Duct Leakage Testing in Frederick County: What to Expect

How a duct leakage test works

Setup and register masking. The technician masks all supply and return registers in the home — covering them with tape or foam to prevent airflow through the register openings during the test. This isolates the duct system so that all airflow measured during pressurization comes from actual duct leakage, not intentional openings at registers.

Pressurization to 25 Pascals. A calibrated fan (duct blaster) is connected at the air handler location, typically at a return grille or the air handler cabinet access. The fan pressurizes the duct system to 25 Pascals above surrounding air pressure. CFM25 — the airflow required by the fan to maintain 25 Pascals — is the standardized duct leakage measurement used in energy codes, building programs, and contractor specifications.

Total leakage vs. leakage to outside. Two different measurements are used depending on the purpose of the test. Total leakage measures all air escaping the duct system, including leakage into unconditioned spaces (attics, crawl spaces) and leakage back into the conditioned space through imperfect connections at registers. Leakage to outside (sometimes written as Qn,out) isolates only the leakage from ducts in unconditioned spaces — the leakage that has the highest energy cost, because that air is lost from the conditioned space entirely. For energy compliance purposes, leakage to outside is the more meaningful number.

Test duration. A duct leakage test typically takes 30–60 minutes including setup, masking, measurement, and result interpretation. If the test is part of a pre-Aeroseal evaluation or an energy audit, the technician will also typically inspect accessible duct sections during the visit to identify visible leakage points that can be manually addressed before or instead of Aeroseal treatment.

• Registers are masked before the test to isolate the duct system.
• Calibrated fan pressurizes the duct system to 25 Pascals.
• CFM25 is the standardized leakage measurement used in energy codes.
• Total leakage and leakage to outside are two different measurements — leakage to outside is the more energy-relevant number.
• Test takes 30–60 minutes including setup and interpretation.

What results tell you — and what to do with them

Interpreting CFM25 per 100 sq ft. The standard reporting unit is CFM25 (total or leakage to outside) divided by the conditioned floor area in 100 sq ft increments. IECC 2021, adopted in Maryland, requires total duct leakage of 4 CFM25 per 100 sq ft or less for new construction. A well-sealed existing system might measure 2–6 CFM25 per 100 sq ft; a typical older Frederick County home measures 15–30 CFM25 per 100 sq ft or more.

What typical existing Frederick homes show. Homes built before roughly 2000 with original ductwork and no previous sealing commonly show total leakage of 100–300 CFM25 for a 2,000 sq ft home — equivalent to roughly 5–15 CFM25 per 100 sq ft, or 15–30% of the system's design airflow. Homes with more recent construction or previous sealing work may show lower numbers; homes with heavily degraded or pest-damaged ductwork may show higher numbers.

What to do with a high result. A high duct leakage test result warrants one of three responses depending on the system condition: (1) Manual sealing of accessible collar connections and register boots if most of the system is accessible. (2) Aeroseal treatment if leakage is widespread but the ductwork is physically intact and connections are mostly inaccessible. (3) Partial or full duct replacement if ductwork is physically damaged, undersized, or degraded beyond what sealing can address. The test result tells you the magnitude of the problem; a follow-up visual inspection identifies which solution is appropriate.

• IECC 2021 new construction requirement: 4 CFM25 per 100 sq ft total leakage or less.
• Typical older Frederick home: 15–30 CFM25 per 100 sq ft — significantly above code threshold.
• High result options: manual sealing (accessible system), Aeroseal (inaccessible but intact), or repair/replacement (damaged ductwork).

When duct leakage testing is required in Frederick County

New HVAC installation under permit. Maryland adopted IECC 2021, which requires duct leakage testing for new construction and, in some interpretations, for significant HVAC system replacements under permit. Specifically, IECC 2021 Section R403.3.3 requires duct systems to be tested for leakage and meet the 4 CFM25 per 100 sq ft threshold. Whether this applies to equipment replacements in existing homes is determined by the jurisdiction's specific code adoption and interpretation — ask your contractor to confirm requirements for your specific project.

Home energy audit programs. Maryland's EmPOWER program and some utility rebate programs require duct leakage testing as part of a whole-home energy audit before certain rebates are issued. If you are pursuing utility rebates for duct sealing or HVAC work, a certified duct leakage test is typically a required documentation step.

Before Aeroseal treatment. Any reputable Aeroseal contractor will perform a pre-treatment duct leakage test before recommending or beginning treatment. The pre-test establishes the baseline leakage that the treatment is designed to improve, and the post-test verifies the actual improvement achieved. A contractor who offers Aeroseal without a pre-test cannot document what improvement, if any, the treatment provided.

Pre-purchase evaluation. If you are buying a home in Frederick County and want to understand the duct system condition before closing, a duct leakage test during the inspection period is a legitimate request. Many home inspectors are not equipped to perform a duct leakage test — it requires specialized equipment — but HVAC contractors and certified energy auditors can provide one. The result gives you a quantified baseline for the duct system condition, not just a subjective assessment.

• New construction: IECC 2021 requires duct leakage testing — 4 CFM25 per 100 sq ft threshold.
• HVAC replacement under permit: required in some jurisdictions under Maryland's IECC adoption.
• EmPOWER and utility rebate programs: testing typically required for documentation.
• Pre-Aeroseal: pre- and post-treatment tests required for verified results.
• Pre-purchase: testing during inspection period provides quantified duct system baseline.