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Energy-Efficient Building

Passive House Air Tightness n50 Blower Door Simulator

Real-time Passivhaus design tool that converts the Blower Door Q50 (50 Pa leakage flow) into n50, equivalent leakage area (ELA), infiltration heat loss and annual heating demand. Adjust building volume, envelope area, U-value and ventilation rate to see compliance with Passivhaus, EnerPHit, LEED Platinum, BELS and Japan baseline standards.

Parameters
Standard
Choose the n50 upper limit to test against
Climate zone
Reference label; heat balance uses the outdoor temperature
Building volume V
m³
Envelope area A_env
m²
Walls + roof + floor + windows facing outdoor air
Blower Door Q50
m³/h
Total leakage flow at 50 Pa pressure differential
Mechanical ventilation
ach
Design ventilation rate of the HRV/ERV
Average U-value
W/m²K
Area-weighted U-value of the whole envelope
Outdoor temperature T_o
°C
For the heat-loss calculation (indoor 20 °C assumed)
Results
—
n50 (1/h)
—
Equivalent leakage area (cm²)
—
Natural infiltration (ach)
—
Infiltration heat loss (W)
—
Envelope heat loss (W)
—
Heating demand (kWh/m²·y)
—
House section & Blower Door pressure gradient

The Blower Door fan creates a 50 Pa pressure difference across the envelope; coloured arrows show leakage paths and the gauge tracks the live n50.

n50 vs Q50 (constant volume)
n50 limits by standard
Theory & Key Formulas

$$n_{50} = \frac{Q_{50}}{V_{\text{building}}},\qquad ELA = \frac{Q_{50}}{C_d\sqrt{2\Delta P/\rho}}\times 10^{4}\;[\text{cm}^{2}]$$

n50 is the air-change rate at 50 Pa, Q50 the Blower Door leakage flow (m³/h), V the heated volume. ELA is the equivalent leakage area (Cd = 0.6, ΔP = 50 Pa, ρ = 1.225 kg/m³). Passive House requires n50 ≤ 0.6 /h.

$$n_{\text{nat}}\approx \frac{n_{50}}{20},\qquad Q_{\text{infil}}=\dot m\,c_p\,\Delta T = \frac{n_{\text{nat}}}{3600}\,V\,\rho\,c_p\,\Delta T$$

Persily's approximation for long-term natural infiltration and the resulting infiltration heat loss. c_p = 1006 J/kgK, ρ = 1.225 kg/m³, ΔT is indoor-outdoor temperature difference.

$$Q_{\text{env}} = U_{\text{avg}}\,A_{\text{env}}\,\Delta T,\qquad E_{\text{heat}} \approx \frac{(Q_{\text{infil}}+Q_{\text{env}})\cdot H_{\text{hr}}}{A_{\text{floor}}\cdot 1000}$$

Envelope conduction loss and annual heating energy estimate. H_hr ≈ 5,000 K·h heating season, A_floor ≈ A_env/2. Passive House limits this to 15 kWh/m²·year.

Passive House Air Tightness n50 & the Blower Door Test

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I keep hearing that "a Passive House must have n50 ≤ 0.6". What does that 0.6 actually mean, and how does it compare with normal houses?
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Good question. n50 is "how many times the indoor air is replaced per hour when the whole building is held at a 50 Pa pressure difference". 0.6 /h is essentially as airtight as a building gets. By contrast a typical Japanese house comes out at n50 = 5 to 10, and even a HEAT20 G2 house lands around 2. Passivhaus, defined by Wolfgang Feist in Germany in 1991, sits more than ten times tighter — and over 50,000 buildings have been certified worldwide since.
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How do you measure it? The idea of "air-tightness testing" sounds like very specialised equipment.
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It uses a "Blower Door" — a calibrated fan mounted in the entrance door. The indoor-outdoor pressure difference is varied between 10 and 60 Pa at several points and the airflow Q is fitted to Q = C·ΔP^n. From the curve we extract the leakage at 50 Pa, Q50 (m³/h), and divide by the building volume V to get n50. ISO 9972 and ASTM E779 are the standards; Method 1 averages pressurisation and depressurisation. With the left-panel defaults (Q50 = 300 m³/h, V = 350 m³), notice n50 = 0.86 in the top-right stat card.
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Oh I see — but that fails Passivhaus (n50 ≤ 0.6), right? How would we drop it to 0.6?
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Exactly — getting from 0.86 to 0.6 is the headline challenge of Passive House design. You would need to bring Q50 from 300 to 210 m³/h. In practice that means (1) a continuous internal air-barrier membrane (Pro Clima Intello, Siga Majpell), (2) sealing every joint and window perimeter with air-tightness tape (Tescon Vana), (3) gasketted boxes for sockets and service penetrations, and (4) certified Passive House triple-glazed windows. EnerPHit (retrofit) is easier at n50 ≤ 1.0.
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The verdict also says "heating demand 62 kWh/m²·year", but Passive House requires 15. Does air-tightness alone make that much difference?
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Air-tightness alone does not — at the current defaults the envelope U = 0.30 W/m²K dominates with 2,700 W of conduction loss against just 103 W of infiltration loss. To certify, you would also have to push U down toward 0.10-0.15, use triple glazing, and recover 80% of exhaust heat with an HRV. But the converse matters too: with a leaky envelope the HRV efficiency collapses. Both have to be in place for 15 kWh/m²·year to be reachable.

Frequently Asked Questions

n50 is the number of building-volume air changes per hour when the whole building is pressurised (or depressurised) to 50 Pa during a Blower Door test. Passivhaus certification requires n50 ≤ 0.6 (1/h) — roughly one-tenth to one-twentieth of a typical home (n50 = 3 to 10). It is obtained by dividing Q50 (leakage flow in m³/h) by the building volume V. EnerPHit retrofits use 1.0, LEED Platinum 1.5, and Japan's general housing has no equivalent value, using the equivalent leakage area C-value at 5 Pa instead.
A Blower Door is a calibrated fan fitted into an entrance door. The indoor-outdoor pressure difference is varied between 10 and 60 Pa and the flow Q is measured at several points, then fitted to the power law Q = C·ΔP^n to extract Q50. The main standards are ISO 9972 (formerly EN 13829) and ASTM E779, with Method 1 (mean of pressurisation and depressurisation) and Method 3 (depressurisation only). Vents, chimneys and drain traps must be sealed; missing one opening can multiply n50 several-fold.
ELA represents all building leaks as a single thin-plate orifice with the same flow. From Q = Cd·A·√(2ΔP/ρ), with Cd = 0.6, ρ = 1.2 kg/m³ and ΔP = 50 Pa, the area A is back-calculated. For Q50 = 300 m³/h this gives ELA ≈ 154 cm², about one postcard's worth of total leakage. Passive House practice keeps ELA below an A4 sheet of paper.
Persily's approximation gives a long-term natural infiltration rate of n_nat ≈ n50/20. Reducing n50 from 5.0 to 0.6 lowers n_nat from 0.25 to 0.03 ach, which in a cold climate (ΔT = 25 K) cuts infiltration heat loss by about a factor of 8. For a 350 m³ dwelling this is roughly 1,500 kWh/year of heating energy, making the Passivhaus 15 kWh/m²·year heating-demand limit realistic when combined with an 80%-efficient HRV.

Real-World Applications

World's first Passive House — Kranichstein, Darmstadt (1991): Built by Wolfgang Feist as his own row-house, it achieved n50 = 0.3 and a 10 kWh/m²·year heating demand. It used what were then state-of-the-art argon-filled triple-pane windows (U_w = 0.7), an envelope U = 0.10, and an 80% HRV. Performance has not degraded after 30+ years, demonstrating the long-term reliability of Passive House construction.

Linz Solar City, Austria (600 dwellings): Europe's largest Passive House housing estate, with every unit verified at n50 ≤ 0.6. Multifamily air-sealing is especially difficult at party walls, but the project used the Pro Clima system tape plus plaster finish to achieve compliance. Similar certified Passive Houses are now appearing in Tokyo, Nasu and Sapporo (e.g. KEN HOUSE, Nasu PH).

Massachusetts public housing — Stephen Wessling Architects: A 14-unit row of US public housing certified Passive House with n50 = 0.5. To control cost, the team relied on standard OSB and Siga Majvest exterior air-barrier rather than imported systems, while running a Blower Door test on every unit. Energy bills dropped to roughly a quarter of conventional public housing, dramatically lowering housing costs for low-income tenants.

EnerPHit retrofits of existing buildings: Reaching new-build PH (n50 ≤ 0.6) on an existing building is essentially impossible, so EnerPHit (n50 ≤ 1.0) is the retrofit standard. External insulation combined with an internal air-barrier layer typically brings old houses from n50 = 4-8 down to 0.7-1.0 and qualifies for public retrofit subsidies in Germany and Austria. In Japan there is growing interest in pushing legacy-code housing toward BELS A plus EnerPHit-level air-tightness.

Common Misconceptions & Pitfalls

The single biggest misconception is that "a tighter building stops window condensation". The truth is the opposite — a tighter envelope traps interior water vapour, and without mechanical ventilation (HRV/ERV) you get more condensation and mould, not less. Passivhaus certification requires an HRV that holds winter indoor humidity to 30-60%; air-tightness and ventilation only work as a pair. Tightening the envelope while running careless ventilation leads to sick-building syndrome, mould and high CO₂ in one stroke.

Another trap is assuming that "n50 = 1.0 and n50 = 0.5 are basically the same". The ratio is just 2×, but in terms of natural infiltration it is 0.05 vs 0.025 ach — a year of heating in a cold climate (ΔT = 25 K) differs by 400-500 kWh per unit. Combine that with an 80% HRV and the effective recovery drops into the 60% range at n50 = 1.0, putting the 15 kWh/m²·year Passive House target out of reach. The reason 0.6 is "mandatory" is essentially that it is the precondition for the HRV to do what it is designed to do.

Finally, "Japan's C-value and n50 measure the same thing, just with different units" is false. C (cm²/m²) is the equivalent leakage area per unit floor area, computed at 5 Pa. n50 is air-changes per volume at 50 Pa. The correspondence depends on building shape, so a generic conversion does not exist. Japan's HEAT20 G2 / ZEH target of C ≤ 1.0 is considerably looser than Passivhaus n50 ≤ 0.6, and to chase PH certification you need to measure n50 directly with a Blower Door test.

How to Use

  1. Enter building envelope volume in m³ (typical Passivhaus: 800–2000 m³)
  2. Input total building envelope area in m² (walls, roof, floor, windows)
  3. Enter measured Blower Door Q50 leakage flow in m³/h at 50 Pa pressure difference
  4. Specify mechanical ventilation rate in ach (typical: 0.3–0.5 ach)
  5. Simulator calculates n50, equivalent leakage area, natural infiltration, and annual heating demand

Worked Example

A 1200 m³ Passivhaus with 450 m² envelope area, measured Q50 = 360 m³/h, and mechanical ventilation of 0.4 ach: n50 = 360 ÷ 1200 = 0.30 1/h (meets Passivhaus standard ≤0.6 1/h). Equivalent leakage area = 10.8 cm² at 50 Pa. Natural infiltration at winter 10 Pa wind = 0.018 ach. Infiltration heat loss = 28 W. With U-value envelope losses of 340 W and 80 m² south glazing (g=0.50), annual heating demand = 12.5 kWh/m²·y, well below Passivhaus limit of 15 kWh/m²·y.

Practical Notes

  1. Passivhaus Standard n50 ≤0.6 1/h; EnerPHit retrofit ≤1.0 1/h. Poor air tightness (n50 > 3.0) requires detailed leakage tracing via thermography.
  2. Blower Door testing must occur after drywall closure but before final finishes; seal temporary openings (exhaust, supply ducts) with foam and plastic.
  3. Equivalent leakage area helps identify construction flaws: cracks, unsealed penetrations, and ductwork gaps are primary culprits in residential builds.
  4. Natural infiltration at 10 Pa (typical winter wind) dominates heating loss if mechanical ventilation fails; redundant supply/exhaust prevents depressurization.