(And Why Repeatability Matters)

It’s not uncommon for duct leakage tests to show slightly different numbers when a system is tested under pressurization versus depressurization. Many technicians performing duct blaster testing or duct leakage testing during blower door diagnostics notice that pressurized and depressurized results can differ.

That doesn’t necessarily mean the data is wrong.

It means the system is behaving like a real duct system.

When air moves through ductwork, leakage pathways don’t behave perfectly symmetrically. Under positive pressure, air is pushed outward through openings. Under negative pressure, outside air can be drawn into the system.

Materials inside the duct system can respond differently depending on the direction of airflow:

• Flex duct jackets can shift slightly
• Tape seams can lift or compress
• Insulation can move
• Mastic (“duct butter”) edges can flex
• Loose connections may open or tighten depending on pressure direction

In some systems, pressurization will show higher leakage.
In others, depressurization may show more — for example, if loose tape, liner, or insulation gets pulled inward and opens the leak path further.

On new systems, properly assembled and sealed ductwork should produce nearly identical numbers regardless of direction.

On older systems, some variation is expected simply because materials age, move, and react to pressure differently.

As long as test pressure is stable and there are no obvious mechanical defects during visual inspection in the attic or ceiling space, modest differences between the two directions generally do not indicate bad data.

Large discrepancies usually correspond with something visible: a loose connection, disconnected run, large gap, or poorly sealed joint.

The Real Issue in Duct Testing: Repeatability

The bigger challenge in duct leakage testing is not pressure direction.

It’s repeatability.

Across the country, technicians are sealing registers and returns with plastic and tape before running tests. But every one of those setups can be slightly different:

• Was the tape sealed tightly?
• Was the plastic stretched the same way?
• Were the perforations spaced the same?
• Did the tape adhere the same to the wall or ceiling?
• Did the technician cover the entire grille consistently?

Those small differences change the test setup — and that can affect the result.

Why Repeatable Sealing Matters

Reliable testing requires consistent conditions.

That’s exactly why systems like Vent Caps™ and the Duct Ninja™ are so valuable.

Instead of relying on:

• Different tape
• Different plastic
• Different sealing methods
• Different technicians

…you get a consistent, repeatable seal every time.

The result:

✔ Accurate testing conditions
✔ Replicable results from house to house
✔ Faster setup and teardown
✔ Safer work in finished homes
✔ Less waste

And unlike tape and plastic:

They’re fully reusable.

No trash bags.
No tape residue.
No piles of plastic in the landfill.

Just consistent testing — anywhere in the country.

Accurate.
Safe.
Environmentally responsible.
And repeatable.