So far every answer has been a lump sum of joules. But heat through a window never finishes — it runs as a steady rate, joules every second, and a joule per second has a name you know from light bulbs: the watt. The formula is $P = \dfrac{kA\,\Delta T}{d}$ (read aloud: P equals k A delta-T over d). $k$ is the material's conductivity (glass conducts eagerly, foam barely), $A$ is how much wall is leaking, $\Delta T$ is how hard the heat is being pushed — and the thickness $d$ sits underneath, because a thicker wall resists.

That $d$ underneath is the entire physics of insulation: double the thickness, halve the leak. Two field notes for the questions below. First, keep rate and amount apart — watts are joules per second, and this lesson asks for the rate. Second, thickness arrives in millimetres and the formula runs on metres — a 4 mm pane entered as 4 makes the window a thousand times thicker than it is, in the flattering and wrong direction.