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

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.