XPS and PU Foam Boards vs Perlite Concrete for Roof Insulation in India
If you ask which material has the lowest thermal conductivity, foam boards win: XPS is about 0.034 W/m·K and PU foam about 0.025 W/m·K. Our lightest perlite concrete is 0.08 W/m·K.
But nobody insulates a roof with a k-value. You insulate it with a system: the insulation, the slope, the screed that protects it, the waterproofing, and everything that happens to it over 30 or 50 years. That is where the comparison changes.
To be clear: for the same insulation, you need more thickness of perlite concrete than of foam board. To match 50 mm of XPS, you need about 125 to 130 mm of the 400 grade. On most Indian terraces that depth is already there, because the slope fill has to be built anyway. Perlite concrete simply makes that slope fill do the insulating.
The roof system, side by side
| XPS or PU boards | Perlite concrete insulation |
Thermal conductivity | 0.025 to 0.034 W/m·K | From 0.08 W/m·K (400 grade, lab-tested) |
Slope to outlets | Boards are flat; slope needs a separate fill | Slope formed in the same pour |
Protection on top | Needs a screed, usually about 50 mm | Hard surface; a thin protective finish is enough |
Joints | Many board joints, each a path for heat and water | One continuous layer |
Walkable | Not without a screed | Yes, once cured |
Fire | Combustible | Non-combustible |
Lifespan | Must stay protected from sun and water | Inorganic; does not rot or decay |
Thickness for the same insulation | 50 mm XPS | About 125 to 130 mm of the 400 grade |
Holds a screw | No | Yes |
Why the screed matters
A foam board cannot take foot traffic, tiles or the sun. So it is always covered with a sand-cement screed, which weighs about 100 kg on every square metre at 50 mm. That screed adds dead load and has no insulation value worth counting. And the board still needs a separate slope fill under or over it, which is often brickbat coba.
With perlite concrete, the insulating layer *is* the slope fill. One pour gives slope, insulation and a hard base for waterproofing.
Fire safety
XPS and PU foams are combustible. On podiums, terraces with services, hospitals and tall buildings, that is a serious design question. Perlite concrete is made from an inorganic volcanic mineral and cement. It is non-combustible.
How much insulation do you need?
The Eco Niwas Samhita (ENS) 2024 sets a roof U-value limit of 1.2 W/m²K for residential buildings. A 250 mm RCC slab alone does not meet it. A 100 mm layer of 400-grade perlite concrete brings the roof well inside that limit. Try your own build-up on the project calculator.
When boards make sense
On a flat roof that already has a good slope, where weight and fire are not concerns and a heavy screed is acceptable, boards can be a valid choice. On most Indian terraces, where the slope still has to be made and the roof must last as long as the building, a poured insulating concrete is simpler and more durable.
Frequently asked questions
Is XPS better than perlite concrete for roof insulation?
XPS has a lower k-value, but it needs a separate slope fill and a protective screed, it is combustible and it has many joints. Perlite concrete does slope, insulation and base in one non-combustible layer.
Can perlite concrete be laid over existing roofs?
Yes, over a sound existing roof, after cleaning and preparing the surface.
What is the k-value of perlite concrete?
Lab-tested by PSG TECHS COE INDUTECH (ASTM C518-21): 0.08 W/m·K at 400 kg/m³, 0.10 at 500, 0.11 at 600, 0.12 at 700, 0.19 at 900 and 0.34 at 1,200.
Related reading: Terrace and roof heat insulation in India · EPS concrete vs perlite concrete
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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