# Rock Wool Insulation — Density, Fire and Acoustics

> Rock wool is mineral fibre spun from molten rock and bound into slabs, rolls or loose fill; it insulates thermally, absorbs sound and is classified A1 non-combustible under EN 13501-1. Its density has to be specified for each application, and its non-combustibility is why it is used for cavity barriers and ventilated facade cavities.

## What it is

**Mineral fibre spun from molten rock**, bound into slabs, rolls or loose fill.
It insulates thermally, absorbs sound, and does not burn.

That last property is why it appears in positions where nothing else is
acceptable.

## Specification at a glance

| Property | Typical specification |
| --- | --- |
| Standard | IS 8183, bonded mineral wool |
| Thermal conductivity | Approximately 0.035 — 0.040 W/mK |
| Fire class | A1, non-combustible, to EN 13501-1 |
| Melting point | Above 1,000°C |
| Density range | 48 — 150 kg/m³ by application |
| Facing | Plain, tissue faced, or foil faced |

## Density by application

| Application | Density |
| --- | --- |
| General thermal infill | 48 — 64 kg/m³ |
| Ventilated facade slab | 80 — 100 kg/m³ |
| Cavity barriers | 100 — 150 kg/m³ |
| Acoustic partition infill | 48 — 80 kg/m³ |
| Roof deck board | 140 — 160 kg/m³ |

## Where it is the only correct answer

- **Cavity barriers** at every compartment line in a ventilated facade
- **Insulation in a ventilated cavity** on high-rise and high-occupancy
  buildings
- **Fire-rated partitions and shafts**, where the tested system names it
- **Acoustic separation** between tenancies and around plant rooms
- **High-temperature plant insulation**, where organic foams cannot go

## Thermal trade-off, stated honestly

Rock wool insulates less well per millimetre than PIR — roughly 0.037 against
0.022 W/mK — so reaching the same U-value needs a thicker build-up. On
envelopes where depth is constrained, that is a real cost. Many industrial
projects therefore run PIR across general areas and rock wool at fire-critical
lines, which is a legitimate and common specification.

## Where projects go wrong

- **Density unspecified.** The single most common error.
- **Facade slabs too soft**, so they slump in the cavity over time and leave
  gaps at the top of each storey.
- **Cavity barriers substituted** for a cheaper general-purpose slab.
- **Left exposed to monsoon rain** before the outer skin is fixed.
- **Compressed to fit.** Compressing the slab reduces its thermal performance.

## Standards referenced

Bonded mineral wool requirements under **IS 8183**. Reaction-to-fire
classification to **EN 13501-1**. Opaque assembly U-value under **ECBC 2017**.

## Frequently asked questions

### What density of rock wool should be specified?

It depends entirely on the application. Roughly 48 to 64 kg/m³ suits general thermal infill, 80 to 100 kg/m³ suits ventilated facade slabs where the board must hold its shape and resist wind scour in the cavity, and 100 to 150 kg/m³ suits cavity barriers and acoustic partitions. A specification that names rock wool without a density is unenforceable, because performance changes completely across that range.

### Why is rock wool mandatory in some facade positions?

Because it is mineral and non-combustible, classified A1 under EN 13501-1. In a ventilated cavity the insulation sits in the air path, so a combustible insulation behind a combustible panel creates the exact assembly behind catastrophic facade fires. Cavity barriers at compartment lines are rock wool for the same reason: they must survive the fire they are stopping.

### Does rock wool lose performance when wet?

Yes, while it is wet. Water displaces the trapped air that does the insulating, so a saturated slab performs poorly until it dries. It does not rot or lose structure, and it dries out, which is why it is tolerated in ventilated cavities where a drained path exists. It should never be installed wet or left exposed to rain before the outer skin goes on.

## Sources

- [EN 13501-1](https://standards.cencenelec.eu/ords/f?p=CEN:105) — Reaction to fire classification
- [IS 8183](https://standards.bis.gov.in/website/published-standards/department-wise) — Bonded mineral wool
- [ECBC 2017](https://beeindia.gov.in/) — Building envelope — opaque assembly U-value

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Source: https://hagerstone.com/materials/rock-wool-insulation