# Rock Wool vs Glass Wool: Temperature, Acoustics and Where Each Belongs

> Both are mineral wool insulations made as fibres bound into batts or slabs. Rock wool is spun from molten rock, is available at higher densities and withstands much higher service temperatures. Glass wool is spun from molten glass, is lighter and cheaper for a given thermal performance, and is common where temperatures are moderate.

## Specify by property, not by name

The single most useful thing on this page: **"rock wool" and "glass wool" are
categories, not specifications.** Two products from the same category can have
very different density, thermal conductivity, service temperature and fire
classification.

A usable specification states density, thickness, declared thermal conductivity,
maximum service temperature, facing and required fire classification. A
specification that says only "50 mm rock wool" leaves every one of those open.

## Side by side

| Factor | Rock wool | Glass wool |
| --- | --- | --- |
| Made from | Molten rock | Molten glass |
| Density range | Higher densities available | Generally lower |
| Service temperature | Substantially higher | Normal building range |
| Thermal performance per cost | Good | Usually better |
| Acoustic mass | Higher at high density | Good at high density |
| Water repellency | Generally better | Compresses and slumps if saturated |
| Weight to handle | Heavier | Lighter |
| Shape retention | Holds form well | Can sag in vertical use |
| Typical use | High temperature, acoustic, dense slabs | Roofs, walls, ducts, general |

## Where each one naturally lands

**Rock wool** goes where the temperature is high or the acoustic requirement is
serious — around process equipment, in plant rooms and machine enclosures, in
partitions that must genuinely reduce sound transfer, and where a dense slab
must stay in place for decades without slumping.

**Glass wool** goes where conditions are ordinary and the requirement is thermal
— building roofs and walls, HVAC ductwork, general envelope insulation. It gets
there lighter and at lower cost, which is a legitimate advantage, not a
compromise.

## The facing changes everything

A great deal of confusion on site comes from ignoring what the insulation is
faced with.

Foil facings, tissue facings and factory claddings affect vapour control,
handling, appearance and — importantly — the fire classification of the finished
product. An unfaced mineral core and the same core with a combustible facing are
different products in a fire test.

Classification is reported under **EN 13501-1** for the complete product. When
substituting or value-engineering, the classification must be re-checked, not
assumed to carry over.

## Performance in the building, not the lab

Insulation only delivers its declared performance if it is installed to fill the
space without gaps, is not compressed below its design thickness, and stays dry.

Gaps matter more than most people expect: a small unfilled area conducts
disproportionately, and thermal bridges at fixings, junctions and structural
members can undo a significant part of the declared performance of the field
area. **ECBC 2017** sets the envelope requirements the assembly has to meet, and
that assessment is on the constructed build-up.

## Mistakes that cost real money

- **Specifying by name with no density or conductivity.** It permits anything.
- **Assuming a faced product shares the core's fire classification.** It may not.
- **Compressing insulation to fit.** Thickness is performance; squashing it
  removes the performance you paid for.
- **Using a normal-temperature product near hot equipment.** Check the stated
  maximum service temperature for the specific product.
- **Treating wet insulation as a material failure.** It is a water-ingress
  failure; replacing the insulation without fixing the leak repeats the cost.
- **Ignoring thermal bridging at fixings and junctions.** A well-insulated field
  with poorly detailed junctions underperforms on the whole-assembly figure.

## Standards referenced

Bonded mineral wool to **IS 8183**; reaction-to-fire classification to
**EN 13501-1**; envelope requirements in **ECBC 2017**; sound absorption
measurement to **ISO 354**. Required fire classification, declared thermal
values and service temperatures must be taken from the specific product's
documentation and confirmed against the project's requirements by the relevant
consultant.

## Frequently asked questions

### Which insulates better thermally?

At equivalent density and thickness they are broadly comparable, and both are specified by declared thermal conductivity rather than by material name. Glass wool usually reaches a given thermal performance at lower weight and lower cost, which is why it is common in general building insulation. Rock wool's advantages are elsewhere — temperature range, density and acoustic mass.

### Which is better for acoustics?

Higher-density products absorb and block sound better, and rock wool is available in higher densities, so it is often specified where acoustic performance is the driver — machine rooms, plant enclosures, partitions requiring real sound reduction. Dense glass wool also performs well. The important specification is the density and the tested absorption data, not the material category.

### What is the temperature difference in practice?

Rock wool tolerates substantially higher service temperatures, which is why it appears around boilers, ovens, furnaces, hot pipework and high-temperature process equipment. Glass wool is suited to normal building and HVAC temperatures. Each product has a stated maximum service temperature and that figure, not the generic material name, is what should be checked against the application.

### How do they behave in fire?

Both are mineral and non-combustible in their base fibre, and both are classified by test, commonly to EN 13501-1. The binder and any facing — foil, tissue, cladding — affect the classification of the finished product, so a faced product does not automatically share the classification of the unfaced core. Ask for the classification of the exact product and build-up being supplied, and have the requirement for your building confirmed by the fire consultant.

### What happens if they get wet?

Both lose thermal performance when wet, and both need a vapour and weather strategy. Rock wool is generally more water-repellent and recovers better after incidental wetting; glass wool is more easily compressed and slumps when saturated. Neither is a solution to a leak — wet insulation in a roof or wall is a maintenance problem that will recur until the water path is fixed.

### Which is easier to install?

Glass wool is lighter and easier to handle and cut, which matters on large areas and overhead work. Rock wool slabs are heavier but hold their shape better, which helps in vertical applications and where the insulation must stay put without sagging over time. Both require appropriate protective equipment during handling.

## Sources

- [IS 8183](https://standards.bis.gov.in/website/published-standards/department-wise) — Bonded mineral wool — specification
- [EN 13501-1](https://standards.cencenelec.eu/ords/f?p=CEN:105) — Reaction to fire classification of construction products
- [ECBC 2017](https://beeindia.gov.in/) — Building envelope — insulation
- [ISO 354](https://www.iso.org/standards.html) — Acoustics — measurement of sound absorption

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Source: https://hagerstone.com/compare/rock-wool-vs-glass-wool-insulation