Heat insulating concrete for Indian roofs: tested k-values, thickness and build-up
Updated: 3 days ago
If you are specifying insulation for a roof in India, most of what you will be offered is a petrochemical foam board. Heat insulating concrete is a different approach, and this page sets out what it actually does, with the tested figures rather than adjectives.
What heat insulating concrete is
Perlite concrete is made by mixing expanded perlite aggregate with cement, or with cement and sand, along with an air-entraining admixture. The aggregate is a volcanic mineral expanded at about 900 degrees Celsius, until each grain is mostly trapped air. That trapped air is what resists the heat.
Finished densities run from about 300 to 2,000 kg per cubic metre, against roughly 2,400 for conventional concrete. The lighter the mix, the better it insulates and the less load it puts on your slab.
The construction-grade perlite aggregate used in the systems described here is Kelite, a product by Keltech Energies. It is expanded at two plants in different parts of the country, at Vishwasnagar near Udupi and at Garamsur near Nagpur. Perlite India specifies, engineers and executes the application.
The tested k-values
Thermal conductivity is tested to ASTM C518-21, at a mean temperature of 0 degrees Celsius. These are the measured values, grade by grade, where the number in each grade name is its density in kg per cubic metre:
Kelite 400: 0.0865 W/m.K
Kelite 500: 0.1025 W/m.K
Kelite 600: 0.1150 W/m.K
Kelite 700: 0.1254 W/m.K
Kelite 900: 0.1952 W/m.K
Kelite 1200: 0.3447 W/m.K
Densities between these grades sit on the same curve, but those values are interpolated rather than measured. When we give you a figure we will tell you which of the two it is, and the test reports are available on request.
Read the list in the right direction. A lower density gives a lower k-value and therefore better insulation, and it also puts less dead load on the slab. A higher density gives you more strength and a harder wearing surface, and insulates less. The grade is chosen from what the application needs, not from the best-looking number on the list.
How the thickness is decided
We will not tell you that this beats XPS or PU foam board on conductivity, because it does not. Those are lower-k materials and any supplier claiming otherwise is not reading his own test report.
What we do instead is work backwards from the thermal resistance your design already calls for. Thermal resistance is simply thickness divided by conductivity, so once you fix the resistance you need, the required thickness of insulating concrete follows from the k-value of the grade you choose.
Then the two systems can be compared honestly, on the things that actually differ: dead load on the slab, the finish you are left with, fire behaviour, and how many separate trades have to come to site.
Where it sits in a roof build-up
The insulating concrete is laid over the structural slab and finished to the fall you need. One material gives you both the slope and the insulation, instead of an insulation board plus a separate screed laid over it to create the slope.
The waterproofing system sits above or below as your consultant specifies. The insulating concrete is not a substitute for it and we do not offer it as one.
Minimum section thickness is about 25 mm, so very thin details are ruled out.
What else it gives you
Dead load: an insulating grade at 400 kg per cubic metre against roughly 2,400 for conventional concrete
Finish: poured in place, levelled with an aluminium straight edge, setting to a hard, smooth, monolithic surface
Toppings: tiles, vinyl, wood or carpet go on directly, and drywall tracks screw straight into it, which most lightweight concretes will not hold
Zero settlement
Non-combustible, which is a property of the mineral itself, per published Perlite Institute data
No shelf life, and nothing in it to break down twenty years into the building's life
Some acoustic benefit alongside the thermal performance
Where it is not the right answer
The scope is non-structural. Roof insulation, floor screeds, sunken and podium fills, insulating plaster on the internal face of external walls. It is not a structural lightweight concrete and should not be specified as one.
For sunken fills it is a lighter filler replacing brickbat fill. It does not settle, but it makes no waterproofing or leak-prevention claim, and your waterproofing stays exactly as specified.
And if your reason for moving away from brickbat coba is purely commercial rather than thermal, the grade to look at is the 1200 kg per cubic metre screed, not a light insulating grade.
A delivered example
Jupiter Hospital has an insulating roof screed in Kelite concrete, specified, engineered and executed by Perlite India.
Across projects our application work runs to more than 3 million square feet of lightweight concrete, in hospitals, data centres, airports, IT parks, malls, and commercial and residential buildings. Much of that was delivered before the Kelite tie-up, using other material, so we do not badge all of it as Kelite. The application engineering is ours. The product history is not.
Get a build-up checked
Send us the roof section you are working on and we will work out the thickness and density that match the thermal resistance your design needs, along with the specification wording and a BOQ format you can use directly.
Write to info@theperlite.in or call +91 98600 63626.
Kelite, a product by Keltech Energies. Specified, engineered and executed by Perlite India.
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