Blower-Door and Duct-Leakage Results on a 2,000 ft² House
A 2,000 ft² single-story house with 8 ft ceilings gets a blower-door test: 2,400 CFM50. For duct testing, the air handler is off and all registers are taped; a duct blaster reports 180 CFM25 total leakage. With the house held at 25 Pa and the registers still sealed, leakage to outside measures 130 CFM25. System airflow at the plenum is 1,200 CFM. The homeowner complains of dust, a back bedroom that is hard to cool, and high summer humidity. The return duct runs through a vented attic; the supply trunks are in the same attic but buried under blown insulation. There is one central return with a 20×25 grille and no door undercuts in the bedrooms.
1. Calculate ACH50 and the duct leakage percentages, and compare them with common program thresholds.
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Calculations:
- Volume = 2,000 ft² × 8 ft = 16,000 ft³.
- ACH50 = (2,400 CFM50 × 60) ÷ 16,000 = 9.0 ACH50.
- Total duct leakage = 180 ÷ 1,200 = 15%.
- Leakage to outside = 130 ÷ 1,200 ≈ 10.8%.
Against common program targets (roughly 3–5 ACH50 for new construction, total duct leakage ≤10%, leakage to outside ≤5%), this house fails on all three. Even against looser existing-home guidance (about 15% total and 10% outside), it sits at the limit and should be improved.
2. Explain which leakage matters most and connect it to the dust, humidity, and the back-bedroom complaint.
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Leakage to outside drives the comfort complaints.
- 130 of the 180 CFM25 (72%) is outside the envelope, so the system is drawing attic air into the return or pushing conditioned air out of the supply trunks.
- Return leakage in a vented attic pulls hot, humid, dusty air directly into the airstream - this explains the dust and high summer humidity and adds latent load.
- Supply leakage in the attic pressurizes the attic and depressurizes the house, pulling outdoor air through the envelope.
- The back bedroom with no return path and a closed door becomes positively pressurized; supply air cannot reach it, so it never satisfies the thermostat.
3. Set the remediation priority and the verification tests that prove the work.
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Seal outside-connected leaks first, then fix pressure balance.
- Seal return ducts and supply trunk joints in the attic with mastic and mesh; buried ducts still leak at joints and boots.
- Seal register boots to drywall and any chase penetrations.
- Provide a return path for each bedroom: transfer grilles, jump ducts, or door undercuts sized to the supply CFM; keep room-to-room pressure within about 3 Pa.
- Re-test: target leakage to outside ≤5% (about 60 CFM25), total ≤10%, and confirm an improved blower-door ACH50.
- Verify room-by-room airflow and that the thermostat satisfies; confirm humidity improves before recommending any equipment.
Document before-and-after numbers - it is the only way to prove the work.