The joke writes itself: duct tape is the one thing you should not use on ducts. It is also true, and the reason is worth understanding, because it explains why a joint somebody sealed five years ago is open again.
What duct tape actually is
A cloth or scrim backing carrying a pressure-sensitive rubber adhesive. It is designed to grab instantly, hold at moderate temperature, and come off later. Every one of those properties is right for a temporary hold and wrong for a permanent joint inside a building.
What a New Jersey attic does to it
An unconditioned attic here goes below freezing in January and well past a hundred degrees in July, every year, for the life of the house. That cycling drives the plasticiser out of the adhesive.
What is left is a stiff, shrunken film. It curls at the edges first, then releases along the length, and the joint it was holding is open again. Two summers is a realistic lifespan; we have opened attics where the tape is still visibly present and holding nothing at all.
Crawlspaces do the same job by a different route. There the enemy is moisture rather than heat, and the adhesive lets go as the backing takes on water.
What mastic does differently
Duct mastic is a thick, fibre-reinforced paste, brushed or gloved onto the seam. It cures into a flexible solid that is bonded to the metal rather than stuck to it.
Three properties matter:
- It moves. Duct expands and contracts with temperature. A cured mastic joint flexes with it instead of cracking.
- It fills. A boot connection or a plenum seam is not a neat straight gap. Mastic fills an irregular joint that no tape can bridge, which is exactly where most leakage is.
- It stays. It does not dry brittle and it does not depend on adhesive tack.
Where a joint needs mechanical strength as well as sealing, sheet-metal screws or a draw band go in first, and the mastic goes over the top. That combination is what the seam should have had on the day the system was installed.
What about foil tape?
UL-listed foil tape is a real product and a legitimate one, unlike cloth duct tape. On a clean, dry, accessible, flat metal joint it performs well.
The practical problem is that duct joints are rarely clean, dry, accessible and flat. On a dusty crawlspace boot at an awkward angle, tape is a compromise, and the surface preparation it needs to work is exactly what is hardest to do in that position. Mastic tolerates the real conditions better, which is why it is what we default to.
How a joint is actually made
- Clean the surfaces so the sealant bonds to metal, not to dust.
- Mechanically fasten the connection where it needs strength: screws on rigid, a draw band on flex.
- Brush mastic over the joint generously enough to bridge the whole gap, out onto both surfaces.
- Support the run so the joint is not carrying load. A repaired duct that is resting on a joist edge will work itself apart again.
- Insulate afterwards where the run passes through unconditioned space.
- Photograph it, because it is about to be invisible for the next twenty years.
The one place tape genuinely belongs
Holding something temporarily while the mastic cures, or marking a run. That is the extent of it.
Why this matters more than it sounds
Duct sealing is the cheapest airflow work available. It does not touch the equipment, it does not require a permit in most cases, and it addresses loss that is happening every hour the system runs. Doing it with a material that has a two-summer lifespan converts a permanent fix into a recurring one.
If you have joints in your attic or crawlspace that were taped, they are worth looking at. What an inspection covers is on the repair service page, and the symptoms that suggest you have leakage are here.
