This is the boss. No calculator — every R-value in it sums to a clean number and every reciprocal is one you already know: RSI 2 is U-0.5, RSI 2.5 is U-0.4, RSI 4 is U-0.25, RSI 5 is U-0.2. Learn those four and half of building science is mental arithmetic.

One wall, one long problem, and every answer feeds the next: stack the resistances, invert to a U-factor, take the design loss, then rule on the wall. Write each line down as you go — a film forgotten on line one walks all the way to the verdict.

And note what the last line actually asks. It does not want another number; it wants a DECISION, the way a real code check does. Watch the direction of the comparison too: a U-factor cap is a CEILING, and lower is better. The arithmetic is only there to earn you the right to make the call.

Everything in the chapter rides in this one wall. 5★ under par here is the chapter crown. Clock in.