A bare concrete terrace can cross 60°C on the surface. The slab soaks that up all afternoon and gives it back to the room below well after sunset, which is why top-floor flats stay hot at midnight even when the air outside has cooled.
Most heat problems in Indian buildings come down to two things: what the roof is made of and what the walls are made of. Get those right, and the AC has far less to do.
Start With a Cool Roof
A cool roof reflects most incoming sunlight and sheds the heat it does absorb. The number to ask for is the Solar Reflectance Index (SRI). Bare grey concrete usually scores in the 30s or 40s. A good white coating can reach 100 or higher when new.
Your realistic options for a cool roof India project:
Reflective coatings. White elastomeric or acrylic paints are the cheapest fix and the easiest retrofit on an existing terrace. They need a clean, sound surface and a primer that suits the substrate.
Cool tiles. Broken China mosaic and high-albedo tiles suit terraces people actually use, since coatings scuff under foot traffic.
Insulated roof build-ups: XPS boards or PUF panels under the waterproofing slow heat moving into the slab. This is the strongest option in new construction.
Vegetation. Soil and plants cool a roof well, but they come with weight, waterproofing, and upkeep. HydroArch’s post on buildings with green roofs covers the types in detail.
Two things people get wrong. First, reflectivity falls as dust, algae, and monsoon staining build up, so plan on washing the roof yearly and recoating every few years. Second, a coating alone helps less than a coating over insulation. Treat them as a pair when the budget allows.
Policy is moving the same way. Telangana’s Cool Roof Policy for 2023 to 2028 pushes cool roofs across Hyderabad, and Ahmedabad’s heat action plan has promoted them for low-income housing. Check the latest targets and any incentives with your local authority before you specify.
Walls: What Actually Resists Heat
Heat moves through a wall at a rate set by its thermal conductivity. Lower is better. These are typical ranges, and actual values vary by manufacturer, so ask for test reports.
| Material | Approx. conductivity (W/mK) | Notes |
| AAC blocks | 0.10 to 0.20 | Light, easy to lay, needs proper plaster and waterproofing |
| Hollow clay bricks | 0.4 to 0.6 | Air pockets slow heat; good local availability |
| Fly ash bricks | 0.6 to 0.9 | Similar to clay: eco benefit, modest insulation |
| Solid concrete blocks | 1.0 to 1.7 | Cheap and strong; the weakest thermally |
| Cavity wall (two leaves + air gap) | Depends on build-up | Best results when the gap is ventilated or filled with insulation |
AAC is the clear winner on insulation alone, but it is not a cure-all. A west wall in AAC with a large unshaded window will still overheat, because the glass lets in far more heat than the wall ever will.
Glazing is the other weak point. For heat-resistant building materials in India projects with large windows, look for a low Solar Heat Gain Coefficient (SHGC) and pair the glass with external shading. Low-E double glazing costs more upfront but cuts the cooling load noticeably on west and south façades.
Exterior finish color matters too. Pale exteriors reflect more sun than dark plaster or dark stone cladding, at no extra cost.
The wider principles behind all this, such as orientation, shading, and ventilation, are covered in HydroArch’s guides on passive design strategies and climate-responsive architecture. This post sticks to the materials themselves.
Retrofitting an Existing Building for 45°C
Most buildings that will face 45°C summers are already standing. If you cannot rebuild, work in this order, since each step costs more and delivers less than the one before:
1. Roof first. Clean, repair cracks, and then apply a reflective coating. Add insulation if the roof is being re-waterproofed anyway.
2. Shade the west and south windows. External louvers, shade sails, or even a temporary pergola beat curtains and window film.
3. Lighten the exterior. Repaint dark walls in a light, high-reflectance color.
4. Improve airflow. Open a high vent or add an exhaust path so hot air can leave the top floor.
5. Upgrade glazing last. Replace glass only where shading cannot fix the problem.
Steps one to three are cheap, quick, and often enough to bring a top-floor room down by several degrees. Ahmedabad’s cool roof pilots reported indoor drops of a few degrees. Your results will depend on the building, so measure before and after with a simple data logger.
What to Do Next
Do not buy insulation, paint, or glass on a supplier’s word alone. Ask for three numbers on every product: SRI for roofs, conductivity or U-value for walls, and SHGC for glass. Then check that the specification for your roof and west façade is settled before the architect finalizes anything else. Those two surfaces decide most of your cooling bill.
Frequently Asked Questions
1. How much cooler does a cool roof make a building in India?
Results depend on the building and the product. Pilot programs in cities like Ahmedabad have reported indoor temperature drops of a few degrees on top floors. Insulated roofs generally do better than coating alone.
2. Which is better for hot weather: AAC blocks or red bricks?
AAC blocks have much lower thermal conductivity, so they resist heat better. Hollow clay bricks are a good middle option where AAC is hard to source. Whichever you choose, shade the windows and treat the roof.
3. How long does a reflective roof coating last?
Most last several years, but their reflectivity fades as dirt and algae collect. Wash the roof once a year and recoat when it stops looking bright white.
4. Can I apply a cool roof coating over an old terrace?
Yes, if the surface is clean, dry, and free of cracks and leaks. Repair the waterproofing first, or the coating will fail early.
5. Is double glazing worth the cost in India?
On large west or south-facing windows, yes. Choose low SHGC glass, and shade it from outside for the best result.
6. Does painting the outer walls white really help?
It helps modestly. A pale finish reflects more sun and costs nothing extra when you repaint anyway. It works best alongside a treated roof and shaded windows.