DUCT SEALING PROS · OUR STANDARDS
How Duct Leakage Is Tested: Our Standards on Every St. George Job
How duct leakage is tested decides whether a sealing job worked. We measure leakage before we seal, again when we finish, and give you both numbers, so the result is something you can check instead of something you have to trust.
How duct leakage is tested, step by step
A duct leakage test measures how much conditioned air escapes from your duct system before it reaches a room. The idea is simple: close the system off, pressurize it, and measure how much air a fan has to supply to hold that pressure. Every bit of air the fan supplies is air leaking out through gaps. That is how duct leakage is tested on every job we do.
- Seal off the registers. We cover every supply register and return grille so the only openings left are the leaks.
- Connect a calibrated fan. The fan attaches to the duct system, usually at the air handler or a return grille, and a gauge reads the pressure inside the ducts.
- Pressurize the ducts. We bring the system to a standard test pressure and record the airflow needed to hold it.
- Record the number. That reading is your baseline, the leakage you have before any work starts.
The same test, run again after sealing, gives the second number. Our page on duct leakage testing covers what to expect on the day.
What we measure and what it tells you
| Measurement | What it tells you |
|---|---|
| Total duct leakage | How much air escapes anywhere in the system, found with the pressure test. |
| Before-and-after change | Whether the sealing worked, shown as the drop between the two tests. |
| Leak locations | Joints, boots, takeoffs and the air handler connection, found by inspection in the attic or crawl space. |
| Airflow at registers | Whether rooms that ran hot now receive the air they should. |
The before-and-after change is the result you are paying for. A lump-sum quote with no test numbers gives you nothing to compare, which is why our guide to comparing duct sealing quotes tells you to ask for them.
The materials we use, and the ones we refuse to use
Ductwork in a St. George attic lives in air that can reach 120°F or more, so the sealant matters as much as the test.
- Mastic: a thick paste brushed over seams and joints, applied thick enough that it will not crack.
- UL 181B-FX foil tape: the listed tape for duct connections. We never use cloth duct tape, which heats up, cools down and lets go.
- Aerosol sealant: sprayed from inside pressurized ducts to reach leaks you cannot touch, where the system suits it. See aerosol duct sealing.
Our mastic and foil tape sealing page shows where each material goes.
Limits we tell you about up front
NREL found aerosol sealing combined with manual patching sealed 70 to 90% of leakage, compared with 40 to 50% for manual sealing alone. That is a strong result, but holes larger than about ½ inch need to be repaired by hand before aerosol is used, because the spray cannot bridge them.
Aerosol manufacturers also claim their seals last for decades. Those are company claims, not independent findings, so ask any contractor for the warranty in writing. We would rather you hold us to a written promise than to a sales line.
If your ducts test tight, we will say so and you keep your money. If they are crushed or disconnected, we will explain why sealing alone will not fix them.
What you receive when the job is finished
When the second test is done, we give you the before-and-after leakage numbers and explain them in plain language: what we found, what we sealed, and whether anything is left that sealing could not reach. Knowing how duct leakage is tested means you can read those numbers yourself and ask better questions of any contractor.
If you want to compare us with another company, put both quotes next to our duct sealing cost guide. The right quote lists testing, repairs, sealing and the retest as separate lines.
Frequently asked questions
What is a good duct leakage number?
It depends on the size of the home and the system. We explain your number in plain terms and show how much it dropped, because the change matters more than a single figure.
Do you test again after sealing?
Yes, every time. The second test is how you know the work did what it was supposed to do.
Can I watch the test?
Yes. Plan to be home, and we will show you the gauge and what the reading means.
Is how duct leakage is tested different for aerosol sealing?
The test itself is the same. Aerosol sealing is done from inside the pressurized ducts, so the pressure test is also what drives the sealing process.
Ready to find out where your cool air is going?
Call us and tell us which rooms are giving you trouble. We will explain what a duct leakage test involves and book your duct sealing services St George crews can finish before the next heat wave.
Duct Sealing Pros
3430 S River Rd, St. George, UT 84790
(435) 281-0395