Blower Door and Duct Leakage Testing, Diagnostic Tools, Energy Modeling, Utility Bill Analysis, and Prioritizing Improvements
Testing, Diagnostics, Modeling, and Prioritization
This domain tests whether you can measure building performance, interpret the numbers, and choose improvements in a defensible order. Expect blower door calculations, duct leakage, diagnostic tools, utility bill analysis, and cost-effectiveness screening.
Core rules
- A blower door measures CFM50 at 50 Pa. ACH50 = CFM50 × 60 ÷ conditioned volume. Natural infiltration is estimated as ACH50 divided by an n-factor, often around 15–20.
- Duct leakage is measured at 25 Pa with a Duct Blaster and reported as CFM25, total and to outside. Pressure pans help locate returns and leaks.
- Diagnostic tools include manometers, infrared cameras, moisture meters, combustion analyzers, and anemometers. Interpret IR with a temperature difference and account for emissivity and reflections.
- Utility bill analysis should normalize for weather using HDD/CDD and separate base load from seasonal load.
- Prioritize health and safety first, then air sealing, insulation, and equipment. Use savings-to-investment ratio (SIR), simple payback, and client goals, not just raw savings.
Worked example
A 2,000 ft² house with 8 ft ceilings has 16,000 ft³. CFM50 = 3,200. ACH50 = 3,200 × 60 ÷ 16,000 = 12.0. With an n-factor of 18, estimated natural infiltration is 0.67 ACH. Comparing CFM50 alone to another house is misleading; compare ACH50 or CFM50 per square foot.
Traps
- Treating CFM50 as a direct natural leakage rate.
- Measuring duct leakage at 50 Pa instead of 25 Pa.
- Ignoring duct leakage to outside, which is the most important portion.
- Using utility bills without weather normalization.
- Recommending measures without a health-and-safety screen.