Mayor Bill de Blasio has set an ambitious goal of reducing greenhouse gas emissions by 80 percent by 2050. But one fact makes it extremely difficult to achieve: In New York City, heat literally goes out the window.

Residents in apartment buildings across the city know the drill all too well. Come cold weather, steam boilers run on overdrive so that heat reaches the farthest apartments, usually on the top floors. As a result, overheated residents open their windows on even the coldest days.

According to a new study, cutting back on energy waste — and cutting carbon emissions in the process — can take just a few simple fixes, like installing a tiny plate in each radiator’s valve to slow the release of steam; adding insulation and a temperature sensor; and affixing a control knob on the exterior of each radiator. The report, to be released on Friday, was written by Energy Efficiency for All, which is affiliated with the Natural Resources Defense Council, and the consulting firm Steven Winter Associates.

An orifice plate, which resembles a bottle cap, “can be inserted into the radiator’s hand-valve in approximately five minutes, by handymen or plumbers,” the report says, adding, “The plates never wear out,” and even help the system run better.