Heat does one thing in a solid: it walks from hot to cold, and it walks faster through some materials than others. Fourier's law prices that walk. $P = \dfrac{k A \Delta T}{d}$ — read aloud P equals k A delta-T over d, with $\Delta$ the Greek letter delta, said “delta” and meaning “the change in”.

Name every letter before it goes to work. $P$ is the rate of heat flow in watts — joules per second, a power, not an amount. $k$ is the material's thermal conductivity in W/(m·K): how many watts cross one metre of it per kelvin of difference. $A$ is the area the heat crosses, in square metres. $\Delta T$ is the temperature difference between the hot face and the cold face, in kelvin — and because it is a DIFFERENCE, a kelvin and a degree Celsius are the same size, so no conversion is ever needed here. $d$ is the thickness along the heat's path, in metres.

Read the shape of it. Three things on top all push heat through: a leakier material, a bigger wall, a colder night. One thing underneath fights back, and it is the thickness. That single fact is the entire insulation industry.

Two habits. First, $k$ is quoted per METRE, so a thickness written in millimetres must be divided by a thousand before it enters — 140 mm is 0.140 m, and forgetting that makes a wall look a thousand times better than it is. Second, push the units through the rearrangement every time: $\mathrm{\frac{W}{m\cdot K} \times m^2 \times K \div m}$ leaves watts, so the arrangement survives. Units guide, they never confess — if they refuse to cancel into the answer's units the rearrangement is wrong, no appeal, but units that do work out never prove you right.