Retrotec discusses RESNET duct testing code, as well as offering rules of thumb, and answering audience questions.
RESNET Duct Leakage Standards
Per ANSI/RESNET/ICC 380 and RESNET Chapter 8:
Total Duct Leakage: Allowable limit is ≤ 8 CFM at 25 Pa per 100 ft² of floor area (or ≤ 80 CFM total).
Duct Leakage to Outdoors: Must be ≤ 4 CFM at 25 Pa per 100 ft² (or ≤ 40 CFM total).
These tests should be conducted post-construction using a duct tester and a blower door (for outdoor leakage). The typical test pressure is 25 Pa, applicable in both pressurization and depressurization modes.
Field-Friendly Rules of Thumb
Retrotec offers several practical tips:
Seal before testing: Use mastic on all fittings before testing to save time and improve results.
- Test ducts in conditioned space: If 100% of ductwork is contained and visible pre-drywall, outdoor leakage testing may be waived or simplified per RESNET 380 Addendum L.
- Balance tester mode: Use depressurization for better register sealing, pressurization for flexibility—values should match either way.
- Check test setup: Use a pressure pan to identify major leaks quickly before full testing.
Audience Questions Answered
Q1: Can a single gauge be used?
Yes—but only if you manually alternate measurements between duct and house pressures. Two-gauge setups are easier and faster, avoiding hose swapping.
Q2: What if ducts and air handler are within conditioned space?
Testing duct leakage to outdoors may be waived if all ducts are verified within conditioned volume pre-drywall under RESNET Addendum L.
Q2: What if ducts and air handler are within conditioned space?
The code allows either, but results must align with the pressure used. Adjust code limits accordingly.
Bottom Line
Total leakage ≤ 8 CFM/100 ft²; outdoor leakage ≤ 4 CFM/100 ft²
Seal and visually verify ducts early—do registration and testing for smoother compliance
Use depressurization mode when possible; keep hoses properly connected when relying on a single gauge
