The long version, for duct leakage testing in Florida. Every question we get asked on site, answered once, properly. For the short version with your own building department named on it, the duct leakage testing pages carry a condensed FAQ per city.
Code note. This FAQ is written around the 2023 Florida Building Code, Energy Conservation, Eighth Edition. The Ninth Edition takes effect December 31, 2026, and when it does we will publish a separate version for that cycle and link the two together rather than overwriting this page. That is deliberate. A home is tested against the code its permit was issued under, and permits pulled under the 2023 code will still be reaching final inspection for years after the new cycle begins. If your permit predates the changeover, this is still the page you want.
Table of Contents
The requirement
Is duct leakage testing required in Florida?
Usually, yes. Under the Florida Building Code, Energy Conservation, duct systems have to be tested and shown to be within a total leakage limit, with the test report going to the code official. There are exceptions, the main one being a duct system that never leaves conditioned space, but on a typical Florida house with an air handler in the garage or attic, the test is required.
What are the passing numbers?
Total leakage, measured at 25 pascals:
- 4 cfm25 per 100 square feet of conditioned floor area when the air handler is installed. This is the post-construction, or final, test.
- 3 cfm25 per 100 square feet when the air handler is not yet installed. This is the rough-in test.
Florida’s forms express these as Qn (leakage divided by conditioned floor area), so you will see them written as 0.04 and 0.03. Same numbers, different notation.
Those are the R402 prescriptive numbers, and on their own they are misleading, because most Florida homes are not permitted prescriptively. Most are permitted on the R405 performance path, and R405 does not require a duct leakage test at all unless the energy calculation claims credit for something better than the default 0.08 Qn. Where credit is claimed, the target is whatever the calculation claims. It is frequently tighter than 0.04, and occasionally looser.
Your energy calculations are the source of truth for your passing number. Not this page, not the prescriptive table, and not whatever the last house you built was held to. If the number on your energy form and the number you were expecting disagree, the form wins.
Why is the rough-in threshold tighter than the final?
Because the air handler is not in the system yet. The cabinet itself leaks, and the allowance goes up by 1 cfm25 per 100 square feet to account for it. Test without the air handler and you are measuring only the duct work, so the standard expects a better number.
When is duct testing not required?
Two exceptions carry most of the weight. The test is not required if all ducts and the air handler are inside the building thermal envelope. And for additions or alterations, it is not required where the ducts extended from an existing heating and cooling system through unconditioned space total fewer than 40 linear feet.
There is a third, and it is the one that catches people out. On the R405 performance path under the 2023 code, a duct leakage test is not required if the energy calculation claims the default duct leakage value of 0.08 Qn. Claiming the default means the calculation has assumed a leaky system and taken no credit for a tight one, so there is nothing to verify. The moment that calculation claims anything better than default, the test is back on.
Does “inside the envelope” mean inside the attic?
Only if the attic itself is inside the envelope, which means a sealed, unvented attic with the insulation at the roof deck rather than the ceiling. Ducts in a conventional vented attic are outside the thermal envelope, even though they are under the roof, and that system gets tested. This is the single most common misunderstanding we run into on this topic.
What standard is the test run to?
ANSI/RESNET/ICC 380, the same standard named for envelope testing. It sets how the system is sealed off, what pressure it is taken to, and how the result is calculated.
Who is allowed to perform the test?
The same qualification list as envelope testing: an individual as defined in Section 553.993(5) or (7), Florida Statutes, an individual licensed under Section 489.105(3)(f), (g) or (i), or an approved third party. The report carries the tester’s license or certification number and issuing authority.
Total leakage and leakage to outside
What is the difference between total leakage and leakage to outside?
Total leakage measures everything escaping the duct system, wherever it goes, including into conditioned space, where it is arguably still doing its job. Leakage to outside measures only what escapes to unconditioned space: the attic, the garage, outdoors. Which one your permit is actually graded on depends on the compliance path. Under R402, the prescriptive path, the compliance test is total leakage. Under R405, the performance path, it is generally leakage to outside. Because most Florida homes are permitted under R405, most homes require a leakage to outside test rather than the total leakage test people expect.
Which one does my permit need?
Total leakage, for the prescriptive thresholds above. If the house is going through the R405 performance path, the energy calculation may specify a different duct leakage value, and that is what has to be met, so check the Form R405 before testing rather than after.
Why does total leakage still get measured?
The question usually gets asked the other way round, so it is worth correcting. Leakage to outside is not an occasional extra. On most Florida homes it is the measurement that decides compliance, because most homes are permitted under R405.
Total leakage still gets measured for good reasons. It is the compliance number on a prescriptive R402 permit. It is quicker to set up, because it does not require the house itself to be pressurised alongside the duct system. And as a first look it tells you how much the system is losing altogether, which is a sensible starting point before working out how much of that is going into the attic.
What it will not tell you is where the air is going. A system can pass total leakage while losing most of it into a 130 degree attic, and the total number on its own will never show you that. That is the case for measuring both.
Rough-in versus final
Should I do a rough-in test or just the final?
You only need one for compliance. But a rough-in test happens while the duct work is still accessible, which is the whole argument for it. Finding a disconnected boot before drywall costs twenty minutes. Finding it after the ceiling is closed, painted and trimmed costs considerably more than the test did.
It is also significantly easier to fix at rough-in. Everything is open and reachable, so a correction is a matter of getting to the connection rather than getting to it through a finished, painted and trimmed ceiling.
When is the rough-in test done?
After the duct work is installed and sealed but before it is covered, so before drywall, with the boots in place and the registers off. The air handler may or may not be set; the threshold you are tested against depends on which.
Is rough-in testing about to matter more?
Yes. Under the 2026 code, the R405 performance path loses the exception that previously let some projects skip duct testing, so systems that were never tested before will be. On those jobs, finding a problem at rough-in rather than at final becomes considerably more valuable. We have written that change up in 2026 Florida Building Code Energy Conservation: What Builders Need to Know.
The test itself
How long does a duct leakage test take?
Typically 30 to 60 minutes for a single system. Most of that is setup, sealing every register and return, rather than the measurement, which takes a few minutes. A house with two or three air handlers takes proportionally longer, because each system is tested separately.
What equipment is used?
A calibrated duct testing fan connected to the system, usually at the air handler or a return, and a digital manometer. Every supply register and return grille gets temporarily sealed so the only way air can leave the system is through a leak.
What needs to be ready before we test?
The duct system complete and sealed, all boots installed and connected, and access to the air handler. For a final test, the registers and grilles should be on. For a rough-in test, the system should be finished but still exposed. And someone should know where the second air handler is, if there is one. That discovery has ended more visits early than anything else.
Can the duct test and the blower door test be done in one visit?
Yes, and on a final that is the sensible way to do it: one trip, one report package, one line item on the schedule. They are separate tests with separate equipment and separate thresholds, but there is no reason they need separate visits.
Failing, and where the leaks are
What happens if the system fails?
It gets sealed and re-tested. As with the envelope test, a failing number is a measurement to improve rather than a violation. The question that decides how expensive it is: can you still reach the duct work?
Where does duct leakage usually come from?
The connections, nearly always. Boot-to-drywall joints at the ceiling, the plenum-to-air-handler connection, branch takeoffs at the trunk, the return platform, and any joint where mastic was skipped in favour of tape alone. Long straight runs of flex are rarely the problem. Everywhere two things meet usually is.
Is tape enough to seal a duct?
Foil tape rated for the purpose holds up; ordinary duct tape does not, and it is worth being blunt about that. Mastic on the connections is what tends to separate a system that passes comfortably from one that passes barely. The distinction shows up clearly in the numbers.
One more thing worth saying plainly, because it comes up on nearly every job. Suretape is sold and marketed as a duct sealing product, and sealing is exactly what it is intended to do. In our experience it rarely does it consistently. We see it applied correctly and still failing to hold a seal often enough that we do not treat its presence as evidence a connection is sealed. Mastic on the connections is what we want to see.
Can a failed duct test hold up the Certificate of Occupancy?
Where duct testing is required on the permit and the result is not documented, final approval can wait on it, the same mechanism as the envelope test. The practical risk is higher here, because fixing duct leakage after the ceiling is closed is genuinely disruptive.
Does duct leakage matter beyond compliance?
In Florida, more than most places. Supply leakage into a hot attic is conditioned air you paid for, dumped where nobody lives. Return leakage pulls hot, humid attic air into the system, which shows up as a house that cannot hold humidity and an air handler that runs longer than it should. Both are comfort complaints that arrive months after closing and get blamed on the equipment.
It is worth separating the two sides of the system, because they fail in opposite directions and they cause different problems.
Supply side leakage pushes air out of the house. Conditioned air escapes the supply ducts before it ever reaches the rooms, while the return keeps pulling the same volume out of the living space. The house ends up at negative pressure relative to outside, and it makes up the difference by drawing air in through every gap it has. In Florida that replacement air is hot and humid, which is why a supply leak so often turns into a humidity and moisture problem rather than a comfort complaint. A negatively pressured house is a serious problem, not a minor inefficiency.
Return side leakage does the reverse. A leaking return pulls attic air into the system, so more air is delivered into the house than is taken out of it and the house runs positive. It also feeds the equipment air off a 130 degree attic, which is nothing like the air the system was sized and selected to cool. The unit works harder, runs longer, and still delivers less than it should.
Neither of these is a code violation as long as the total number passes. Both of them show up on the power bill and in how the house actually feels.
Reports and scheduling
How fast is the report?
Same day, delivered digitally, like the envelope report. If both tests were done in one visit, both reports come together.
What is on the duct leakage report?
Address and permit information, conditioned floor area, which system was tested and whether the air handler was installed, the measured cfm25, the resulting Qn, the threshold it was measured against, the standard, the equipment and calibration, and the tester’s signature with license or certification number and issuing authority.
Do multiple systems need multiple reports?
Each system is tested and documented separately, because each has its own leakage relative to the floor area it serves. How they are packaged for submission depends on what your building department prefers.
Still have a question?
For jurisdiction-specific questions (who reviews it, how they want it submitted) the city and county pages name the authority having jurisdiction for each. For anything about your own project, send us the details.
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