A blower door test report fits on a single page: a flow reading, a normalized air change number, an effective leakage area, and a list of leaks. The page is only useful if someone reads it correctly, and the difference between a pass and a fixable fail is usually interpretation, not more testing. The discipline starts with knowing what each number means and where the result sits against benchmarks for the climate and the home type, and the same skill set powers interpreting blower door test results on every project.
The measurement itself is standardized. A fan panel seals into an exterior doorway, the house is depressurized to 50 pascals, and the gauge records the airflow the fan must move to hold that pressure. Raters working under RESNET and BPI protocols follow the same procedure on every house, so a result from one job can be compared with a result from another.
Three figures carry most of the information: CFM50, ACH50, and effective leakage area. Each answers a different question, and the useful one depends on whether the goal is comparing houses, sizing ventilation, or planning repairs.
The Numbers a Blower Door Test Produces
CFM50 is the raw number. It is the airflow, in cubic feet per minute, needed to hold the house at 50 pascals below outdoor pressure, and it depends on both the tightness of the shell and the size of the house. A big house with average construction can post a higher CFM50 than a small, leaky one, which makes the raw figure hard to judge on its own. The equipment and protocol that produce it are covered in full in guides on air leakage testing for residential construction.
CFM50 versus ACH50
ACH50 removes the size bias. Divide the measured flow by the conditioned volume, multiply by 60, and the result is air changes per hour at the test pressure. Two examples show the difference:
- A 1,200 CFM50 reading in a 16,000 cubic foot house: 1,200 x 60 / 16,000 = 4.5 ACH50.
- The same 1,200 CFM50 in a 30,000 cubic foot house: 2.4 ACH50, a passing result for many programs.
The same raw number can pass or fail depending entirely on volume, which is why programs quote ACH50 limits and why owners should compare normalized results rather than gauge readings.
Effective Leakage Area
Effective leakage area expresses the same leakage as a single hole. A house at 3.0 ACH50 typically carries an ELA equivalent to a sharp-edged opening about the size of a dinner plate. The figure makes the report intuitive: the goal of air sealing is to shrink that imaginary plate down to a saucer, then a coin.
Raters also report leakage at multiple pressures on some jobs, typically 50 and 25 pascals. The lower pressure reading approximates natural conditions more closely, and the ratio between the two readings hints at where the leaks sit, since large holes behave differently from many small cracks as the pressure changes.
Reading Results Against Benchmarks
A number without context says little. The same 3.0 ACH50 that fails a Passive House target comfortably passes ENERGY STAR in cold climates, and a 7.0 ACH50 that looks alarming on paper is common in existing homes. Trade publications return to this point constantly, and guides on getting the most out of blower door results walk homeowners through the same comparison against realistic targets.
Benchmarks at a Glance
| ACH50 range | House type | What the range means |
|---|---|---|
| Under 1.0 | Very tight, Passive House class | Needs balanced mechanical ventilation year-round |
| 1.0 to 3.0 | Tight new construction | Meets ENERGY STAR in climate zones 3 through 8 |
| 3.0 to 5.0 | Moderate new construction | Meets ENERGY STAR in zones 1 and 2 |
| 5.0 to 10 | Typical existing home | Sealing can cut heating and cooling load noticeably |
| 10 to 20 | Leaky older home | Drafts, high bills, and uneven room temperatures |
| Over 20 | Very leaky | Priority candidate for a full air sealing program |
Climate zone changes the target before the test even starts. A 4.0 ACH50 house that fails in Vermont passes in Florida, so the same report gets judged against different limits depending on the address.
A common rule of thumb estimates natural infiltration at roughly ACH50 divided by 20, so a 6.0 ACH50 house exchanges its air about 0.3 times per hour under ordinary conditions. That is a planning aid, not a measured value, but it explains why tight houses need mechanical ventilation while leaky ones get it for free.
Acceptance Criteria and Verification
Construction professionals face the same interpretation problem across every material they test. A single concrete core or rebound reading means nothing until it is weighed against acceptance criteria, and the same discipline applies to concrete in-situ test results before anyone signs off on a slab. Air leakage numbers deserve the same treatment: the blower door result only matters when it is compared with the limit that applies to the project.
Why Criteria Matter
Acceptance criteria turn a measurement into a decision. The program or code sets the limit, the test determines compliance, and the margin between the measured value and the limit tells the team how close the call was. A house at 2.9 ACH50 against a 3.0 limit passes, but it leaves no room for seasonal drift, so a careful rater flags it for a retest under different weather.
Programs also differ in how they count the result. Some accept a single blower door reading, while others average multiple runs or require the duct leakage test alongside it. Reading the fine print of the criteria prevents a false pass.
The report should carry the context needed to judge it: rater name and credentials, test date, outdoor temperature and wind, the equipment used, and the condition of the house during the test. Reports without that context are hard to defend when a buyer or an inspector asks questions.
Test Timing and Repeatability
Blower door results are snapshots, not fixed properties of the house. Concrete strength readings are read on a schedule for the same reason, and the 3, 7, and 28 day strength results that govern acceptance exist because the material changes over time. Air leakage changes across the construction schedule too: a rough-in test on open framing reads completely differently from the final test behind drywall.
Seasonal and Weather Effects
Wind pushes against the shell and shifts the pressure signal. Temperature differences stack on top of the fan’s 50 pascal reference, and humidity changes how air moves through materials. The same house can test at 3.5 ACH50 on a windy winter morning and 3.0 on a calm spring day, which is why protocols restrict when testing is valid and why retests should aim for similar conditions.
Repeatability matters most for verification. Plan the retest with the same setup as the first run:
- Record the original conditions: date, weather, house setup, and rater.
- Match those conditions for the retest as closely as possible.
- Run the same procedure with the same class of equipment.
- Compare the before and after numbers against the same benchmark.
If a repair is supposed to drop the house from 4.0 to 3.0 ACH50, the before and after tests need comparable weather, the same rater, and the same house setup, or the improvement gets lost in the noise.
Using Results to Drive Repairs and Retesting
The report earns its keep in the repair phase. Builders use the leak list to assign work, and experienced crews rely on field guides for interpreting the results when they walk the house with a smoke pencil. The combination of a measured number and a visible smoke trail turns abstract CFM50 values into concrete jobs: a gasket here, a can of foam there.
The leak list ranks work by impact. The biggest flows usually sit at the band joist and the top plates, and crews who start there close most of the gap before touching anything else.
- Band joists: spray foam or rigid foam with sealed edges.
- Top plate penetrations: caulk or foam around every wire and pipe.
- Recessed lights: airtight trims or foam gaskets.
- Window and door rough openings: backer rod and sealant.
- Attic hatches: weatherstripping and a rigid foam cover.
- Exterior outlets: foam gaskets behind the plates.
Standardized procedure is what makes any test result trustworthy. The cube test procedure that governs concrete sampling exists for the same reason the blower door protocol governs air leakage measurement: deviate from the method and the number stops meaning anything. Crews should know which procedures their results depend on.
From Test Report to Tighter Building
The report matters less than what the team does with it. The lesson that applies to cube and cylinder acceptance criteria applies to air leakage too: results only change the building when someone acts on them. File the report, share the leak list with the crews, and treat the retest as a scheduled step rather than an optional follow-up.
A tight house changes how the mechanical systems are designed. Once the envelope holds 3.0 ACH50, the ventilation system must bring in outdoor air deliberately, and the heating and cooling loads drop enough to shrink the equipment. Owners who read the report with those consequences in mind get far more from the test than a pass or fail stamp.
Track the results over time. A second test after a remodel, a new window installation, or an attic addition tells the owner whether the envelope held its performance, and a third test a few years later turns the report into a maintenance history for the house.