Isover vs Rockwool

Isover and Rockwool are two of the most recognised names in the insulation industry, with both brands being used and trusted by professionals all across the country. So, which is better between the two?

To answer this, we need to consider what you’re insulating and why you’re insulating it. 

Generally speaking, Rockwool is often the better choice due to its fire-proofing, acoustic performance and moisture resistance. On the other hand, Isover is better tailored to those on tighter budgets. 

Brief Overview

Not everyone has the time to read an in-depth comparison guide, so here is a quick summary of both brands.

Understanding the materials

The biggest difference between the brands is the material they use; Isover is made from glass mineral wool, whereas Rockwool is made from rock-based fibres. 

Isover

Isover insulation is made from glass mineral wool, manufactured primarily from recycled glass and sand, where the fibres are bound together to create an incredibly lightweight, flexible roll & slabs. 

This structure traps air, which provides excellent thermal and acoustic insulation while remaining breathable.

Rockwool

Rockwool is made from mineral wool - a material created by melting volcanic rock (basalt and recycled slag) and spinning it into fine fibres. The result is an insulation product that’s naturally fire-resistant, sound-absorbing, and water-repellent.

It’s heavier and denser than Isover, but that density is what gives Rockwool its superior acoustic and fire protection properties.

Thermal performance

There are two things to bear in mind when it comes to thermal performance: thermal drift and the purpose of insulating. 

Both brands have a good thermal conductivity rating:

• Isover thermal conductivity: typically 0.032–0.044 W/mK
• Rockwool thermal conductivity: typically 0.035–0.040 W/mK

On paper, Isover would appear to have a slightly better thermal performance per millimetre of thickness; however, when you consider thermal drift, Rockwool performs better.
 

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).

Isover 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, Rockwool 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 Rockwool far more resistant to thermal drift and the better choice for thermal performance. 
 

Why are you insulating your home?

This is an important question to ask yourself as the answer will help you know what products you need to buy. 

The main reason people insulate their homes is to improve thermal performance; however, there are other reasons, which both Isover and Rockwool lend themselves better to, such as acoustic performance or fire resistance. 

If you’re insulating your home just for thermal performance, it would be wiser to use a different brand with a better thermal conductivity rating, as that would allow you to achieve target U-values with much thinner boards. Some of these include: 

• Celotex insulation (thermal conductivity up to 0.022W/mK)
• Ecotherm insulation (thermal conductivity up to 0.22W/mK)
• Kingspan insulation (thermal conductivity up to 0.019W/mK)
• Recticel insulation (thermal conductivity up to 0.022W/mK)

*For reference, these are previous thermal conductivity ratings: 

• Isover: typically 0.032–0.044 W/mK
• Rockwool: typically 0.035–0.040 W/mK

Using two insulators alongside each other

The brands mentioned previously — Celotex, Ecotherm, Kingspan, and Recticel — are made from PIR insulation, which is exceptionally thin yet highly effective for improving thermal performance. 

This narrow thickness allows them to be used in conjunction with Isover or Rockwool, allowing you to achieve your target U-values for energy efficiency while also enhancing fire safety, sound insulation, and durability.
 

Moisture performance

Rockwool is the superior choice for moisture resistance. This is because it’s made from stone fibres, which are hydrophobic, meaning they repel water. It’s also vapour-permeable, meaning trapped moisture can escape instead of accumulating inside the structure, making it highly effective in humid environments like basements, external walls, or flat roofs.

Isover, on the other hand, doesn’t handle moisture well. This is because it’s made from fibreglass, which absorbs moisture easily. Once the insulation gets soggy, it will likely lose all of its thermal properties, as well as lead to mildew and rot. 
 

Fire resistance

For fire performance, Rockwool is the better option. 

Both brands achieve a Euroclass A1 fire rating, meaning they do not contribute towards the spread of fire or toxic smoke. However, unlike Rockwool, Isover is not 100% fireproof and will melt under extreme temperatures. 

This is why Rockwool is the better option. As it’s made from volcanic rock, it does not melt and can withstand extremely high temperatures over 1,000°C. This is why it’s used in applications where fire safety protocols are in the highest demand, such as partition walls and ceilings.  

Acoustic performance

Both brands perform exceptionally well for soundproofing; however, Rockwool has the edge.

Rockwool, being made from stone wool, has a dense and fibrous structure, which makes it exceptional at absorbing sound waves. This results in a dramatic reduction in both airborne noise (voices, music) and impact noise (footsteps, vibrations),and is commonly used in recording studios, apartment blocks, and internal partitions.

Compared to Rockwool, Isover does offer a strong acoustic performance. The flexible and lightweight structure of the insulation effectively reduces noise transmission in stud walls, ceilings, and floors. This is often more than enough for residential applications; however, in high-demand environments, Rockwool is the better option. 

Sustainability

Whilst sustainability doesn’t have an impact on the performance, it understandably plays a role in many customers' minds. 

Isover is manufactured from up to 82% recycled glass at a facility using 100% renewable electricity. Their product packaging is also entirely recyclable. For more information, please visit Isover’s website. 

Rockwool is manufactured from natural volcanic rock, which is naturally abundant and recyclable. For more information, please visit Rockwool’s website.
 

Installation

Rockwool

Rockwool is considered less itchy to install than Isover; however, it’s still important to wear protective gear such as gloves, a mask, and long sleeves to prevent loose fibres from contacting your skin, eyes, or lungs. 

Because Rockwool is slightly heavier and more dense, installation can be a bit more labour-intensive, but it’s often easier to get a snug, professional fit. 

It’s supplied in semi-rigid slabs that can be friction-fitted tightly between studs or joists without the need for staples or netting. These hold their shape well, cut cleanly with a serrated knife, and are less likely to slump or leave gaps over time. 

Isover 

Installing Isover is relatively straightforward, coming in lightweight rolls or batts that are easy to cut with a utility knife and fit between joists or studs. 

Where it differs from Rockwool is that the material is soft and compressible, which means it will require stapling or otherwise supporting to prevent sagging in walls and ceilings.

Additionally, Isover is made from fibreglass, which is why it’s incredibly important to wear protective equipment, as loose glass fibres can easily cause skin irritation if exposed. 
 

Cost

The cost of the project is often the biggest deciding factor in choosing your insulation. While it’s important to consider the upfront costs, the long-term performance is going to have the biggest impact. 

Upfront costs - Price per Square Metre

• Isover insulation ranges between £5-20/m²
• Rockwool insulation ranges between £5-30/m²

Long-term costs

Both brands are great for saving money on your energy bills, with possible reductions up to 50%, which could be anywhere from £300 to £500 a year (based on an average home). This being said, if you have the space for it, mineral wool is going to be more cost-effective in the long-term due to its superior resistance to sagging, moisture and thermal drift.
 

Which is right for me?

As mentioned at the start, the choice between the two comes down to what you’re insulating and why you’re insulating it.

Rockwool is the right choice for those wanting the best acoustic performance possible, indestructible fire-resistance, and a structure that holds form over time (especially if exposed to damp or installed in walls, basements or floors).

Isover is the right choice for those looking to save as much money as possible and need a quick, reliable solution in the short term.

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.

“Rockwool is the right choice for me”

If Rockwool is the brand you need, these are two products we recommend to customers:

• Rockwool RWA45 - Suitable for internal walls, floors and ceilings. This is a dense, multi-purpose slab offering excellent soundproofing and fire resistance properties. 
• Rockwool Hardrock - Suitable for a wide range of flat roof applications. This is a dual density, non-combustible insulation board featuring excellent thermal and acoustic properties 

“Isover is the right choice for me”

If Isover is the brand you need, these are two products we recommend to customers: 

• Isover APR: A denser glass wool solution offering exceptional acoustic performance in partitions, walls and floors. 
• Isover Cavity Wall Slabs: A highly dense and durable slab featuring a water repellent binder, ideal for external masonry cavity walls. These are available as 0.032W/mK and 0.036W/mK options.