10.5 Duct Leakage, Blower Door Testing, and Building Envelope Performance
Key Takeaways
- A blower door depressurizes the house to 50 pascals and reports airflow as CFM50, which divided into the conditioned volume and multiplied by 60 gives ACH50.
- ACH50 equals CFM50 times 60 divided by conditioned volume in cubic feet; a house at 1,800 CFM50 with 16,000 cubic feet tests at 6.75 ACH50.
- Duct leakage is measured at 25 pascals with a duct blaster; total leakage includes all leaks, while leakage to outside is measured with the blower door running to neutralize house pressure.
- Duct leakage in unconditioned space is the costly kind: return leakage pulls attic or crawlspace air into the system and supply leakage pressurizes those spaces.
- Tightening a building envelope without verifying combustion appliance draft can create a depressurization hazard, which is why blower door work and CAZ testing are performed together.
10.5 Duct Leakage, Blower Door Testing, and Building Envelope Performance
The Duct & Envelope sheet of the HVAC Excellence Competency and Task List is short but dense. It requires the technician to understand the basics of building science, identify and set up the components of a blower door, perform blower door and duct leakage tests, calculate conditioned volume, ACH50, and duct leakage rates, determine maximum allowable testing static pressure, and carry out carbon monoxide testing and safe-entry protocol. It backs the Duct and Envelope Testing Specialty Certification and the Master Specialist Duct Leak Testing and Envelope Leak Testing hands-on exams.
1. The Building Science Behind the Test
A building is a pressure boundary, and air moves across it whenever a pressure difference exists. Three drivers:
- Stack effect — warm indoor air rises, exits high, and draws replacement air in low. The neutral pressure plane sits where indoor and outdoor pressure are equal; above it the house is positive to outdoors, below it negative. This is why attic leaks dominate winter heat loss and why a basement is where cold air enters.
- Wind — positive pressure on the windward face, negative on the leeward and roof.
- Mechanical — exhaust fans, clothes dryers, unbalanced duct leakage, and closed-door pressures created by the air handler.
Air leakage carries heat and moisture. Moisture is the destructive part: warm humid air moving into a cold cavity condenses on the first surface below dew point, producing rot and mold. That is why air sealing matters more than insulation R-value in many retrofits.
The house is a system. Tightening the envelope reduces infiltration, which reduces the makeup air available to natural-draft combustion appliances. Envelope work and combustion-safety testing (Section 8.4) are never done separately.
2. The Blower Door
Components: an adjustable aluminum frame with an airtight nylon panel that seals into an exterior door opening; a variable-speed calibrated fan with interchangeable flow rings; a two-channel digital manometer measuring house-to-outdoor pressure on one channel and fan pressure (converted to flow) on the other; and reference tubing run to the exterior.
Setup procedure
- Set the house to test conditions: exterior doors and windows closed and latched; interior doors open (so the whole conditioned volume is one zone); dampers, fireplace flues, and exhaust fans off; combustion appliances turned off or set so they cannot fire during the test; ashes cleaned or covered.
- Do not seal intentional openings for the standard single-point test — the goal is to measure the house as it operates.
- Install the frame in a door that is not near a large obstruction, mount the fan, and run the reference hose outdoors away from wind and away from the fan discharge.
- Take a baseline house pressure with the fan sealed, to correct for wind and stack effect.
- Depressurize the house to −50 pascals with respect to outdoors and record the fan flow.
The primary result is CFM50 — the airflow required to hold the house at 50 Pa, which equals the leakage area's airflow at that pressure.
The ACH50 calculation
Conditioned volume is the volume inside the pressure boundary — conditioned floor area times average ceiling height, including conditioned basements and, where the boundary is at the roofline, conditioned attics. It excludes vented crawlspaces, vented attics, and unconditioned garages.
Worked example. A 2,000 sq ft house with 8-foot ceilings plus a 900 sq ft conditioned basement with 7-foot ceilings: The blower door reads 2,100 CFM50:
Interpreting the number
| ACH50 | Character |
|---|---|
| Below 1.0 | Passive-house tight; mechanical ventilation mandatory |
| 1.0–3.0 | Very tight new construction; mechanical ventilation required |
| 3.0–5.0 | Current code-level new construction in most climate zones |
| 5.0–10.0 | Typical existing house |
| Above 10.0 | Leaky; large air-sealing opportunity |
The IECC requires 3 ACH50 or less in climate zones 3 through 8 and 5 ACH50 or less in zones 1 and 2, verified by test. Below roughly 3 ACH50 a house cannot be assumed to ventilate itself, and ASHRAE 62.2 mechanical whole-building ventilation becomes a requirement, not an option (Section 1.1).
A leaky structure versus a tight structure — a task-list comparison — comes down to control. A leaky house ventilates unpredictably: too much in January, almost none on a mild day, and always through the dirtiest path available. A tight house ventilates by design, through a filtered, measured, often heat-recovering path.
3. Duct Leakage Testing
Duct leakage is measured with a duct blaster: a small calibrated fan connected to the duct system (usually at a return or the air handler cabinet) with all supply and return registers temporarily sealed.
The reference pressure is 25 pascals, chosen to approximate operating duct pressure. The result is CFM25.
Two distinct tests
| Test | Setup | What it measures |
|---|---|---|
| Total duct leakage | Duct blaster only; registers sealed; house pressure irrelevant | All leakage, including leaks into conditioned space |
| Leakage to outside | Duct blaster and blower door running simultaneously, both holding the house and the ducts at the same pressure | Only leakage that crosses the pressure boundary — the leakage that costs money |
Running both fans neutralizes the pressure difference between the ducts and the house, so leaks into conditioned space move no air and only leaks to unconditioned space register. Leakage to outside is the number that matters for energy and for safety.
Expressing the result
Duct leakage is normalized to conditioned floor area: The IECC post-construction limit is commonly 4 CFM25 per 100 sq ft of conditioned floor area for total leakage, and rough-in testing allows a tighter number because the air handler may not be installed. A 2,000 sq ft house at 4% would be allowed 80 CFM25.
Why duct leakage is worse than envelope leakage
- Return leakage in an attic or crawlspace pulls unconditioned, often contaminated air directly into the supply airstream — attic air at 130°F in summer, crawlspace air carrying moisture, radon, or pesticide.
- Supply leakage in an attic dumps conditioned air outside the boundary and depressurizes the house, drawing replacement air through every other leak — including, potentially, through a natural-draft appliance's flue.
- Leakage occurs at connections, boots, plenum seams, and the air handler cabinet. Seal with mastic or mastic-backed tape, not cloth duct tape, which fails within a few years.
Related airside measurements the sheet requires
- Determine maximum allowable testing static pressure — do not exceed the duct system's construction class or the manufacturer's limit, and never pressurize flex duct beyond its rating.
- Measuring static pressure and finding CFM with an anemometer are covered in Section 10.1; the same instruments serve both competencies.
- Duct R-value: insulation requirements depend on location — commonly R-8 for ducts in unconditioned attics and R-6 elsewhere outside the conditioned space in most climate zones. Sizing insulation also prevents surface condensation on cold supply ducts in humid spaces.
4. Carbon Monoxide Testing and Safe Entry
The Duct & Envelope sheet devotes seven separate lines to carbon monoxide, opening with the blunt statement that carbon monoxide is deadly.
- Sources: any incomplete combustion — furnaces, water heaters, boilers, ranges, fireplaces, generators, vehicles in attached garages, and any appliance that is backdrafting.
- Health effects: CO binds to hemoglobin roughly 200 times more readily than oxygen, forming carboxyhemoglobin and starving tissue of oxygen. Symptoms progress from headache and nausea to confusion, unconsciousness, and death, and they mimic flu — which is why CO poisoning is routinely misdiagnosed.
- Safe entry protocol: test the air before entering. Wear a personal CO monitor. At 35 ppm ambient, ventilate and investigate; at 70 ppm, evacuate occupants; at 100 ppm and above, evacuate immediately and do not re-enter without ventilation and monitoring. Never rely on smell — CO is odorless, and the odors sometimes present come from other combustion byproducts.
- Instruments: a low-level personal CO monitor for occupied space, and a combustion analyzer measuring undiluted flue CO (Section 7.3 and 8.4). Consumer UL 2034 alarms do not annunciate until roughly 70 ppm sustained for over an hour, so they are a life-safety backstop, not a diagnostic tool.
- Do the CAZ test whenever you tighten a house. Air sealing that reduces ACH50 from 9 to 4 has removed the makeup air a natural-draft water heater was relying on. Verify draft and spillage before you leave.
A blower door test on a house with 2,400 square feet of conditioned floor area at 9-foot ceilings reads 1,620 CFM50. What is the ACH50, and how should it be characterized?
What is the difference between a total duct leakage test and a leakage-to-outside test, and why does the distinction matter?
After air sealing reduces a house from 9 ACH50 to 3.5 ACH50, what additional testing is mandatory before the job is complete?