A Passive Radiative-Cooling Roof Assembly for Heat-Wave Housing: Computed Cooling Budget and Price per Degree of Indoor Temperature Reduction

SIMULATED, NOT MEASURED. A passive radiative-cooling roof assembly for heat-wave housing — a high-emissivity sky-window coating (solar reflectance 0.95, long-wave emissivity 0.95) over a low-cost fibre-cement substrate with reflective foil underlay, plus an optional 200 kg water night-storage mass — is specified and its cooling budget computed for a 16 m2 room under dry, midlatitude, and humid clear-sky cases. Net cooling flux at peak sun with the surface at ambient temperature: 103.2 W/m2 (dry), 50.9 W/m2 (midlatitude), 4.8 W/m2 (humid). Daily-mean indoor reduction vs a dark roof: 3.75 K, 3.71 K, 3.61 K; vs an ordinary white roof the assembly adds 0.87 K, 0.83 K, 0.79 K. Assembly cost USD 18.00/m2 (USD 288.00 per room), i.e. USD 76.75-79.80 per degree of daily-mean indoor reduction vs the dark roof. Humidity limit printed in the paper: in the humid case sub-ambient daytime cooling effectively vanishes (equilibrium depression 0.4 K at peak sun). All numbers are outputs of the deposited computation script; no hardware was built or tested.