bear the 181B-FX mark for pressure-sensitive tape or 181B-M for putty. You can use sealing putty or aluminum foil tape.
Below you will find a list of some of the benefits of both. Response to Sean: Although you are posting your question on a blog about adhesive putty and adhesive tape, it seems that your question refers to another blog. Hardcast recommends Foil-Grip butyl tape for use with galvanized steel ducts, duct plates or flexible ducts. If you are using putty to seal joints in the ducts of a fiberglass plate, use fiberglass mesh tape for all joints.
However, it was so broken that the grill was closed with putty, and now that I've sealed it with putty (not very well as you can see in the image, another layer is needed on the fiberglass tape), I have no idea how to put the grill back together without caulking it again. Aluminum foil tape (also known as a “roll camera”) is easier and less dirty, but it tends to fail faster due to delamination and dirty, oily ducts. However, I am skeptical about the durability of the seal when putty is used in the joints between the sections of the duct. Duct tape tends to work better on round ducts than on sharp, uneven corners of rectangular ducts. I intend to seal all the ducts in my house, since, according to the HVAC guys, they are made of 100% fiberglass panels joined together and connected with adhesive tape on the aluminum back.
The UL-181 used in the seams of fiberglass panels has to use fiberglass mesh and putty to be a UL-181 fastener. If yours are different, you may need to replace the boots or seal the gaps with canned spray foam, putty, or adhesive tape. With silicone putty to seal duct seams, others have used that technique, especially for longitudinal locking joints in round galvanized ducts. At that point, you can attach the duct to the cover and properly seal the seam before repairing the roof.
