On a January morning, the lobby is where people first feel a building. It's where visitors stamp off the snow, tenants wait for the elevator, and the front desk staff spends the whole shift. When that lobby is drafty, people notice. When it gets cold enough, pipes can freeze.
The heating system usually takes the blame, but it isn't always the problem. Consider stack effect.
If you've ever lit a fireplace, you've seen it at work. Warm air rises up the chimney, and cold air is drawn from every corner of the house toward the hearth.
Most tall buildings don't have chimneys, but they have something similar: stairwells and elevator shafts. In winter, warm air rises up these shafts. That leaves the lowest floors at lower pressure than the outdoors, and cold air gets pulled in through every door, gap, and crack it can find.
We can't change the physics. Tall buildings will always see this pressure difference in winter. But there are good ways to fight the chill.
Two sets of doors in a row mean the lobby is never open directly to the outdoors, so it doesn't take an icy blast every time someone walks in. Energy codes require vestibules at the entrances of many new buildings.
They do the same job as a vestibule. Instead of a blast of cold air, the lobby gets a small puff.
Even with a vestibule, the pressure difference pulls air in through windowsills, door jambs, and cracks in walls. The older the building, the worse this tends to be.
Tight doors between the elevator lobby and the reception area help. Even sliding doors make a difference.
Modern HVAC systems can adjust automatically in winter, bringing in additional heated make-up air when needed to counter the pressure.
A dedicated cabinet unit heater in the vestibule helps stop drafts before they reach the lobby.
Placing lobby return air openings close to the floor helps draw warm air down from the ceiling. This matters most in tall lobbies.
Most buildings need a combination of these approaches. The right mix depends on the building's height, entrances, age, and how people use it. Our mechanical engineers can help you find the right combination for yours.
Aaron Frantz, PE, CDP, is an Associate and Lead Mechanical Engineer at Peter Basso Associates.