Rockwool vs Celotex - Which is better?

Rockwool and Celotex are two of the most popular insulation brands on the market - each with standout benefits as to why you would choose one over the other. So with this being said, which is the better choice?

To answer this, you need to assess what you’re insulating and why you’re insulating.

Generally speaking, Celotex boards offer a greater thermal performance whilst being significantly thinner. On the other hand, Rockwool has superior fire-proofing, acoustic and moisture-resistant properties.

In this guide, we’ll break down everything you need to know about Rockwool and Celotex so you can make a decision that’s right for you.

Brief Overview

We know not everyone has the time to read through in-depth guides, so to make your life easier, we’ve put together a quick side-by-side comparison comparing the two.

Understanding the materials

Rockwool is made from mineral wool - a material created by melting volcanic rock (basalt and recycled slag) and spinning it into fine fibres. This creates a dense, fibrous material that’s naturally fire-resistant and offers excellent sound absorption. 

Celotex is made from PIR (Polyisocyanurate) foam, which has a closed-cell structure that’s highly effective at preventing heat transfer. The boards feature a foil facing on one side to improve thermal performance and act as a vapour barrier.

Thermal Performance

Celotex outperforms Rockwool when it comes to thermal efficiency. With a much lower Lambda value compared to Rockwool, you can achieve target U-values with much thinner boards. 

• Celotex thermal conductivity rating: 0.022W/mK 
• Rockwool thermal conductivity rating: 0.035W/mK

In simple terms, you’ll need a thicker layer of Rockwool to achieve the same insulation value as a thinner Celotex board. This makes Celotex a better choice when you’re working with limited space, like insulating a loft conversion or internal walls.

Fire Safety

If fire safety is required, Rockwool is what you need. 

This is because it’s naturally non-combustible and achieves a Euroclass A1 fire rating - the highest possible rating. Rockwool does not contribute towards the spread of fire, nor produce toxic smoke, and can withstand high temperatures over 1,000°C. 

Rockwool is often used as a fire barrier in cavity walls, floors and ceilings for this very reason. 

Celotex, being a foam product, is combustible and achieves a Euroclass rating of F - the worst rating for combustibility. This means it will burn and emit toxic gases if exposed to flames. In fact, building regulations often require it to be covered with plasterboard or another fire-resistant layer if installed. 

However, just because it has a Euroclass F rating, does not mean it’s unsuitable for home use, rather it just means it will not stop the spread of fire. 

As Celotex boards are incredibly thin, you can use them in conjunction with other fire-resistant materials to achieve both the target thermal performance and fire-resistant properties. 

Acoustic performance

Rockwool excels as a sound insulator, reducing noise transmission and improving the overall soundproofing with ease. Its dense, fibrous structure absorbs sound waves, reducing airborne noise (like voices and music) and reverberation within spaces, making it ideal for partition walls, floors, and ceilings where noise control is important.

Celotex is not suitable for use as a sound insulator. Whilst excelling as a thermal insulator, its rigid foam structure doesn’t effectively block or absorb sound.

Moisture & Vapour Performance

Rockwool performs exceptionally well at managing moisture and vapour within a building. Its dense yet breathable structure is highly vapour permeable, which allows moisture to pass through and escape instead of becoming trapped. 

At the same time, its water-repellent fibres prevent liquid water from being absorbed, which makes it ideal for use in areas prone to humidity or condensation.  

Celotex, on the other hand, falls short in this area. Its rigid foam structure has high water vapour resistance, which effectively acts as a vapour barrier. Without careful detailing and ventilation, this can lead to moisture becoming trapped within the wall or roof, increasing the risk of condensation, mould and long-term damage.
 

Installation

Rockwool is supplied as large rolls or slabs, which both require a large cavity to fit into. They can be easily cut down and fitted into irregular spaces - but it should be done carefully using a sharp knife and wearing protective gear to avoid itchy fibres getting on your skin. 

Celotex is supplied as boards - these are lightweight, rigid and easy to handle. Its slim profile and ability to be cut to size make it ideal for fitting into straight, flat cavities. However, its rigidity limits its adaptability when dealing with uneven surfaces or awkward spaces, where achieving a perfect fit can be more challenging.

Which is cheaper?

While cost is an important factor, the choice between Celotex and Rockwool shouldn’t be based on price alone, as they serve very different purposes.

If your primary goal is achieving high thermal performance, Celotex is the more cost-effective option. Its excellent thermal efficiency and incredibly thin profile mean you can meet target U-values using far less material and space. 

However, if you’re insulating for acoustic performance or fire resistance, Rockwool is the better choice. Although it comes at a higher upfront cost, it delivers superior sound absorption and natural fire resistance, making it the right solution for you. 

If you need any help with your insulation project, please reach out to us and we’ll be happy to help. UKFR is a family-run business with extensive flat roofing & insulation knowledge to help you find exactly what you need. 

Speak to us today.