The staircase as a chimney: ventilating through height

In a two-storey house there is almost always only one opening that runs through it from bottom to top: the staircase. If there is a way out for the air at the top, it is a chimney that has already been built. If it ends against the roof, it is a tube that carries the heat of the ground floor up to the bedrooms and leaves it there.

The difference between the two is usually one high window. But that window works at some hours and not at others, and in some cases it does more harm than good. Here is the physics, told with numbers, the hours at which it helps in Cali, and what is worth checking before opening up a wall.

What drives it: height and temperature difference

Warm air is lighter than cool air. When the column of air inside is warmer than the one outside, it rises, leaves through the high opening and is replaced by air coming in through the low one. The World Health Organization's guide to natural ventilation puts it in a short formula, and two things in it matter: the push grows in proportion to the height between inlet and outlet, and in proportion to the temperature difference between inside and out. Twice the height, twice the push.

That is why the staircase works and the courtyard of a single-storey house barely does. In the central courtyard we did the sum for three metres of height and one degree of difference: one tenth of a pascal. A two-storey staircase with its outlet six metres above the inlet, and two degrees of difference, gives about four tenths: four times as much, and close to the push of a gentle breeze of one metre per second.

When it works in Cali, and when it runs backwards

The formula has a consequence almost nobody mentions: if it is cooler inside than outside, the flow reverses. The same WHO guide says so: air comes in through the high openings and leaves through the low ones. In Cali that happens in mid-afternoon. IDEAM's Universidad del Valle station (IDEAM is Colombia's weather service) has a mean maximum of 30.1 °C and a mean minimum of 19.5 °C in the 1991-2020 normals. At three in the afternoon a well-shaded house can be cooler than the street, and an open high window lets hot air in at the top.

• —It works best at night. Outside, the temperature drops towards the 19.5 °C mean minimum while the house still holds the heat of the day. It is warmer inside: the chimney draws the right way and empties the heat from walls, floors and slabs. This is when it pays off most.
• —In the morning, while it stays cool outside, it still helps and leaves the house ahead for the rest of the day.
• —In mid-afternoon, it depends on the top floor. If the upper floor, under a hot roof, is hotter than the street, the high outlet still relieves it. If the house is cooler than outside, the high outlet is closed.

It is the same timetable the rule asks for. Annex 1 of Resolución 0194 de 2025, Colombia's sustainable-building rule, says that in hot dry and hot humid climates natural ventilation should only be allowed when the outdoor air is no warmer than the comfort temperature and has low humidity, "mainly during the night hours". A high outlet that cannot be closed is no use in Cali: it must open at night and close in the afternoon.

How it is designed

1. 1A low inlet, on the cool side. A window or a grille on the ground floor, in the shade and away from the facade that takes the afternoon sun. If the incoming air is already hot, the chimney only moves it about.
2. 2A clear path to the staircase. A staircase shut in a box with a door ventilates nothing. Doors that can be left open, or fitted with grilles, or a staircase open to the living area.
3. 3The outlet as high as possible. Above the roof if you can: a lantern with side windows, a rooflight with a side opening, a high window in the stair wall. Height is the engine.
4. 4On the side the wind does not hit. If the outlet faces the wind, the breeze pushes inwards and beats the chimney, which is weaker. Wind direction is observed on the plot.
5. 5Inlet and outlet of similar size. The smaller one governs. An insect mesh reduces the free area, and it has to be counted.
6. 6Sheltered from the rain. Louvred shutters, eaves or a screen. In April the Universidad del Valle station averages 196.6 mm: the high outlet will take downpours.
7. 7Openable and closable from below. Because of the timetable above, because of wind-driven rain, and because if an air conditioner is ever switched on, an open chimney draws out the cold air you are paying for.

When it does not work, or is not a good idea

• —Too little height. In a single-storey house there is no chimney worth the name: there is cross-ventilation, and that is a different design.
• —No temperature difference, or the wrong way round. A hot afternoon with a cool house reverses the flow. A still night with the house already cool moves almost nothing: the WHO warns that in low wind the thermal push may not be enough to move the air inside.
• —Humidity. Night air is cooler but relatively more humid, and the rule asks for low humidity to ventilate. In the most humid months (May averages 75.0% relative humidity at the Universidad del Valle station), watch for condensation on cold walls and in wardrobes.
• —Smells and noise. The chimney carries upwards everything it meets: the smell of the kitchen if it sits at the foot of the stairs, and the noise of the living room. Decide where the kitchen goes before opening up at the top.
• —Smoke. In a fire, the open staircase is also the path smoke takes to the bedrooms. In a detached house the staircase is usually open anyway; in a building, the shared staircase is the escape route and is nobody's chimney: it is not altered on your own initiative. What Titles J and K of the NSR-10, Colombia's building code, require of staircases and smoke we have not been able to read, and we do not state it here. For what can be done in a flat, see renovating under shared ownership.
• —Security. An opening above the roof is a way in for anyone walking on it. A grille or mesh, with the same free area that was worked out for the air.

If your house already exists

Before opening a wall, try what touches nothing: leave the windows already at the top of the staircase open at night, swap a solid door for one with a grille, fit shutters to the low window so it can stay open. Sometimes that is enough.

If it is not, a new high window or a lantern on the roof is another matter. Decreto 1077 de 2015 defines minor repair work (the kind that needs no building permit) as work that does not affect the load-bearing structure or the volumetric characteristics of the house. A new opening in a load-bearing wall touches the structure, and a lantern on the roof changes the volume. Both fall outside minor repair work: they call for checking whether a permit is needed and, in the case of the wall, for the opinion of a civil engineer, who is the one who signs off the structure's integrity. What happens when it is done without that is in I built without a permit, and what touching the structure of an existing house involves, in structural strengthening and the NSR-10.

And if what you have in mind is adding a storey, that is the moment to decide where the staircase goes and how high it rises: the chimney is drawn with the extension, not afterwards.

At the studio

In the proposal for the headquarters of the Tribunales de Cundinamarca, a 2025 public competition in Bogotá, a void runs through the building and the design statement lists cross-ventilation and the stack effect among its strategies. It is another scale and another climate (Bogotá is cold), and it is a competition proposal, not built work: we have no measurements from it. We cite it because it is the same idea this article brings to a house: a vertical void with a high outlet, thought out from the first drawing.

The chimney is the vertical half of the matter. The horizontal half (every room with two facades so the air can cross) is in the central courtyard, and the general climate framework in the bioclimatic house, climate by climate.