Lesson 2.4Lesson 2.4 · Hot-Dry Strategies
Night Flushing
The thick wall stored the day's heat — and the only window to give it back is the cool desert night.
A sealed heavy house gets hotter, not cooler
A heavy desert house keeps you cool on day one. But seal it tight every night to "keep the heat out" and by day three the rooms are warmer than the street — the mass has filled with heat and has nowhere to dump it.
The fix is counter-intuitive: at night, when the desert drops to 24 °C, open the house up and let cool air strip the stored heat from walls and floors, so the mass starts each morning empty. Mass without a night flush is a battery you never discharge.
Open at dusk, shut at dawn. The shutter is the air-conditioner, and the clock is the controller.
The mass as a 24-hour battery
The building's mass is a thermal battery running on a daily charge-and-discharge cycle, and night flushing is the discharge stroke.
Through the hot day the house is closed and shaded; the mass charges slowly and keeps the interior cool. In the evening, as the outside air drops below the inside, you begin to open up and the mass starts to release. Through the cool night full cross-ventilation drives air across the exposed mass, discharging its stored heat to the night sky. By dawn you close the house before the day heats — and the mass is empty, ready to absorb again.
Timing is a discipline, not a gadget. Open too early, while it is still hot outside, and you let heat *in*; close too late, after sunrise, and you trap warmth. Open at dusk, shut at dawn — a daily algorithm run with shutters.
Closed and shaded by day, wide open by night. The mass must start every morning empty.
The night-flush simulator — a reusable instrument
Picture the lesson's simulator running three days side by side: a red curve for the outdoor air and a blue curve for the indoor mass. Set the night ventilation rate in air changes per hour (ACH) and watch whether the mass resets each morning or climbs.
Three presets tell the whole story. "Sealed" ratchets up — the blue line creeps higher every day because the mass never gets to discharge. "Good flush" holds steady — a strong night purge empties the mass by dawn, so day three looks like day one. "Humid night" collapses — even at high ACH the indoor line refuses to fall, because the night air is too warm to carry heat away.
This is not a one-lesson toy: the same charge-discharge logic underlies every mass-and-ventilation decision in the hot-dry and composite modules.
Sealed climbs. Good flush holds. Humid night collapses — even wide open.
Why it only works in the dry zone
Night flushing demands two things, both gifts of the hot-dry climate. First, a genuinely cool night — a temperature gap for heat to flow out into. The desert's big day-night swing provides it; Mangalore's 28 °C night does not. Second, the ability to open safely at night — which a dry, low-rainfall, secure site allows.
On a humid coast the strategy fails twice over: the night is too warm to carry heat away, and opening up invites humidity, insects and monsoon rain. This is why the four hot-dry strategies are one integrated system. Mass defers the heat, the courtyard and jaali shade and breathe, evaporation actively cools, and night flushing resets the mass — they work only as a set, and only in a climate whose night actually falls.
Three altitudes on the same idea
Read the band that fits you — or all three.
The cheapest cooling habit you own costs nothing. Through the hot day, keep windows shut and curtains drawn to hold the heat out; once the evening cools, open everything up — a fan helps — and let the night air wash through; then shut the house again before the morning heat arrives. Done on schedule, this keeps a heavy masonry house comfortable through a desert summer with no air-conditioning at all. The catch is the word schedule — the discipline is what does the work, not the building alone.
Treat night flushing as a system, not a feature. Provide secure, insect-screened operable night openings sized for genuine cross-flow across *exposed* mass — mass boxed behind plaster, panelling or furniture cannot discharge. Aim for an effective 10–30 ACH at night, and pair the flush with the courtyard (Lesson 2.2) as a cool-air reservoir and stack outlet. Automate the openings on an indoor-outdoor temperature differential where you can, and — before committing to the whole strategy — confirm the climate qualifies by checking the night minimum against the mass temperature.
Stored heat in the mass: Q = rho * c * V_mass * dT. Flush removal rate: P = rho_air * (n * V_room / 3600) * c_air * dT_air, where n is air changes per hour, rho density, c specific heat. Worked: a 40 m3 room at n = 15 ACH with dT = 6 C gives mass flow 1.2 * (15 * 40 / 3600) = 0.2 kg/s, so P = 0.2 * 1005 * 6 = 1.2 kW. Over an 8-hour night that is about 34 MJ — enough to strip a day's charge from a moderate masonry room. Double the ACH and you roughly double the rate; drop below about 10 ACH and the flush can no longer keep pace, so the indoor temperature ratchets up day on day.
“To keep a house cool in summer, keep it sealed and shaded all the time.”
Run the method yourself
Run the simulator and the maths once before crossing into Module 3.
- 1Run the "sealed" preset for three days and note the indoor peak climbing — your do-nothing baseline.
- 2Switch to "good flush" and raise the ACH until the mass resets each morning. What rate does it take?
- 3Run "humid night" and work out why it collapses even at a high ACH.
- 4For a 40 m3 bedroom, use P = 1.2 × (n × V / 3600) × 1005 × dT to estimate the heat removed over an 8-hour night, and ask whether it is enough to discharge a masonry room.
↳ Use the worksheet below to record your answers.
Take it with you
The discharge stroke of the hot-dry engine
That completes Module 2: you can read a hot-dry site and build for it end to end — size mass for the right lag, proportion a courtyard, tune a jaali, judge evaporative cooling against the wet-bulb limit, and run a night-flush cycle. The hot-dry house is India's most self-sufficient. Now we cross the country and **invert everything**. In warm-humid India — Kerala, the Konkan, the Bengal delta — the night never cools, so mass becomes a liability; the air is saturated, so evaporation is useless. The only strategy that survives the crossing is air movement. Module 3 rebuilds the toolkit for the humid coast, beginning with the move that replaces mass as the central idea: cross-ventilation.