Terrace Heat Insulation: Why Top Floors Get So Hot, and How to Fix It
Ask anyone living on a top floor in India and they will tell you: the ceiling stays hot long after sunset. In hospitals, offices and data centres, that same heat shows up as a bigger cooling bill. This guide explains why, and how heat-insulating concrete fixes it.
Why the top floor is the hottest
A concrete terrace sits in direct sun all day and its surface gets far hotter than the air.
Concrete is a good conductor of heat (thermal conductivity around 1.5 W/m·K), so the heat travels down through the slab.
The slab stores that heat and releases it into the room below for hours, well into the night.
The fix is to stop the heat before it reaches the slab, with a layer that resists heat flow.
Why brickbat coba does not solve it
Brickbat coba is laid on most terraces for slope, not insulation. Brick and mortar are dense and conduct heat well, so coba mainly adds weight (about 226 kg/m² at 115 mm) without much resistance to heat.
How heat-insulating perlite concrete works
Perlite is a volcanic mineral that expands when heated into light granules full of tiny air cells. Mixed with cement, it makes a concrete that resists heat flow. The lighter the mix, the better it insulates:
Insulating grades go down to about 300 to 400 kg/m³
Thermal conductivity as low as 0.08 W/m·K, close to 19 times lower than dense concrete
Because it is a concrete, it does the slope and the insulation in one layer, and gives a hard base for the waterproofing.
The result on a typical terrace
In our worked comparison for a typical RCC terrace, published in our product brochure, perlite insulating concrete let 63% less heat into the room below than brickbat coba. The ceiling surface ran at about 30.7 °C instead of 37.2 °C.
Perlite concrete vs XPS and PU foam boards
XPS boards and PU foam insulate well per millimetre. When you compare, match the thermal resistance: work out the perlite concrete thickness that gives the same resistance as the board, then compare the full build-up. A board system still needs a screed to slope and a protection layer on top; perlite concrete gives slope, insulation and a hard base in one pour, is non-combustible, and has no joints between boards.
Where it is used
Terraces of residential towers, hospitals, IT parks, malls and data centres
Roofs over air-conditioned floors, where every degree saved is cooling saved
Steel deck roofs and deck slabs in industrial buildings
Frequently asked questions
How much cooler will my top floor be?
It depends on the existing roof, the thickness and density chosen, and the building's use. Send us your roof drawing and we will calculate the heat reduction for your terrace.
Can I add it on an existing terrace?
Yes. The old coba is removed and perlite concrete is laid to slope. Because it is lighter than coba, the roof usually ends up carrying less load than before.
Does it need waterproofing on top?
Yes. Perlite concrete forms the insulating, sloped base. A waterproofing membrane or coating is laid over it as usual.
Is it fire safe?
Perlite is non-combustible. Source: Perlite Institute (USA) published technical information; Keltech Energies Ltd is a member.
Related reading: What is perlite concrete? · Brickbat coba alternative
About Perlite India. Perlite India designs and lays perlite lightweight insulating concrete for roofs, floors, sunk fills and podiums, with 3M+ sq.ft laid across India. Supported by the Perlite Construction Division of Keltech Energies Ltd, a member of the Perlite Institute (USA).
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