Is Duct Sealing Worth It?
For most homes with ducts outside the living space, yes. ENERGY STAR puts typical duct losses at 20 to 30 percent of the air the system moves, which shows up as higher bills and rooms that never feel right. Sealing goes straight at that loss.
Measure it. A duct leakage test before and after sealing shows exactly how much air you got back.
Mastic, Metal Tape or Duct Tape: What Holds Up
| Material | Where it works | Verdict |
|---|---|---|
| Mastic sealant | Seams, joints, boots and irregular gaps | Recommended by ENERGY STAR |
| Metal tape | Straight seams and flat joints | Recommended by ENERGY STAR |
| Cloth duct tape | Nowhere on ducts | Avoid — not long-lasting, per ENERGY STAR |
Joints Worth Sealing First
- Connections where registers meet floors, walls and ceilings, which ENERGY STAR names as common leak spots
- Joints on ducts in the attic, crawl space, unfinished basement or garage
- The connections at the air handler and filter box, which Virginia's code requires sealed
- Disconnected runs, which need ductwork repair before any sealant goes on
From Dusty Seam to Sealed Joint
Find the Leaks
An inspection or leakage test shows which joints matter.
Clean the Surface
Mastic and tape grip poorly on dust, so the seam gets wiped first.
Refasten Loose Sections
Screws or clamps hold the joint; sealant alone doesn't.
Apply Mastic or Metal Tape
Brush mastic over the seam, or press metal tape flat along it.
Insulate Exposed Ducts
ENERGY STAR advises insulating ducts you can reach in attics, crawl spaces, unfinished basements and garages. Attic runs get extra attention on the attic ductwork page.
Retest
A second leakage reading confirms the work.
Sealing Joints Isn't the Same as Coating Duct Interiors
Some cleaning companies offer to spray a sealant coating inside ducts after cleaning. That's a different product with a different purpose. The EPA reports that it, NADCA and other groups don't recommend the routine use of sealants to cover the inside of ducts. Leak sealing works on the joints and seams, where the air actually escapes.