Glass Wool vs Rock Wool: 7 Differences That Decide the Spec

Glass wool and rock wool are both mineral-fiber insulants, both A1 non-combustible, and both widely used in industrial and building applications. But they are not interchangeable. The wrong choice can mean melted insulation, sagging blankets, or money spent on performance you do not need.

This guide compares the two materials across the seven differences that actually matter when you write a spec. If you are deciding between a glass wool blanket and a rock wool blanket, start here.

Quick Decision Table

1. Service Temperature

Temperature is the single most important difference.

• Glass wool blanket: typically rated for –50 °C to 350 °C, with some industrial grades reaching ~450 °C for short periods. Above this, the binder begins to degrade and the fibers lose their spring.
• Rock wool blanket: typically rated for –50 °C to 650 °C, with some high-duty grades reaching ~750 °C. The basalt-based fibers have a higher melting point and retain their structure at temperatures where glass wool would slump.

If your process runs above 350 °C continuously, rock wool is the safer spec. For the exact rock-wool ceiling, see our guide on what temperature rock wool can withstand.

2. Fire Performance

Both materials are classified A1 non-combustible under EN 13501-1. Neither will propagate flame or produce significant smoke. The difference shows up under prolonged direct flame or as a fire barrier.

Rock wool's higher density and melting point make it the preferred material for fire-rated walls, fire-stop collars, penetration seals, and fire isolation zones. Glass wool provides excellent fire safety for ducts, partitions, and ceilings where the temperature stays within its service range, but it is not normally chosen as a primary fire barrier.

For a deeper look at glass-wool fire behavior, read is glass wool fireproof.

3. Density & Compressive Strength

Density follows directly from the fiber diameter and production process.

The higher density of rock wool makes it better for applications where the insulation must support its own weight over long spans, such as large steel-framed roofs or vertical wall cavities. Glass wool's lower density reduces structural load and makes it easier to cut and fit around services.

4. Thermal Conductivity

At room and moderate temperatures, glass wool usually has a slight edge.

• Glass wool: ~0.031–0.040 W/m·K at 50 °C mean temperature
• Rock wool: ~0.035–0.045 W/m·K at 50 °C mean temperature

The gap narrows as temperature rises. More importantly, rock wool keeps its conductivity stable at temperatures where glass wool would degrade. Specifying insulation by a single low-temperature conductivity value is a common mistake: always check the mean-temperature curve across the actual service range.

5. Acoustic Performance

Glass wool's fibers are finer and more uniform, giving it excellent sound absorption across mid and high frequencies. It is the usual choice for acoustic ceilings, duct liners, plant rooms, and building partitions where speech and machine noise dominate.

Rock wool absorbs sound well, especially at lower frequencies, and its higher mass improves airborne sound insulation. It is often used in cinemas, studios, and industrial enclosures where low-frequency energy is the problem.

For rigid board applications, compare glass wool board and rock wool board with the project's acoustic target in mind.

6. Water Resistance & Moisture Behavior

Both materials can be supplied with water-repellent treatment, typically reaching ≥ 98 % water repellency. Untreated, the behavior differs:

• Glass wool drains water relatively well after short-term wetting and dries faster because of its open, fine-fiber structure. However, prolonged soaking collapses the blanket and destroys thermal performance.
• Rock wool absorbs less water by mass but can hold moisture in its denser fiber matrix. Once saturated, it takes longer to dry.

In practice, both materials should be protected with a vapor barrier or cladding in wet locations. The choice between them should be driven by temperature and mechanical loads, not by small differences in water absorption.

7. Cost & Installation Speed

Glass wool is generally the lower-cost option, both in material and labor for large areas. Long rolls can be unrolled quickly across roof decks and ductwork. Rock wool is more expensive per square metre and often supplied in boards or shorter rolls that require more cutting. The trade-off is durability and temperature headroom.

How to Choose: A 3-Step Framework

Use this sequence when writing the spec:

1. Check the temperature. If continuous service is above 350 °C, eliminate glass wool unless the hot face is protected by a higher-temperature material.
2. Check the load and span. If the insulation must carry its own weight over a wide cavity or roof span, favor rock wool's higher density.
3. Check acoustic and budget priorities. For maximum sound absorption at the lowest cost, glass wool wins. For fire barriers and high-temperature duty, rock wool wins.

Common Mistakes

• Specifying glass wool on a 500 °C steam pipe — the binder burns out and the blanket compacts.
• Using rock wool for light acoustic ceiling tiles — heavier, harder to handle, and more expensive than necessary.
• Ignoring facing and cladding — both materials need protection from weather and liquid water, regardless of water-repellent treatment.
• Comparing only material price — installation labor, longevity, and reline frequency often outweigh the per-metre cost difference.

Bottom Line

Glass wool and rock wool solve different problems. Glass wool is the lighter, more acoustically absorbent, lower-cost choice for temperatures up to about 350 °C. Rock wool is the higher-temperature, higher-density, fire-barrier choice for service up to 650–750 °C.

At Rosewool we manufacture both glass wool blankets and boards and rock wool blankets, boards, and pipes since 1982, with ISO 9001, CE, and SGS certification. Contact our team for a spec matched to your temperature, acoustic, and fire requirements.