It is a common winter phenomenon in New York City: As you try to pull open a lobby door, it seems soldered shut. Wrestle it open, and you are greeted by a shrill whistle and a blast of air.

That is the sound of heat — and money — escaping.

“It’s hard to open doors in winter because there’s negative pressure,” said Russell Unger, executive director of Urban Green Council, the New York chapter of the United States Green Building Council, which released a study Tuesday night about preventing heat loss in city buildings. “Doors don’t naturally whistle. It’s a sign of a building leaking a lot of heat.”

The science is simple: Heat is continuously being lost through leaks in buildings, especially out of the tops of elevator shafts. Then, as a lobby door is opened, cold air rushes in and must be heated. The process repeats each time the door is opened.

According to the study, the amount of warm air lost in winter through the elevator shafts of the city’s 4,000 multifamily buildings that are at least 10 stories tall could fill 29,000 Empire State Buildings.