In a tall building, air moves for reasons the HVAC system did not choose. Temperature differences, wind and open shafts all push air up, down and through the envelope. Architects shape these forces long before the mechanical drawings start. This guide covers the main ones and the design responses.
The forces at work
| Force | What it does | Design response |
|---|---|---|
| Stack effect | In cold weather, warm air rises inside the building and leaks out at the top while cold air enters low. In hot weather the effect reverses and is weaker | Tight envelope, vestibules at ground floor, compartmentalized shafts |
| Wind pressure | Pushes air in on the windward side and pulls it out on the leeward side | Careful intake and exhaust placement, tight facades, pressure control |
| Shaft pathways | Elevator, stair and service shafts connect floors, so air moves between them | Seal shafts, control stair pressurization, door hardware that closes |
| Solar and perimeter loads | Glass facades swing between hot and cold depending on orientation | Separate perimeter zones, shading, good glazing |
What happens to occupants
- Doors at the base are hard to open or whistle.
- Lower floors feel drafty in winter and upper floors feel overheated.
- Odors and smoke can move between floors through shafts.
- Zones on the same floor but different facades never agree on temperature.
Design measures that help
- Air tightness. Specify and test the envelope. Leakage drives stack effect problems.
- Vestibules. Double doors at entries reduce the air pushed in at ground level.
- Zoning. Zone by facade and by floor group, and give perimeter zones their own control.
- Shaft design. Compartmentalize shafts, seal penetrations and coordinate with life safety design.
- Pressure control. Design the supply, return and exhaust so the building stays slightly positive to outdoors where appropriate.
- Coordination. Make sure the mechanical and fire protection engineers see the same shaft layout.
Testing after construction
Measured airflow at each floor, outdoor air at each air handler and building pressure relationships show whether the design works. See why air balancing belongs in the design and how to fix temperature imbalances in large buildings.
A worked scenario
A tower owner receives complaints that the lobby door is hard to open in winter and the top floor is too warm. The building has a leaky podium, open service shafts and perimeter zones that share a thermostat with the interior. Measured pressure shows a strong inward flow at the lobby. The fixes are to seal the shaft penetrations, add a vestibule, and split the perimeter zones from the interior, in that order, with before and after pressure readings. The pattern is common in tall buildings.
Common mistakes
- Treating stack effect as a mechanical problem. It starts with envelope and shafts.
- Sharing one zone between facades. The loads differ by hour.
- Ignoring door hardware. Doors that do not close let air through.
Questions architects ask about tall building airflow
Can the HVAC system overcome stack effect?
Partly. The system can control pressure, but a tight envelope and sealed shafts do most of the work.
How do we test it?
With envelope air tightness testing, pressure measurements across doors and shafts, and a measured air balance.
Who coordinates life safety and HVAC?
The engineers do, with the architect making sure the shaft layout is shared. Document it.
Related reading: Multifamily Building IAQ: A Property Manager's Guide.
To measure airflow and pressure in a tall building, call Ghazi Hussain at (908) 323-9780 for a quote.
