Blower Door Reads High Baselines on a Windy Afternoon
You arrive at a 1,900 square foot single-family house at 2 p.m. on a day with sustained 18 mph winds and gusts to 30 mph. The house has a two-story open foyer and a wood-burning fireplace with a glass door insert. You set up a single blower door in the front entry and begin a multipoint test. The baseline (with-reference) reading is unstable: three consecutive measurements at the same fan setting vary by 40 percent, and the zero-flow baseline shows 9 Pa of apparent pressure with the fan off. The homeowner mentions the fireplace damper was left open after a fire last night. Your program requires a valid test with a stable baseline before you can report ACH50, and the report is due tomorrow morning.
1. Diagnose the unstable baseline and decide whether to proceed, adjust conditions, or reschedule.
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Diagnosis: Two things are happening. First, the 9 Pa baseline with the fan off is a strong signal of a real pressure differential across the envelope, not just gauge noise - an open fireplace damper on a windy day is a large, low-resistance path that couples the house to outdoor pressure and to wind-driven stack effects. Second, 18 mph sustained wind with 30 mph gusts produces fluctuating exterior pressure on the shell that a single-point blower door cannot average out, which explains the 40 percent variation between consecutive readings.
Order of corrections:
- Close the fireplace damper and any glass doors per the manufacturer's instructions. This alone may collapse the 9 Pa baseline.
- Re-check the zero-flow baseline. If it returns to near zero and holds, continue.
- If wind-driven fluctuation persists, take multiple readings and average them, or wait for a calmer period. Many programs set a wind limit (often around 15 mph) precisely because above it the uncertainty is too large.
Decision: Do not report a number from the unstable data. Close the damper, re-baseline, and if the wind still prevents a stable reading, reschedule for early morning when winds are typically lower. Tell the client why, and document the aborted attempt. A report due tomorrow is not a reason to publish an invalid ACH50.
2. After closing the damper, the zero-flow baseline is 1 Pa and stable, but the fan is now pulling harder to reach 50 Pa. The multipoint data at 50 Pa gives 2,850 CFM. Compute ACH50 and assess whether the fireplace correction changed the building's leakage character.
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ACH50 calculation:
- Volume = 1,900 ft² × ceiling height. With a two-story open foyer, use the actual conditioned volume rather than assuming 8 ft. If the average conditioned height is 9 ft, volume = 17,100 ft³.
- ACH50 = (CFM50 × 60) / volume = (2,850 × 60) / 17,100 = 171,000 / 17,100 = 10.0 ACH50.
If the true average height is 8 ft (15,200 ft³), ACH50 = 171,000 / 15,200 = 11.25. The volume assumption changes the answer by more than 10 percent, so measure or verify the conditioned volume and state it in the report.
Did closing the damper change the leakage character? No - it removed an intentional, controllable opening that was distorting the baseline. The envelope's unintentional leakage is what ACH50 should describe. Leaving the damper open would have inflated CFM50 with a path the homeowner can close at will, making the number non-representative and non-repeatable. Closing it makes the test describe the house as normally operated.
Report note: Record that the damper was found open and closed before the valid test, along with the baseline values before and after. That documents why an earlier reading was discarded and protects the result if it is questioned.
3. The homeowner asks whether 10 ACH50 is a good result and whether the open foyer is hurting them. Answer both without overstating what the test shows.
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Is 10 ACH50 good? It depends on the climate and program, but as a general reference, modern tight construction often lands in the 2–5 ACH50 range, older homes commonly run 8–15, and many efficiency programs set targets around 3–5 ACH50 for new construction or major retrofits. At 10 ACH50 this house is typical of older stock and is likely above a tight-construction target. Present it as a benchmark comparison, not a pass/fail verdict, unless the program defines one. Also note that ACH50 is a reference-pressure metric, not the air change rate the house sees in normal operation - actual infiltration at typical weather pressures is much lower.
Is the open foyer hurting them? The test does not isolate the foyer. A two-story open foyer increases conditioned volume and creates a tall stack path, which can raise stack-driven infiltration in cold weather, but the blower door measures the whole envelope as one system. You cannot attribute a specific CFM to the foyer from this test. What you can say is that the foyer's height makes stack effects more pronounced and that upper-level ceiling and attic penetrations above it are worth inspecting.
Next step: Offer a targeted air-sealing walkthrough - attic hatch, rim joist, penetrations, fireplace - rather than promising a specific ACH50 reduction from any single measure.