Duct Leakage and Building Envelope
What leaky ducts and a leaky envelope cost, and how each is measured.
Safety
What it is
Duct leakage is conditioned air escaping the duct system, and unconditioned air being drawn into it, before it reaches the rooms. Envelope leakage is uncontrolled air movement through the building shell. Both waste energy and both change the pressures the mechanical system works against.
Why it matters
- Supply leakage into an attic or crawl space is paid for and never delivered.
- Return leakage pulls in dusty, humid or contaminated air from those same spaces.
- Unbalanced leakage pressurizes or depressurizes the house, which can affect combustion appliance draft.
- Sealing the envelope without addressing ventilation can create moisture and air quality problems.
How each is measured
| Test | What it does | Result |
|---|---|---|
| Duct leakage test | Pressurizes the duct system with a calibrated fan while registers are sealed | Leakage at a reference pressure, often expressed relative to floor area |
| Blower door test | Pressurizes or depressurizes the whole building with a calibrated fan | Air changes per hour at a reference pressure |
| Pressure pan test | Measures pressure at each register with the building depressurized | Identifies which runs leak most |
| Room pressure test | Measures pressure in a room with doors closed and the fan running | Reveals missing return paths |
Check combustion appliances after air sealing
Local requirements
Duct and envelope leakage limits, required testing, verification and documentation vary by jurisdiction, adopted code edition and the program under which the work is performed.
Local Code Notice
Practice & Mastery
- Duct Leakage and Building Envelope Troubleshooting Scenarios2
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