Stack Effect: Why Stairwell Pressurization Fails in Winter

A stairwell pressurization fan that passes its acceptance test in July can still fail on the coldest night of January. The reason: stack effect, the simple physics of warm air rising inside a tall, heated building. So a smoke-control system sized for a mild morning can quietly invert when a polar vortex arrives. This post walks through that physics, the failure cases it has caused, and how to design around it.

TL;DR on stack effect and pressurization

• First, stack effect scales with the indoor-outdoor temperature gap, so winter pressures run 3 to 5 times higher than summer.
• In a 30-story building at −18 °C, total envelope stack pressure can reach ~180 Pa — far above the 12.5 to 25 Pa a fan holds across a stair door.
• Below the neutral plane, stack pressure adds to the fan and can jam doors shut past the 133 N opening-force limit.
• Above the neutral plane, stack pressure fights the fan and can reverse the door pressure, so smoke seeps into the stair.
• Finally, field tests and four landmark fires — MGM Grand, First Interstate, Cook County, Plasco — all show the same chimney behaviour.

A tale of two calculations

Picture a consulting engineer in Atlanta sizing the stair pressurization fan for a new 30-story tower. It happens in July. So the acceptance test runs on a mild 22 °C morning. The fan delivers a textbook 25 Pa across every stair door, all twelve open doors stay under the 133 N force limit, and the system passes NFPA 92. Sign-off, occupancy, move-in.

Six months later the same building sits under a polar vortex. Outside reads −18 °C; inside, +21 °C. Then a trash-room fire starts on the 4th floor. The fan kicks on as designed. Yet on the 28th floor, smoke seeps around the stair door from the corridor. On the 2nd floor, a fleeing tenant cannot haul the door open against what feels like a sealed vault. The fan still runs. However, the physics has changed.

This is the pressurization paradox. A system whose design chases one static pressure target can flip on the coldest day of the year. Stack effect plays the villain here. Despite its age — Wilson and Tamura at Canada’s National Research Council described it in 1968 — it stays a stubborn blind spot in real commissioning.

The physics of stack effect

Stack effect arises because warm air weighs less than cold air. In a heated tower wrapped in cold outdoor air, the inside column of warm air weighs less per unit height than the matching outdoor column. So the pressure difference across the wall grows with height.

For a uniform shaft, the engineering form used in the ASHRAE Handbook of Smoke Control Engineering reads:

$\Delta P_{\text{stack}} = 3460 \, h \left( \frac{1}{T_o} - \frac{1}{T_i} \right)$

Here $h$ means height above the neutral plane in metres, and $T$ means temperature in kelvin. A handy rule: in cold climates, stack effect makes about 4 Pa across each floor of a tall building. For a 30-story building (≈ 100 m) at −18 °C outside and +21 °C inside, the total comes to:

$\Delta P_{\text{stack}} = 3460 \cdot 100 \left(\frac{1}{255} - \frac{1}{294}\right) \approx 180 \text{ Pa}$

That figure represents the total envelope pressure, spread about the neutral plane. It dwarfs the 12.5 to 25 Pa that NFPA 92 asks the fan to hold across a stair door.

The neutral pressure plane

Specifically, the neutral pressure plane (NPP) marks the height where inside and outside pressures match. Below it in winter, outdoor pressure wins and air leaks inward. Above it, inside pressure wins and air leaks outward. With evenly spread leakage, the NPP sits near mid-height. However, leaky lobbies, tight upper floors, and open shafts can shove it up or down. Wilson and Tamura showed that a 183 m building at a 56 K gap holds about 500 Pa of total stack pressure. Of that, a leaky entrance lobby alone can soak up 200 Pa — enough to make the front doors nearly impossible to budge.

Why winter beats summer every time

Summer runs a reverse stack effect: the cooled interior sits below the outdoor temperature. However, the summer gap stays small, often 5 to 15 K. Winter gaps reach 30 to 50 K. Because stack pressure scales with that gap, winter pressures run 3 to 5 times larger. Worse, winter stack pushes smoke upward — the same way occupants climb to flee a lower-floor fire.

What stairwell pressurization should do

The goal stays plain: hold the stair shaft above the corridor pressure at every floor, so leakage flows outward through the door cracks and smoke stays out. Overall, two numbers govern the design. First, the pressure target itself. Second, the door-opening force a human can manage.

NFPA 92 and the IBC set the floor and the ceiling. On the low side, sprinklered buildings need 12.5 Pa across a smoke barrier; non-sprinklered ones need 25 Pa. On the high side, the IBC caps the stair-to-building difference at 87 Pa. That ceiling comes not from smoke physics but from people. Above roughly 133 N (30 lbf) of door force, a child, an elderly tenant, or someone carrying a load cannot open the door.

NFPA 92 does say the target must hold “under maximum anticipated conditions of stack effect and wind effect.” In practice, though, many designers plug in one winter temperature — often a mild one — and so under-build for the worst night.

The pressurization paradox in winter

In a uniformly leaky building with the NPP at mid-height, winter stack effect breaks a running fan in two opposite ways at once.

Lower floors, below the NPP. Here stack pressure pushes the same way as the fan: stair to building. So the two pile up together. On the bottom floor of a 30-story tower at a 39 K gap, stack can add ~60 to 70 Pa on top of the 25 Pa fan target, sailing past the 87 Pa force limit. The result: occupants cannot open the door.

Upper floors, above the NPP. Here stack pressure fights the fan. The stair tends to sit below the warm interior pressure. If the fan cannot win, the door pressure reverses. The result: smoke from a corridor fire slips into the stair.

Ferreira and Cutonilli (CTBUH 2008) modelled exactly this for a 30-story building at −14 °C. With a plain, non-compensated fan, the stair-to-building pressure at the top fell to −2.5 Pa. The differential had reversed. Meanwhile, the door force at the bottom blew past 90 Pa. The fan ran the whole time. It simply failed at both ends.

Field evidence from real winters

The deepest recent field study comes from Strege and Ferreira (2017) in Fire Technology. The Jensen Hughes team measured door pressures in 15 high-rises across Cleveland, Baltimore, Minneapolis, and Philadelphia during January to March 2013. Buildings ran 44 to 150 m tall; outdoor air ranged −12 °C to +15 °C.

Their numbers tell the story. On lower floors, shaft doors saw −2.7 to −24.9 Pa (averaging −12 Pa) — air flowing into the shafts. On upper floors of 13 of the 15 buildings, doors saw +0.5 to +34.9 Pa — air flowing out.

Two buildings already ran stair pressurization. Strikingly, the team found that turning the stair fan on could increase vertical air movement through unprotected elevator shafts. In other words, the fan could worsen smoke spread by piling onto the stack-driven pressure. They also warned that “simplified algebraic calculations may underpredict” the real differentials, and urged multi-zone or CFD analysis instead.

The lesson for practitioners runs blunt. A hand calculation that ignores leakage, HVAC, and wind hands you optimistic numbers. So real winter weather can beat the design target.

When smoke found the stairs

History keeps proving the point.

MGM Grand, Las Vegas, 1980. A first-floor fire killed 85 people in this 26-story hotel. Yet most victims — 61 of 85 — died on the 19th through 26th floors, as far from the flames as possible. NFPA tied those upper-floor deaths to smoke riding elevator shafts, stairwells, and seismic joints, driven by stack effect. The hotel had no stair pressurization. So the MGM Grand became the textbook reason U.S. high-rises now require pressurized stairs and sealed shafts.

First Interstate Bank, Los Angeles, 1988. Fire gutted five floors of a 62-story tower mid-retrofit. Crews arriving on the fire floor found smoke entering all four stair shafts around the doors. When they opened doors to fight the fire, heat and smoke poured into the stairs and rose fast. Observers saw smoke leaking from the elevator shaft 30 floors above the fire. Stack effect carried it there.

Cook County Administration Building, Chicago, 2003. A 12th-floor fire in this 35-story building killed six people in a stairwell. The smokeproof design leaned on smoke louvers that, through maintenance and code lapses, never vented properly. When crews opened the stair door, smoke entered and climbed; doors above stayed locked. NIST later found the six most likely would have survived if the louvers had worked as intended.

Plasco Building, Tehran, 2017. A fire in this 17-story tower spread up the single stair shaft and helped trigger a full collapse 3.5 hours later. Twenty-two people, including sixteen firefighters, died. The building had one staircase and no pressurization. On a cold January morning, stack effect turned that shaft into a chimney.

How three code families handle it

NFPA 92 (US), EN 12101 (Europe), and France’s IT rules all require designers to consider stack effect. They differ in teeth. NFPA 92 names stack and wind but sets no explicit design temperature, so the worst-case choice falls to the engineer.

Europe’s revised EN 12101-13:2022 goes further. It demands an explicit pressure-distribution analysis for buildings 60 m and taller. It also requires distributed supply at least every three storeys above 11 m. France, meanwhile, caps door pressure near 80 Pa, much like the IBC’s 87 Pa. It then leans on tight compartmentation to shorten the height over which stack can act.

The pattern reads clear. The newer the code, the more it forces stack effect out of the footnotes and into the calculation.

Designing for January, not July

Five moves separate a system that works in winter from one that only looks good in summer.

First, inject at multiple points. Klote’s Handbook of Smoke Control Engineering and EN 12101-13 both push supply every two to three floors. This flattens the pressure profile, so no single floor sees a spike or a dip.

Second, modulate the fans. Variable-speed drives reading live pressure can chase the changing outdoor temperature, wind, and door positions. ASHRAE’s RP-1203 and RP-1447 tests showed these compensated systems hold target across far more scenarios than fixed-speed fans.

Third, add vestibules. A small lobby between corridor and stair splits the total pressure across two doors in series. So each door keeps a manageable force while the smoke barrier doubles.

Fourth, compartment the shaft. Smoke-tight doors every 10 to 15 floors cut the chimney into shorter sections, each with a smaller driving height. European practice does this often; NFPA 92 allows it as a performance path.

Finally, commission in winter. NFPA 92 demands acceptance testing but never names a season. So write a winter re-test into the project specification, with pressure profiles logged at an outdoor temperature at or below 0 °C. No code requires it. Yet it remains the only honest proof the design works on a January night.

For the related trap of trusting one mechanical element too far, see our sprinkler design mistakes deep dive. To see how engineers weigh rare-but-severe failures like these, read our F-N curve and societal risk primer. And for the combustion basics behind smoke movement, start with what is fire.

The honest takeaway

Treat the building as a thermodynamic system, not a box with a fan on the roof. Its pressure field comes from the column of warm air it holds. On the coldest night, that column overwhelms a fan sized for a mild morning. The codes already ask for stack effect; the failure lives in design shortcuts and summer-only testing. A smoke-control system tested in July tells you only that it worked in July. The fire that kills people starts at 3 a.m. on a Tuesday in January. The system that saves them must answer to that night, not a mild summer morning.

Cite this article

Dinh, D. C. (2026, May 29). Stack Effect: Why Stairwell Pressurization Fails in Winter. PyroRisk. https://pyrorisk.net/blog/stack-effect-why-stairwell-pressurization-fails-in-winter/

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