Mineral wool vs glass wool

Mineral wool and fibreglass wool are two of the most common materials used in insulation, but which one is better? The answer to this is not straightforward and depends entirely on what you’re hoping to achieve.

Rock mineral wool is considered to be more thermally efficient, and its material composition makes it suitable for fireproof, moisture-resistant and sound-absorption applications.

Fibreglass, on the other hand, is more cost-effective and is seen as a good, quick option to improve your home’s thermal efficiency.

In this guide, we’ll compare the two side by side by key factor to give you everything you need to know to make the right decision.

Overview

Understanding the materials

What is mineral wool?

Mineral wool, also known as stone wool, is a material used to create a thick, dense insulation used for soundproofing, moisture resistance and thermal applications. 

It’s made by melting volcanic rock (basalt and recycled slag) and spinning it into fine, dense fibres, which are naturally fire-resistant. 

As it’s made from volcanic rock, it’s naturally durable, fire-resistant and sound-absorbent, which is ideal for both residential and commercial applications. Being made from volcanic rock also means it’s three times denser than fibreglass.

What is fibre glass wool?

Fibreglass wool is made by melting glass and spinning the resulting fibres into thin strands which are bonded together to form insulation. It’s incredibly lightweight, often available as rolls and slabs, and generally more affordable compared to mineral wool at face value. 

Thermal performance

On paper, fibreglass is slightly more effective per inch of thickness, with an R-value between 0.032 to 0.040 W/mK compared to mineral wool’s 0.035 - 0.045 W/mK. However, in reality, mineral wool is considered to be more thermally efficient - this is due to thermal drift. 

What is thermal drift?

Insulation naturally weakens over time whether due to material composition, exposure to moisture, or the building settling. This gradual decline is known as thermal drift, and it results in the R-value reducing (the measure of thermal resistance).

Fibreglass insulation is light, fluffy, and doesn’t hold its shape particularly well. Over time, it can sag or compress leading to gaps allowing heat to escape. Its thermal performance also drops significantly if it becomes damp, as moisture displaces the air pockets that are crucial to insulation. 

On the other hand, mineral wool is tough, rigid, and dense. It’s designed to bounce back to its original shape even after compression, and thanks to its rigid structure, it stays in place without sagging. It’s also naturally hydrophobic, meaning it repels water rather than absorbing it. This makes mineral wool far more resistant to thermal drift.

Moisture resistance

Mineral wool is more suitable than fibreglass for moisture protection. 

This is because it’s hydrophobic, which means it repels water. As it’s essentially volcanic rock, it won’t absorb water, sag or lose performance even if it’s exposed to a leak. Additionally, it’s naturally porous, which means it’s breathable; this means it’s good at keeping heat in, water out and vapour to escape. 

On the other hand, fibreglass being light and airy means it absorbs moisture easily. When it gets soggy, it becomes subject to mildew and rot, as well as losing all of its thermal properties.
 

Fire-resistance

Mineral wool is significantly better than fibreglass for fire protection. 

This is because mineral wool is non-combustible and does not contribute towards the spread of fire. If a fire does break out, mineral wool doesn’t burn, drip, or release toxic smoke; instead, it acts as a barrier and doesn’t contribute towards the spread of the fire. 

Fibreglass is also non-combustible, achieving a Euroclass Fire Rating of A1 or A2 (depending on product),but it is not 100% fireproof and will melt under extreme temperatures around 500-600C. 

Compared to other materials like PIR insulation, fibreglass stands tall in fire resistance; however, mineral wool is ultimately the better choice. 

Soundproofing

Mineral wool is far superior to fibreglass for acoustic performance in every application, whether used in walls, floors or ceilings. 

This is because mineral wool is incredibly dense and effective at absorbing and blocking sound waves. That density helps reduce noise transfer between rooms and floors, making it ideal for creating quieter, more comfortable living spaces.

In contrast, fibreglass is lighter, less dense, and more compressible, which means there’s more space within the insulation for airborne sound to travel through. While some speciality products offer some sound reduction properties, it’s not as effective as mineral wool is at completely blocking out sound transmission.

Shop acoustic insulation 

Installation 

Mineral wool is considered to be less itchy to install than fibreglass; however, you should always wear protective gear (gloves, a mask, and long sleeves) to ensure you no loose fibres get on your skin, in your eyes or in your lungs during installation. 

Installing mineral wool insulation is slightly more labour-intensive due to its weight and density, but it’s often easier to get a snug, professional fit. 

It’s supplied in semi-rigid slabs which can be friction-fitted tightly between studs or joists without needing staples or netting. It holds its shape well, cuts cleanly with a serrated knife, and is less likely to slump or leave gaps over time. 

If there are any small gaps when installing it, Rockwool acoustic sealant can be used to ensure the property is not at risk of fire and smoke spread. 

Installing fibreglass insulation is relatively straightforward, especially for DIYers. It comes in lightweight rolls or batts that are easy to cut with a utility knife and fit between joists or studs. However, as it’s soft and compressible, it needs to be stapled or supported to prevent sagging in walls or ceilings. 

Cost

Money is often the biggest driving factor when it comes to choosing your insulation. While upfront costs are important, they shouldn’t be the only thing you consider - you need to also consider long-term performance.

Upfront costs - Price per Square Metre

Fibreglass insulation ranges between £5-15/m²

Mineral wool insulation ranges between £5-30/m²

Long-term costs

Both fibreglass and mineral wool are great for energy savings, with potential reductions up to 50%, saving you anywhere from £300 to £500 a year (based on an average home). This being said, mineral wool is more cost-effective long-term due to its superior resistance to sagging, moisture and thermal drift. 

Which is right for me?

The choice between the two comes down to your priorities, budget and area you’re insulating. 

Mineral wool is ideal for those wanting excellent acoustic performance, the best fire resistance, and insulation that holds up well over time, especially in damp or tricky areas like walls, basements, or floors.

Fibreglass wool is ideal if you're on a tight budget and need a quick, reliable, easy-to-install solution for the short term.
 

“I’ve decided mineral wool is right for me, which products should I go for?”

If you’ve decided mineral wool is the right one for you, these are two products we recommend to customers depending on the application:

• Rockwool RWA45 - Suitable for internal walls, floors and ceilings. This is a dense, multi-purpose slab offering excellent sound-proofing and fire resistance properties. 
• Rainscreen duo slab - Suitable for external walls and ventilated façades. This features high-density outer surfaces, lower-density underside, and excellent acoustic performance. 

“I’ve decided glass wool is right for me, which products should I go for?”

If you’ve decided fibreglass is the right way to go, these are two products we recommend to customers depending on the application:

• Knauf loft roll 44: A budget-friendly, easy-to-install insulation ideal for loft applications. 
• Isover APR: A denser glass wool solution offering exceptional acoustic performance in partitions, walls and floors. 

If you need any help with your insulation project, please don’t hesitate to get in touch with us today. We’re proud to be a family-run business, and have extensive knowledge in both insulation and flat roofing.