Diagnostics · The Number That Runs the Job
Blower Door Testing: What the CFM Number Means, and What It Costs You in Heat
The blower door reduces your home's entire leakage story to one measured number — CFM50. This page explains how the test works, how to read your result, and then does the arithmetic most contractors skip: converting that CFM number into gallons of oil, kilowatt-hours, and dollars per winter, using the same heat-transfer math the ASHRAE Handbook of Fundamentals is built on.
The instrument
How the test works
A blower door is a calibrated fan sealed into an exterior doorway with an airtight fabric panel, paired with a digital manometer. Following the practice codified in ANSI/BPI-1200-S-2017 (and the ASTM E779/E1827 test methods behind it), the technician closes the exterior doors and windows, opens interior doors, prepares combustion appliances safely, and runs the fan to depressurize the house to a standard test pressure of 50 pascals — roughly the push of a steady 20 mph wind on all sides of the building at once.
To hold the house at −50 Pa, the fan must exhaust air exactly as fast as air leaks back in through every crack in the envelope. The manometer reads that fan flow directly. The result:
- CFM50 — cubic feet per minute of airflow at 50 Pa. This is the raw size of all your leaks added together, expressed as one hole. (Divide by about 10 for a rough single-hole equivalent in square inches: a 3,200 CFM50 house leaks like a ~2 ft × 1 ft hole in the wall.)
- ACH50 — air changes per hour at 50 Pa: ACH50 = CFM50 × 60 ÷ house volume (ft³). This normalizes for house size so buildings can be compared — and it's the unit the code regulates.
With the house held at −50 Pa, every leak becomes a cold incoming jet, findable by hand, smoke pencil, or infrared camera. That's why the test isn't just a score — it's the treasure map for the air sealing work that follows, and the test-out afterward proves what changed.
| ACH50 | What it describes |
|---|---|
| 0.6 | Passive House certification limit — the practical floor of airtightness |
| ≤ 3.0 | Maximum for new Maine homes — 2021 IECC §R402.4.1.2, verified by mandatory blower door test (see new construction code) |
| 4–7 | Typical conscientious construction of the 1990s–2010s; the 2015-code era target was 4.0 |
| 8–12 | The broad middle of Maine's existing housing stock — big, cost-effective sealing opportunity |
| 12–20+ | Older farmhouses, balloon framing, camps — the equivalent of a window open year-round; sealing results here are dramatic |
The 2021 IECC also allows very small dwellings (<1,500 sq ft) to qualify at ≤0.30 CFM50 per square foot of envelope area, since ACH50 penalizes small volumes. Test results are single numbers, but leakage isn't uniform — the distribution (attic plane vs. basement vs. walls) matters as much as the total, which is why the diagnostic walk during the test drives the work scope.
The math
From CFM50 to dollars: the four-step conversion
CFM50 is measured at an artificial 50 Pa; your house doesn't live at 50 Pa. Converting the test number into real heating cost takes four steps. We'll carry a worked example through all four: a 2,000 sq ft, two-story Maine house, 8 ft ceilings (16,000 ft³ volume), testing at 3,200 CFM50 — that's ACH50 = 12, squarely in the "typical older Maine home" band.
Step 1 — Test pressure → natural infiltration
Average real-world infiltration is estimated by dividing CFM50 by the LBL "N-factor" (from Lawrence Berkeley National Laboratory's infiltration model): a number between roughly 14 and 26 that accounts for climate, building height, and wind shielding. Cold, windy climates and taller houses convert test leakage into natural leakage more aggressively — Maine sits in the coldest LBL climate band, so a two-story Maine house lands around N ≈ 15–17 (versus the ~20 national rule of thumb).
3,200 CFM50 ÷ 16 ≈ 200 CFM of continuous natural infiltration, averaged over the heating season. Two hundred cubic feet of heated air leaving the house every minute, replaced by outdoor air — mostly in through the basement, out through the attic, courtesy of the stack effect.
Step 2 — Airflow → heat load
Warming air takes energy in proportion to flow and temperature difference. For standard air, the sensible-heat equation (ASHRAE Handbook of Fundamentals) is:
The 1.08 bundles air's density and specific heat with the minutes-to-hours conversion. Example: 200 CFM on a 0°F night with the house at 68°F → 1.08 × 200 × 68 ≈ 14,700 BTU/hr — a large space heater running flat out, doing nothing but reheating replacement air. And because stack pressure grows with ΔT, leakage is worst in exactly the hours fuel matters most.
Step 3 — Hourly → seasonal, using degree days
Season-long, the ΔT×time integral is exactly what heating degree days measure. Central Maine runs about 7,500 HDD (base 65°F); Caribou nearer 9,500. So:
Example: 1.08 × 200 × 24 × 7,500 ≈ 38.9 million BTU per year — the heat content of the air this one house exhales each winter.
Step 4 — BTU → fuel and dollars
Oil boiler (138,690 BTU/gal × ~80% efficiency ≈ 111,000 delivered BTU/gal): 38.9M ÷ 111,000 ≈ ~350 gallons/yr — roughly $1,200–1,350 at recent Maine prices.
Propane furnace (91,500 BTU/gal × ~90%): ≈ ~470 gallons/yr.
Cold-climate heat pump (seasonal COP ≈ 2.5 → ~8,530 BTU/kWh): ≈ ~4,600 kWh/yr — roughly $1,150–1,350 at recent Maine electric rates.
So for this ordinary, real-world Maine house, air leakage alone is a four-figure line item every winter — a quarter to a third of a typical total heating bill, consistent with what field studies of leaky homes find.
The homeowner's rules of thumb
What a CFM is worth — and what sealing realistically saves
- Every 100 CFM50 ≈ $35–45 per year in heating cost for a typical central Maine house (the two fuel paths above converge on the same band; Aroostook's deeper HDD pushes it higher). Equivalently, for our example house, each full point of ACH50 ≈ $100–120 per year.
- Professional air sealing typically removes 20–40% of measured leakage in older homes in a one-to-two-day mobilization. On the example house, a 40% reduction (3,200 → ~1,900 CFM50) is worth roughly $450–550 every year — before counting the insulation that follows, the summer cooling and dehumidification savings on the same holes, the ice-dam and moisture benefits, or the comfort change (drafts are the ΔT you can feel; ASHRAE 55 treats them as a measurable comfort defect).
- The savings compound with the insulation. Sealing restores the rated R-value of fibrous insulation that moving air was degrading — the reason seal-then-insulate is the BPI sequence and the Efficiency Maine specification.
Honest fine print
These are planning numbers, not guarantees. The N-factor is a modeled average — a tall house on a windy ridge in Caribou leaks more per CFM50 than a sheltered ranch in Kittery; the calculation is sensible-heat only (moisture adds load in both seasons); and a house sealed very tight then receives deliberate mechanical ventilation per ASHRAE 62.2, which returns a controlled, filtered fraction of the airflow you eliminated — a feature, not a loss, since it's right-sized, recoverable (HRV), and delivered where you want it instead of through the rim joist. Program-grade savings estimates (the kind behind Efficiency Maine projects) calibrate these models against actual fuel bills, in the spirit of BPI's whole-house standards. What the blower door guarantees is the measurement itself: your leakage, before and after, in numbers nobody can hand-wave.
When a blower door test is required vs. smart
Required: every new Maine dwelling — max 3.0 ACH50 with a written report (2021 IECC §R402.4.1.2); additions may instead use visual verification per Table R402.4.1.1 (details). Expected: pre/post testing on Efficiency Maine weatherization projects. Smart: before buying any insulation for an existing home (it sets the scope), mid-construction on new builds (leaks are cheap to fix while reachable), and after any major renovation, so the certificate reflects reality.
Get your number.
An assessment from one of our recommended installers includes the blower door test, the infrared walk, and this exact math run on your house — your CFM50, your fuel, your degree days, your dollars. Then a scope that attacks the biggest leaks first.
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