Rockwool and fibre glass wool sit next to each other on every insulation quotation in India, usually within a few rupees of one another, and they are routinely swapped for whichever is cheaper that month. On a warm water line, nobody notices. On a steam header, the substitution shows up as a hot spot within two seasons.

Start with the temperature

Materials work on an industrial site

Fibre glass wool is comfortable up to roughly 450 °C. Rockwool takes around 750 °C. Ceramic fibre goes to 1260 °C. Those numbers are the whole argument: pick the material whose limit sits comfortably above your operating temperature, allowing for upset conditions, and only then compare rates.

The mistake we see most often is a line specified against its normal running temperature rather than its maximum. A header that normally sits at 320 °C but touches 480 °C during start-up is a rockwool line, not a glass wool one, and no amount of extra thickness changes that.

Density is not the same as performance

Both materials are sold by density, and it is tempting to read a higher number as a better product. Density governs how the material behaves mechanically — whether it will hold its shape on a vertical surface, whether it will sag inside a duct, whether it will take foot traffic — more than it governs thermal conductivity.

  • 16–24 kg/m³ glass wool: light-duty building and duct wrap, horizontal runs only.
  • 48–64 kg/m³ glass wool: general HVAC and moderate process work.
  • 96–150 kg/m³ rockwool slab and LRB: hot process piping, vessels, fire-rated partitions.
The right question is not “which is denser”, it is “what is this surface going to do to the material for the next ten years”.

What happens when either one gets wet

Neither material tolerates standing water. Both lose most of their thermal value when saturated, and both will hold that water against the steel underneath. The difference is recovery: rockwool generally dries and recovers better, glass wool tends to slump and stay compressed.

This is why we spend as much of a quotation on cladding, bands, sealants and detailing as on the wool itself. A perfectly specified insulation system with a badly lapped sheet is a wet system within one monsoon.

Where the cost difference actually shows up

On a material-only comparison the two are close. On an installed comparison, rockwool pipe sections on a large-bore line are usually faster to fit than cutting slab, and glass wool rolls are faster on a large flat area than rockwool slab. The labour line moves more than the material line, which is one reason a supply-and-install quotation from one company is easier to compare than two separate ones.

If you want this decided for your own line list rather than in general terms, send it to us. We will come back with material, density, thickness, finish and a rate against each line.