Heat

Heat and heat levels for the various Buildings is one of the Game Mechanics in Frostpunk.

Description
Generating heat allows you to combat the cold and keep your city functioning. Each 10C drop reduces heat levels by 1, whereas things like building insulation and heat from the Generator raise heat levels.

Summing up all the heat level modifiers for a building determines its temperature level:

For example, a tent in -40C weather within the heat zone of a level 1 generator would have the following modifiers: The sum is 0 so the building is Chilly.
 * -2 Weather (-1 per 10C below the initial -20C)
 * +1 Building insulation
 * +1 Heat from generator

Trivia

 * The reason why the temperature is comfortable at 0°C is that the people in our city are already wearing warm clothing, and have a device called a lamp outlined in one of the Relics so a temperature of 0°C is warm to them.