Now we cash the R-value in. $\dot{Q} = \dfrac{A \, \Delta T}{R_{tot}}$ — read aloud Q-dot equals A delta-T over R-total. $\dot{Q}$ is the heat loss rate in watts (the dot means “per second” — it is a rate, not a quantity of energy), $A$ is the assembly's area in square metres, $\Delta T$ is the inside-to-outside temperature difference in kelvin, and $R_{tot}$ is the total R-value in m²·K/W.

Drop the area and you get the same idea per square metre: $q'' = \dfrac{\Delta T}{R}$ — q double-prime equals delta-T over R, the heat flux, in W/m². The two primes are the trade's way of writing “per unit area”. Flux is what a thermographer measures and what a designer compares between assemblies, because it does not care how big the wall is.

The design day is where the numbers come from: an indoor set point, usually 20 to 22 °C, against the outdoor temperature the local code says to size for. Watch the sign — 21 °C indoors against −14 °C outdoors is a 35 K difference, not 7. The minus sign in front of the outdoor figure is the single most expensive character in a heat-loss spreadsheet.

Estimate before you compute, every time. A code wall runs somewhere near ten watts a square metre on a cold day, so 40 m² of it is a few hundred watts — about a hair dryer's worth, running all winter. Hold that scale in your head and a slipped decimal announces itself before the calculator does.