Blower Door Result Below the Program Threshold on a Complex Enclosure
A single-family retrofit is being verified under a program that requires a post-retrofit air leakage rate of 3.0 ACH50 or lower for the climate zone. The house is a 2,400 sq ft single story with 9 ft ceilings and a conditioned crawlspace connected by an open hatch. The technician sets up a single blower door in the front door with a three-point test and reports 1,850 CFM50. During the test the homeowner opens an interior door to the attached garage to retrieve a tool, and the technician notes the garage-to-house door has no weatherstripping and a 3/8 inch undercut. The crawlspace hatch was propped open with a paint can. The technician's report lists 2.9 ACH50 and recommends no further air sealing.
1. Compute the reported ACH50 from the field data and evaluate whether the pass result is defensible.
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Volume is 2,400 sq ft x 9 ft = 21,600 cubic feet. ACH50 = CFM50 x 60 / volume = 1,850 x 60 / 21,600 = 111,000 / 21,600 = about 5.14 ACH50. That is far above the 3.0 threshold, so the reported 2.9 cannot come from these numbers; either the CFM50, the volume, or both were altered or mis-transcribed. The correct conclusion from the raw data is a fail, not a pass.
Even before arithmetic, the test conditions are invalid. A door to an unconditioned or semi-conditioned attached garage was opened mid-test, and that door is itself a large uncontrolled leakage path (no weatherstripping, 3/8 inch undercut). A propped-open crawlspace hatch also changes the pressure boundary and the effective volume. A valid test requires a defined pressure boundary, all interior doors in their normal position, exterior openings closed, and no occupant traffic. The technician should re-establish the boundary, seal or account for the garage door, close the hatch, and rerun the test. Only then can a compliance determination be made.
2. The technician argues the crawlspace should be included in conditioned volume because it is 'connected.' How should the verifier treat the crawlspace volume and the hatch, and what documentation is needed?
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Volume used for ACH50 must reflect the space inside the intended pressure boundary and be consistent with how the program defines conditioned space. An open hatch does not by itself make a crawlspace conditioned; it makes the boundary ambiguous and lets air move between zones during the test. The verifier should first determine whether the crawlspace is inside or outside the thermal and pressure boundary per the program's definition and the design documents. If it is outside, the hatch must be closed and sealed for the test and its volume excluded. If it is inside, the hatch may remain in normal position but the volume must be included and documented.
The documentation should record the assumed volume, the basis for including or excluding the crawlspace, the condition of the hatch and any sealing applied, and photographs or notes of the boundary setup. The verifier should also confirm the manometer calibration and the test file (raw pressure and flow readings, not just the final ACH50). If the volume assumption changes the result across the threshold, that decision must be justified in writing, because it is a judgment that affects pass/fail. The verifier should not accept a volume figure that was chosen after seeing the result.
3. The report recommends no further air sealing. What should the verifier require before accepting or rejecting that recommendation?
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The recommendation is premature because the test was invalid and the arithmetic does not support the claimed result. The verifier should reject the report as submitted, require a valid retest under a controlled boundary, and require the corrected ACH50 calculation with the volume basis stated. Only after a valid test should the recommendation be evaluated.
If the valid retest still exceeds 3.0 ACH50, the report must identify the dominant leakage locations with evidence, such as a smoke pencil survey or infrared imaging, and propose targeted air sealing with expected savings. A blanket 'no further sealing' conclusion is not supportable when the measured leakage is above threshold. The verifier should also check that the recommendation is consistent with any combustion safety requirements, since tightening a house with atmospheric combustion appliances can create backdrafting risk. The corrected package should include the raw test data, the volume basis, the leakage location evidence, and a revised recommendation that follows from the data.