Passive Cooling Design: Keeping Homes Cool Without AC

Indian summers are getting harder to live through, and the default answer is an air-conditioner in every room. It works, at a price: high electricity bills, a peak load the grid struggles with on the hottest evenings, hot exhaust air pushed into streets and neighbouring flats, and homes that become unbearable the moment the power fails. Many new houses make the problem worse by design — flat concrete roofs baking all day, large unshaded windows facing west, walls of solid concrete or thin blocks, and rooms with one window and no way for air to leave.

Passive cooling is the set of design decisions that keep a building cool without, or with much less, mechanical cooling. It is not a single product and it is not nostalgia. It is physics: stop solar heat from getting in, let the heat that does get in find its way out, and move air across the body so that people feel comfortable at higher temperatures. Traditional Indian buildings did all three, and the same principles, combined with modern materials such as insulation boards, reflective coatings, efficient glass and BLDC fans, can make a modern house or flat far more comfortable for far less energy.

This article explains how a house gains heat, how climate changes the strategy, and how to apply shading, roof treatment, wall materials, ventilation and fans in the right order — with dimensions, the relevant Indian codes, and indicative October 2026 costs. It is written for people building a house and for people improving a flat they already own, because most of these measures can be added later, and the cheapest of them cost less than a single air-conditioner.

How a house heats up

A building gains heat in five ways. The roof receives the most intense sun for the longest time; on a single-storey house or a top-floor flat it is usually the largest single source of heat, and an uninsulated concrete slab can stay hot well into the night, radiating heat down onto the people sleeping below. Walls, especially those facing east and west, absorb low-angle morning and afternoon sun. Glass lets solar radiation straight in, where it is absorbed by floors and furniture and re-emitted as heat that cannot pass back out through the glass — the greenhouse effect. Hot outside air enters through open windows in the heat of the day and through gaps and cracks. And people, cooking, lights and appliances add internal heat.

Passive cooling deals with these in order of cost-effectiveness. Shade first, because sun that never reaches the building never needs to be removed. Then resist the heat that does arrive — reflective and insulated roofs, better walls, better glass. Then remove heat — ventilation when the outside air is cooler than the inside, especially at night. Finally, cool the people rather than the building, with air movement from fans. Air-conditioning, if it is still needed, comes last and runs for fewer hours at a lower load.

Climate decides the strategy

India does not have one climate, and passive cooling is not one recipe. The National Building Code of India 2016, Part 11, and the Energy Conservation Building Code classify the country into five climate zones: hot-dry, warm-humid, composite, temperate and cold. In a hot-dry climate such as much of Rajasthan and parts of Gujarat including Ahmedabad, days are very hot, nights are cooler and the air is dry — heavy walls that store heat by day and release it at night, shaded courtyards and evaporative cooling all work well. In a warm-humid climate such as Surat, Mumbai, Chennai or Kolkata, the day–night temperature swing is small and humidity is high — air movement is the key, heavy mass helps less, and evaporative cooling adds moisture to air that already holds too much. Composite climates such as Delhi's combine hot-dry summers, humid monsoons and cool winters, and need a balance.

What traditional buildings knew

Pre-industrial Indian buildings had no choice but to cool passively, and their methods remain the best textbook. Courtyard houses — the havelis of Rajasthan, the pol houses of Ahmedabad, the nalukettu of Kerala — used a shaded open court as a reservoir of cooler air and as a chimney to draw warm air up and out. Thick walls of stone, brick or earth delayed the afternoon heat until evening. Jaalis broke sunlight into small patches while letting air through, as in the screens of Gujarat's mosques and the facade of the Hawa Mahal. Stepwells such as Rani ki Vav and Adalaj gave a cool, shaded space beside water many metres below the hot ground. Verandahs, chhajjas and deep eaves kept rain and sun off walls; in Kerala, steep tiled roofs over deep overhangs dealt with both monsoon and sun. Colonial bungalows added tall rooms, high ventilators and wetted khus screens. Aaditya Interior's articles on havelis, pol houses, stepwells, the jaali and colonial bungalows cover each of these in detail; the principles they share are the ones this article applies.

Step one: shade before anything else

Shading is the cheapest and most effective cooling measure, and it must be outside the glass. A curtain or blind inside a window stops glare, but the solar heat has already passed through the glass and stays in the room. An overhang, a fin, a jaali, an external roller blind, a verandah or a tree stops the sun before it reaches the glass. The design rule is that the shading device should block direct sun during the hot months and hours, while admitting daylight and, ideally, low winter sun in climates that have a winter.

Orientation decides the device. South-facing windows are the easiest: the sun is high in the sky through the hot months across most of India, so a horizontal overhang of modest depth shades them well. East- and west-facing windows are the hardest, because the morning and afternoon sun is low and comes in almost horizontally under any overhang; these need vertical fins, a jaali or perforated screen held off the glass, external blinds, or a verandah. North-facing windows receive little direct sun, but in the parts of India south of the Tropic of Cancer the summer sun rises and sets north of east and west, so small side fins help. The simplest overall strategy is to keep glass on the east and especially the west to a minimum, and to put the largest windows on the north and south.

The depth of a horizontal overhang can be worked out from two numbers: the height H from the bottom of the overhang to the window sill, and the vertical shadow angle — the angle of the sun in the plane of the wall section — for the hottest months and hours at that site and orientation. The overhang depth D equals H divided by the tangent of the shadow angle. A sun-path chart for the latitude, or any of the free sun-path tools and building-energy software, gives the angle. The worked examples in Fig. 2 show how sensitive the depth is: for a 1500 mm high window, a shadow angle of 70° needs an overhang of about 550 mm, 60° needs about 870 mm, and 45° needs 1500 mm — which is why a single overhang works on the south but a verandah, fins or screen is needed on the east and west.

Step two: a cool, insulated roof

For a top-floor flat or a single-storey house, the roof is the most important surface in the home. A bare RCC slab absorbs most of the sun that falls on it, heats up through the day and keeps radiating heat into the rooms below until long after sunset. Two treatments work, and they work best together. A cool roof surface — a white or very light-coloured coating, tile or membrane with high solar reflectance — reflects much of the sunlight and keeps the slab surface far cooler than dark concrete or a dark waterproofing membrane. Insulation — a rigid board such as extruded polystyrene (XPS) or polyurethane foam (PUF) laid over the waterproofing, under a protective screed and tiles — slows the heat that is still absorbed.

The Bureau of Energy Efficiency's residential energy code, the Eco-Niwas Samhita 2018 (Part I: Building Envelope), sets a maximum thermal transmittance for roofs of 1.2 W/m²K. An uninsulated 150 mm RCC slab is far above that, typically around 3 W/m²K or more; 40–50 mm of XPS or PUF over the slab brings it comfortably below the limit. Several Indian cities and states — Ahmedabad's heat action work is the best known example, and Telangana has adopted a cool-roof policy — have promoted reflective roofs as a public-health measure against heat waves. Over-deck insulation must be designed with the waterproofing: insulation boards go above the membrane in most details, with a screed laid to falls and tiles or a reflective coating on top.

Step three: walls, glass and materials

Walls matter most on the east and west, and in any home where the walls receive direct sun for many hours. Two properties count. Thermal resistance — how well the wall resists heat flow — is improved by low-conductivity materials such as autoclaved aerated concrete (AAC) blocks, by cavities, and by insulation. Thermal mass — how much heat the wall stores — delays heat flow, which is helpful in hot-dry climates with cool nights, and less helpful in warm-humid climates, where a heavy wall that has absorbed heat all day keeps radiating it into the bedroom at night. Colour is the third lever: a light exterior paint reflects far more sun than a dark one, and an exterior repaint in a light colour — at about ₹17–26 per sq ft of paintable area for weather-shield emulsion — is one of the cheapest thermal upgrades a west-facing home can buy.

The Eco-Niwas Samhita measures the combined performance of walls and windows (excluding the roof) through the residential envelope transmittance value, RETV, and caps it at 15 W/m² for the composite, hot-dry, warm-humid and temperate climate zones. In practice, compliance is reached by a mix of lower-conductivity walls, shading and sensible window sizes — especially limited unshaded glass on the east and west. For large windows, glass with a low solar heat gain coefficient (SHGC) and double glazing reduce heat gain, but they are not a substitute for external shading, which stops the heat before it reaches the glass at all.

The values in the table are approximate, for comparison only: actual U-values depend on the specific product's declared conductivity, plaster thickness and workmanship, and should be calculated for any project that needs to demonstrate compliance. The practical message is simple: in a hot climate, an east or west wall of 200 mm AAC, painted light and shaded where it can be, performs much better than solid concrete or a plain brick wall in direct sun.

Step four: let the heat out — ventilation

Ventilation cools a building only when the outside air is cooler than the inside. In the heat of an Indian summer afternoon that is often not the case, and opening every window lets hot air in. The skill is timing: keep windows and shutters closed and shaded during the hottest hours, and open the house fully when the outside air cools — in the evening, at night and in the early morning. This is night flushing, and it is most effective in buildings with some thermal mass that can be cooled overnight and then absorb heat slowly the next day.

Two mechanisms move air through a building. Cross-ventilation uses wind pressure: air enters through openings on the windward side and leaves on the leeward side. It works best when rooms are one or two rooms deep, when openings are on opposite or adjacent walls, and when the outlet is at least as large as the inlet — a smaller inlet with a larger outlet increases the air speed through the room. The stack effect uses buoyancy: warm air rises and leaves through high openings, drawing cooler air in through low ones. A staircase with openable windows at the top, a double-height living room with clerestory windows, or a ventilated shaft can act as a chimney even when there is little wind. Both depend on openings that can be left open safely — with grilles, mesh and rain protection — at night.

Fans: cool the people, not the room

Moving air cools the body by speeding up evaporation of sweat and heat loss from the skin, so a person under a ceiling fan feels comfortable at a higher temperature than in still air. Research on Indian buildings — notably the India Model for Adaptive Comfort (IMAC), developed by researchers at CEPT University, Ahmedabad, from field studies in Indian offices and homes — has shown that people in naturally ventilated buildings, with fans and operable windows, accept a wider range of indoor temperatures than older international standards assumed. Ceiling fans are therefore part of passive design, not a fallback. Brushless DC (BLDC) ceiling fans typically draw around 28–35 W at full speed, roughly half of a conventional induction fan, and BEE star-labelled models make comparison straightforward. A fan in every room, centred over where people sit or sleep, with the blades at least 2.4 m above the floor, is the most cost-effective comfort upgrade in most Indian homes. When air-conditioning is used, running a ceiling fan with it allows a higher thermostat setting for the same comfort.

Evaporative and earth cooling

In hot-dry climates, evaporation is a powerful cooler: water absorbs heat as it evaporates, which is why khus tatties, courtyard fountains and desert coolers work in Rajasthan and north Gujarat. In warm-humid climates the same technique simply adds humidity and makes rooms feel stickier; in Surat or Mumbai, air movement is a better investment than an evaporative cooler. Earth-air tunnels — pipes buried a few metres deep, where the ground stays close to the annual average temperature, through which air is drawn into the building — have been used in some Indian institutional buildings; they need careful design for condensation, cleaning and fan energy, and are rarely practical for a single flat.

A costed plan: new house and existing flat

Passive cooling is cheapest in a new house, where orientation, window placement, overhangs and the roof build-up are designed from the start. But much can be done in an existing flat. The table orders measures by the value they usually give for the money, with indicative October 2026 costs. The effect of each measure depends on climate, orientation and the existing building, so the figures describe cost, not guaranteed savings.

Where passive cooling is going

Passive cooling is moving from the margins to the mainstream for three reasons. First, heat: more frequent and severe heat waves have led Indian cities and states to prepare heat action plans, following national guidance from the National Disaster Management Authority, and reflective roofs and shaded streets are now part of public-health policy. Second, energy codes: the Eco-Niwas Samhita sets envelope performance for homes, ECBC covers commercial buildings, and some states are bringing these into their building rules; green-building ratings such as GRIHA and IGBC reward passive design. Third, tools: daylight and thermal simulation that once needed specialists can now be run on a single room, and sun-path tools are free on any phone.

New materials are arriving too. Phase-change materials that absorb heat as they melt at a set temperature are being built into boards and plasters. Radiative cooling coatings, which reflect sunlight and emit heat to the sky, are moving from laboratories into products. Insulation boards, high-performance glass and BLDC fans are already mainstream and becoming cheaper. None of these replaces the basic order of shade, resist, ventilate and move air; they make each step more effective. The house of the near future will probably still have an air-conditioner — but it will need it for weeks rather than months, and at half the load.

Rules for a passively cooled home

• Shade first and shade outside the glass: overhangs on the south, fins, jaali or blinds on the east and west.
• Keep west-facing glass to a minimum; put the largest windows on the north and south.
• Make the roof reflective and insulated — aim for a roof U-value at or below the Eco-Niwas Samhita limit of 1.2 W/m²K.
• Use lower-conductivity walls (such as AAC or cavity brick) on sun-exposed east and west faces, and paint them light.
• Plan every main room for cross-ventilation, with outlets at least as large as inlets and air paths through doors and transoms.
• Add high-level openings — a stair chimney, clerestory or vents — so warm air can escape by the stack effect.
• Close and shade in the afternoon; open up and flush the house with cooler air in the evening and at night.
• Fit an efficient BLDC ceiling fan in every room, centred over where people sit and sleep.
• Match the strategy to the climate: thermal mass and evaporation in hot-dry zones, air movement and light construction in warm-humid ones.

Frequently asked questions

How can I keep my house cool without AC in summer?

Shade windows from outside, especially on the west; coat the roof with a reflective cool-roof paint or lay light tiles; keep windows closed and shaded in the afternoon and open them in the evening and at night; use ceiling fans in every room; and paint exposed walls a light colour. In a new house, add roof insulation, AAC walls on the east and west, and windows placed for cross-ventilation.

Does cool roof paint really work?

A white or highly reflective roof coating reflects much of the sunlight that a dark or bare concrete roof absorbs, so the roof surface stays much cooler and less heat reaches the room below. It is most effective on top-floor flats and single-storey houses. Coatings weather and get dirty, so they need cleaning and recoating every few years; combining a cool surface with insulation gives the best result.

Which wall material keeps a house cool?

Lower-conductivity materials such as AAC blocks resist heat flow far better than solid concrete or plain brick walls. In hot-dry climates, heavy walls with night ventilation also help by delaying heat. In warm-humid climates, lighter, well-insulated and shaded walls with good ventilation work better than heavy mass. Light exterior colours help in every climate.

How deep should a window overhang (chhajja) be?

Divide the height from the overhang to the window sill by the tangent of the sun's vertical shadow angle for the site and orientation. As a rule of thumb, south-facing windows need an overhang of roughly 0.3–0.6 times the window height, extended about 300 mm past each side. East and west windows cannot be shaded well by an overhang alone and need fins, a jaali or an external blind.

Is cross-ventilation useful in a humid city like Surat or Mumbai?

Yes — in warm-humid climates, air movement is the most important passive strategy, because the day–night temperature swing is small and humidity limits evaporative cooling. Plan openings on opposite walls, keep the outlet at least as large as the inlet, and add ceiling fans to keep air moving when the wind drops.

What is RETV in the Eco-Niwas Samhita?

RETV, the residential envelope transmittance value, measures how much heat the walls and windows of a home (excluding the roof) let in, accounting for their materials, area, orientation and shading. The Eco-Niwas Samhita 2018 caps it at 15 W/m² for the composite, hot-dry, warm-humid and temperate zones, and separately limits the roof U-value to 1.2 W/m²K.