Testing & Diagnosis
What Does a Duct Leakage Number Actually Mean?
A duct leakage number, reported as CFM25, is the airflow needed to hold the duct system at 25 Pascals of pressure — a raw figure that means little on its own. It becomes meaningful when normalised against conditioned floor area or the system's rated airflow, and interpreted against a code or program threshold, which varies by jurisdiction and is not a universal standard.
Key takeaways
- CFM25 alone is not interpretable — it must be compared to floor area or system airflow to mean anything.
- Common normalisations are CFM25 per 100 square feet of conditioned floor area, or as a percentage of system airflow.
- What counts as an acceptable result depends on the applicable building code, energy program or utility requirement, not a fixed universal number.
- The number tells you how much air is lost; it does not by itself tell you which room suffers or why.
- A before-and-after comparison of the same measurement is the clearest way to know whether sealing work changed anything.
Most likely causes
What you can safely check yourself
- Ask for the leakage result expressed both in raw CFM25 and normalised terms, so you can judge it in context.
- Ask whether the figure quoted is total leakage or leakage to outside, since they answer different questions.
- Check whether a specific code, program or standard was used as the comparison point for the result.
- If sealing work has been done previously, ask for the original test result to compare against.
Stick to what you can see, feel and hear. Leave electrical panels, refrigerant, combustion appliances and equipment internals to a licensed professional.
What a professional should measure
- Reporting CFM25 alongside conditioned floor area, so the result can be expressed per 100 square feet.
- Reporting CFM25 as a percentage of system fan airflow, another common normalisation.
- Comparing a leakage-to-outside figure against whatever threshold the relevant code or program specifies for the project.
- Retesting after sealing work to produce a like-for-like before-and-after comparison.
When duct leakage could be involved
A leakage number by itself, say 120 CFM25, tells you almost nothing — the same figure could be excellent on a large duct system serving a big house and poor on a small, compact system. Normalising it against floor area or system airflow puts it on a scale that can be judged and compared.
Even normalised, what counts as acceptable is not physics, it is policy: building codes, weatherisation programs and utility rebate criteria each set their own thresholds, and those thresholds differ by jurisdiction, construction type and program. A number that clears one program's requirement may not clear another's, which is why the honest answer to 'is this a good number' always starts with 'according to which standard.'
What the number cannot do is point to a specific leak location or explain a specific room's comfort complaint. It confirms whether leakage is present and roughly how much, but locating and fixing individual leaks — and confirming their effect on comfort — requires physical inspection and, ideally, a repeat test after sealing.
When this probably isn't a duct-sealing problem
- A comfortable leakage percentage does not rule out a comfort complaint driven by sizing, balancing or return-air problems.
- A leakage number cannot explain humidity complaints that stem from runtime or equipment sizing rather than air loss.
- The number does not diagnose noise, odour or equipment-cycling issues.
- A result that fails one program's threshold is not automatically a functional problem for the household — context matters.
When to call an HVAC contractor
Duct Daddy specialises in duct leakage testing and duct sealing. We do not perform mechanical HVAC repair, refrigerant work or equipment replacement. Contact a qualified HVAC contractor if any of the following apply:
- The leakage number is acceptable but comfort or efficiency complaints persist, pointing toward equipment or airflow issues.
- Static pressure or airflow readings suggest a restriction rather than a leakage problem.
- The system shows signs of mechanical wear, short cycling or unusual sounds unrelated to duct condition.
- A rebate or program requires a specific test result tied to equipment work that only a licensed contractor can perform.
Why this matters in Southeast Louisiana
Some Southeast Louisiana energy programs and code jurisdictions specify their own leakage-to-outside thresholds for new construction or renovation work — worth checking against the specific program in play rather than assuming a single number applies everywhere.
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Frequently asked questions
- Is there one duct leakage number that counts as 'passing' everywhere?
- No. Acceptable duct leakage varies by building code, energy program and jurisdiction. A number that satisfies one program's requirement may not satisfy another's, so the relevant threshold should always be confirmed for the specific project or code cycle in question.
- Why is duct leakage reported per square foot instead of as a raw number?
- Normalising against conditioned floor area allows fair comparison between homes of different sizes. A raw CFM25 figure says nothing about whether that loss is large or small relative to the system it is serving, which is why per-area or per-airflow figures are the standard way results are communicated.
- Does a low leakage number guarantee good comfort throughout the house?
- No. Low leakage confirms the duct system itself is not losing much air, but comfort also depends on duct sizing, balancing, return air and equipment performance. A tight duct system can still deliver air unevenly if those other factors are not addressed.
- How do I know if sealing work actually improved my leakage number?
- The only reliable way is a retest using the same method after the work is complete, compared directly against the original measurement. That before-and-after comparison confirms the actual change rather than relying on an estimate of what the sealing should have achieved.
Get a number, and a plan for what it means
A duct leakage test gives you a figure specific to your system, explained against the standard that actually applies to your home. Talk to Duct Daddy about what your result would mean.
Related problems
What Is a Duct Leakage Test?
What the test actually measures, how it differs from a general inspection, and what it does not tell you.
How Is Duct Leakage Tested?
The equipment and steps behind a duct pressurisation test, and what each reading measures.
Is Duct Sealing Worth It?
What sealing can and cannot fix, and how to know if your duct system is a good candidate.
Do leaky ducts raise bills?
Yes, leaking supply and return ducts can raise energy use — but the size of the effect depends on where the leaks are and how the ducts are routed.
Sealing vs. Replacement
How to tell whether your duct system needs sealing, replacement, or neither.
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