Attic Insulation — Rock Wool or Glass Wool? What to Consider When Choosing Materials

Attic insulation is a high-impact investment. It boosts comfort, reduces heating bills and improves indoor acoustics. Choosing the right type of mineral wool — and a reliable supplier — is key to getting the full benefit. The two most common options are rock (stone) wool and glass wool. Each has distinct properties. Which should you pick for your project?

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At a glance

• 🧱 Rock (stone) wool — higher density → better acoustics, moisture handling and fire resistance (up to ~1000 °C).
• 🪶 Glass wool — lighter & more flexible → fast installation in tight areas; very good thermal performance.
• 🌬️ Both are vapour‑open — when combined with vapour‑open membranes they help avoid condensation.

Quick comparison

Where does each material work best?

Rock wool is an excellent choice for lived-in attics. It provides strong thermal and acoustic performance — ideal for bedrooms, home offices and kids’ rooms.

Glass wool, thanks to its low weight and flexibility, shines in tight or hard-to-reach spaces, e.g. narrow gaps between rafters. It cuts easily and installs quickly, which helps speed up the build.

Product examples

• ✅ Rock wool: Knauf Rock Mineral, Isover Super-Mata
• ✅ Glass wool: URSA GLASSWOOL, Isover Uni-Mata

Breathability — a “breathing” insulation

Effective attic insulation needs high vapour permeability to avoid condensation. Both rock and glass wool allow water vapour to pass, supporting natural air exchange and reducing the risk of moisture in the roof structure.

• 💡 Rock wool (e.g., Knauf Rock Mineral) has naturally higher moisture resistance, making it a strong pick for rooms exposed to temperature swings.
• 💡 Glass wool (e.g., Isover Uni-Mata) is often protected with hydrophobic coatings to limit water uptake.

Tip: Pair the insulation with vapour-open membranes and good roof ventilation to extend service life.

Fire safety

Fire resistance is a critical factor in attic design.

• 🔥 Rock wool withstands temps up to ~1000 °C, greatly improving safety in case of fire.
• 🔥 Glass wool is also non-combustible, but its high-temperature resistance is typically lower (~500 °C).

If maximum fire protection is your priority, rock wool is the safer bet.

Thermal and acoustic performance

• 🏠 Glass wool (e.g., URSA PureOne) delivers very good thermal insulation and is light, so it doesn’t overload the roof structure.
• 🔇 Rock wool has a higher density, which dampens noise more effectively — great for spaces where good acoustics matter.

Moisture resistance

Moisture is one of the main enemies of attic insulation.

• 💦 Rock wool generally copes better with humidity, helping to prevent mould and mildew.
• 💦 Glass wool, while less moisture-resistant by nature, is additionally protected to reduce water absorption.

Good attic ventilation plus vapour-open membranes are key to long-term durability.

Summary — which insulation for the attic?

• 🔹 Rock wool — better acoustic insulation and higher resistance to moisture and fire. Ideal for lived-in attics.
• 🔹 Glass wool — lighter, easier to install, thermally efficient. Great for tight, complex areas.

The right answer depends on your roof build-up and priorities. Both solutions work, provided they’re installed to spec.

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Footnote: external reference¹