Passive Cooling Through Windows (India): Cool a Home Without the AC — How a window's operable area, cross and stack ventilation, night purge, external shading and low-SHGC glass keep an Indian room comfortable without air conditioning.

Sunlit Indian living room with deep chajja shading, a low operable window catching breeze and a high clerestory venting hot air

A window is the single biggest lever a homeowner has over how hot a room feels. The wall stops most heat, the roof can be insulated once, but the window does three jobs at once: it lets light in, it lets heat in, and it lets air move. Get those three balanced and a room in Chennai or Nagpur can stay comfortable for most of the year on breeze alone. Get them wrong and you buy an air conditioner to fix a window.

This guide is specifically about the window's contribution to passive cooling. The whole-building strategy (orientation, mass, insulation, roof, landscape) lives in our companion pieces Passive Cooling Strategies for Indian Homes and Passive Design by India Climate Zone. Read those for the house. Read this for the openings in its walls. For where the windows actually go, start from the planning pillar Window Placement Guide (India).

A window cools a room four ways: it moves air through (cross), it lets hot air escape upward (stack), it flushes stored heat at night (purge), and it refuses unwanted sun (shading and low-SHGC glass). The art is combining them for your climate.

The five levers a window gives you

Lever	What the window does	Key window move
Cross ventilation	Daytime breeze sweeps heat and moisture out	Inlet and outlet on opposite walls; operable not fixed
Stack ventilation	Hot air rises and leaves high; cool air drawn in low	High clerestory or vent out, low window in
Night purge	Open at night to dump heat stored in walls and floor	Secure operable openings left open overnight
External shading	Keep the sun off the glass before it becomes heat	Chajja, louvre or fin sized to sun angle
Low-SHGC glass	Cut the heat that does reach the glass	Spectrally selective or Low-E DGU

The first four cost almost nothing and depend entirely on how the window opens and where it sits. The fifth costs money. Spend on the first four first.

Lever 1 and 2: cross plus stack ventilation

Cross ventilation needs an inlet and an outlet on different walls so wind pressure on the windward side and suction on the leeward side drive air through. Stack ventilation works without wind at all: hot air is lighter, so it rises and escapes a high opening while cooler air is pulled in through a low one. The two combine beautifully.

Section through a room showing a low inlet window, a high clerestory outlet, daytime breeze arrows crossing the room, hot air rising out the top, and a separate night-purge panel with the cool night air flushing the warm floor slab

To make the window do this:

Put the inlet low and the outlet high. A clerestory or a high vent above the door lets the hottest air leave. NBC 2016 asks for openable area of at least 10 per cent of the room carpet area; for cooling you want as much of that as possible split between a low inlet and a high outlet.
Make the outlet equal to or larger than the inlet. A smaller inlet feeding a larger outlet speeds the breeze (a venturi effect) and you feel more air movement.
Choose openings that open fully. Casement, awning and louvre windows open close to 100 per cent of their area; a sliding window opens only about 50 per cent. For cooling, the operable type matters more than the glass.
Keep the path clear. A tall wardrobe or a closed internal door kills the flow. Transom vents over doors keep stack ventilation alive even when doors are shut.

This is the cooling angle. The airflow physics and the inlet-outlet sizing maths are in our sibling guide Window Design for Cross Ventilation (India); use the Cross Ventilation Analyzer to test a layout.

Lever 3: night purge

By 4 pm in summer your walls and floor slab have soaked up the day's heat and are radiating it back into the room. Night purge uses the cool night air to flush that heat out, so the mass starts the next day cool rather than hot.

Time	Window state	Why
10 am to 5 pm (peak heat)	Closed, shaded, low-SHGC	Keep hot air and sun out
After sunset	Open inlet and outlet	Let night air sweep through
Overnight	Secure operable openings left open	Cool the slab and walls
Early morning	Close before the heat builds	Trap the coolness

Night purge only works in climates with a big day-to-night temperature swing (hot-dry and composite zones such as Jaipur, Nagpur, Delhi). In warm-humid coastal air (Mumbai, Chennai, Kochi) the night is barely cooler and very humid, so you keep windows open for breeze, not purge. The window detail that makes night purge practical is security with airflow: louvres behind grilles, awning windows that shed light rain, or restrictor-fitted casements you can leave ajar safely.

Lever 4: external shading sized to the sun angle

The cheapest air conditioning is shade. Stopping sun before it strikes the glass is far more effective than tinting the glass after, because once solar energy is inside it becomes heat that stays. The shape of the shade follows the sun angle, which depends on which wall the window is on.

Sun-path shading diagram: a south window with a horizontal chajja blocking the high midday summer sun while admitting the low winter sun, an east and west window with vertical fins or louvres against the low morning and afternoon sun, and a sized overhang projection note

Wall	Sun character	Right shading device
South	High midday sun in summer, low in winter	Horizontal overhang or chajja, sized to latitude (blocks summer, admits winter)
West	Low harsh afternoon sun, most heat and glare	Vertical fins, louvres, deep reveals, external blinds, deciduous trees
East	Low gentle morning sun	Vertical fins or operable louvres; overhangs alone do not catch low sun
North	Almost no direct sun	Minimal shading needed; safe for larger glass

A horizontal chajja works on the south because the summer sun is high; the same chajja is nearly useless on the west, where the afternoon sun comes in flat under it. West and east windows need vertical shading or operable louvres. This is why the worst overheating in Indian homes is almost always a large unshaded west window. For the orientation logic in full, see the planning pillar Window Placement Guide (India).

Lever 5: operable area and low-SHGC glass

You cannot cool with a sealed picture window. Fixed glass gives light and view but zero ventilation, so every room you mean to cool passively needs a real operable opening. Use fixed glazing for the view and pair it with a casement, awning or louvre for the air.

When sun cannot be fully shaded (west walls, large openings), cut the heat at the glass with low-SHGC glazing. SHGC, the solar heat gain coefficient, measures how much solar heat the glass lets through; lower is cooler. A Low-E double glazed unit can hold a low SHGC while keeping useful daylight. The Eco-Niwas Samhita ties this to your window-to-wall ratio: the more glass you add, the lower the VLT and SHGC the code demands (and it caps the wall envelope at RETV of 15 W/m2). Pick glass with our companion Best Glass for Hot Climates in India.

Combining the levers by climate

Decision matrix mapping four climate zones (hot-dry, warm-humid, composite, temperate) against which levers dominate, with cross, stack, night-purge, shading and glass cells highlighted or muted per zone

Climate (example)	Lead levers	Window strategy
Hot-dry (Jaipur, Nagpur)	Shading plus night purge plus mass	Smaller openings, deep shade, low-SHGC, open at night
Warm-humid (Chennai, Kochi)	Maximum cross plus stack ventilation	Large operable area, louvres, shaded, breeze day and night
Composite (Delhi, Lucknow)	Seasonal switch	Shade and purge in summer, open and admit sun in winter
Temperate (Bengaluru, Pune)	Cross ventilation plus light	Generous shaded operable windows, gentle year round

A worked example: a hot west bedroom in Nagpur

A 12 ft by 12 ft bedroom (carpet area about 13.4 m2) with one large 6 ft by 4 ft sliding window on the west wall. It bakes from 3 pm.

Before-and-after plan and section of the bedroom: before shows one big unshaded west sliding window; after shows the west glass reduced and shaded with vertical fins, a new low east casement inlet, a high vent or clerestory outlet, and night-purge and low-SHGC labels

Operable area check. NBC asks for openable area of at least 10 per cent of 13.4 m2, about 1.34 m2. A 6 ft by 4 ft slider (about 2.2 m2) that opens only half gives roughly 1.1 m2 effective; the single wall also gives no cross flow. So the room fails on both counts.
Cut and shade the west glass. Reduce the west opening, fit vertical fins or external louvres (a chajja will not stop the low afternoon sun), and specify low-SHGC Low-E glass on whatever west glass remains.
Add a cross path. Put a low casement inlet on a cooler wall (north or east) and a high vent or clerestory outlet near the ceiling on the opposite side, so daytime breeze crosses and hot air leaves high.
Night purge. Leave the inlet and a secure louvre open overnight to flush the slab; close everything by 9 am.

The room now cools itself for most of the year, and the AC, if ever needed, runs briefly against a much smaller heat load. Cost-wise, the fins, the extra casement and the clerestory are a fraction of a lifetime of cooling bills.

Quick do and avoid

Do	Avoid
Inlet low, outlet high, on opposite walls	One window on one wall (no cross flow)
Casement, awning or louvre for full opening	Relying on a half-opening slider for cooling
Shade sized to the wall's sun angle	A horizontal chajja on a west window
Night purge in hot-dry and composite zones	Sealing the house shut at night when air is cool
Low-SHGC glass where sun cannot be shaded	Large unshaded west picture window

References

BIS Guide for Using NBC 2016: https://www.bis.gov.in/wp-content/uploads/2022/08/Booklet-Guide-for-Using-NBC-2016.pdf
IS 3362, natural ventilation of residential buildings: https://law.resource.org/pub/in/bis/S03/is.3362.1977.pdf
Eco-Niwas Samhita 2018 (BEE/ECBC): https://ecbc.in/econiwas.html
BEE ENS Residential Code, Building Envelope: https://beeindia.gov.in/sites/default/files/Residential%20Code_Building%20Envelope_Draft_rev4.pdf