Size a ventilation shaft for a buried room at a minimum of 150 cm² of cross-section per 20 m² of floor, bring it at least 60 cm above ground under a rain hood and insect-and-animal mesh, and feed the room through a low intake and a high exhaust. The natural draft is weak, about 2 Pa for a 3 m shaft when the air differs by 20 °C, and it reverses in summer, when the cool underground air is the denser air, so wind and a solar chimney or cowl often decide the airflow.
A ventilation shaft is a vertical duct that moves air between a buried room and the surface, driven by temperature difference and wind.
The Draft Is Small, So The Shaft Must Not Be A Bottleneck
The stack effect works because warm air is lighter than cool air. A column of air in a shaft at a different temperature from the air outside produces a pressure difference, and that difference moves air through the room. In winter the buried air is warmer than the outside air, so the warmer room air rises out of the high exhaust and fresh air comes in low. In summer the relationship flips: outside air is warmer than the ground-cooled room, so the cool room air is the heavier air, and the flow can run backward, with air falling out of the low opening. The same principle is explained for fires in The Chimney Effect, which notes that wind can match the pull.
What You Need
- Shafts of the right section: the parent article’s minimum is 150 cm² per 20 m² of floor.
- Height: each shaft at least 60 cm above ground.
- A rain hood (a tilted plate on supports) and mesh against animals.
- A low intake and a high exhaust, high on the wall and near the ceiling.
- Optional: a solar chimney (a dark-painted shaft section above ground) or a wind-catching cowl facing the prevailing wind.
Method: Size, Place, Check
Step 1: Compute the area. For floor area A in m², required section = 150 cm² x A / 20.
Step 2: Place the intake low and the exhaust high, away from each other.
Step 3: Raise the shaft tops at least 60 cm above ground and hood them.
Step 4: Mesh them, with openings small enough to stop animals and wide enough not to clog.
Step 5: Check the flow with a strip of light cloth or a thread held at the intake, in both winter and summer, and see which way it moves. Never hold a flame or smouldering stick in a buried room for this.
Worked Example
- Floor area 30 m²: section = 150 cm² x 30 / 20 = 225 cm² per shaft.
- A square shaft of side s has area s²: s = square root of 225 = 15 cm. The parent article pairs “150 cm²” with “roughly 15 x 15 cm” for 20 m² of floor, but 15 x 15 cm is 225 cm², which is the figure for 30 m². Use 150 cm² per 20 m² as the minimum: for a 20 m² floor that is a square of about 12.2 cm, and a 15 x 15 cm shaft meets it with margin.
Draft pressure, for a shaft of height h = 3 m with a temperature difference of 20 K between inside and outside air at about 293 K:
- Operation: Δp ≈ ρ x g x h x ΔT / T, with air density ρ = 1.2 kg/m³.
- ρ g h = 1.2 x 9.81 x 3 = 35.3 Pa. Multiply by 20 / 293 = 0.068: Δp = 2.4 Pa.
Wind pressure is ½ ρ v²; for a 3 m/s breeze that is 0.5 x 1.2 x 3² = 5.4 Pa, more than the stack draft, so the draft alone cannot be relied on to ventilate. That is why the parent article adds a solar chimney or cowl, and why the shaft section is generous.
Common Mistakes
- Shaft too narrow. A small draft meets high resistance.
- Both openings at the same height. No stack effect at all.
- Rain hood sealing the top. Leave an open gap around the hood.
- Assuming the winter flow also holds in summer.
Safety
Combustion and carbon dioxide underground
Carbon dioxide pools at floor level, and any stove, lamp or heater adds carbon monoxide: see Carbon Monoxide Risks. Never run a combustion appliance in a buried room without its own flue and an air supply. Never test air by lowering a flame. If anyone gets a headache or feels sleepy, put the fire out, open both openings and get everyone into fresh air.
What’s Next
- Learn next: Light Wells and Clerestories
- Combine with: Interior Moisture Management — when to open and when to close