Kingspan vs Rockwool - Which is better?

Kingspan and Rockwool are arguably the two biggest insulation brands out there, with both being relied upon across homes all across the UK. 

The question on many people’s minds is often, which is the better one? Unfortunately, there isn’t a definitive answer - instead, you need to focus on what you’re insulating and why you’re insulating it. 

This is because the brands are made from different materials - Rockwool is made from mineral wool, whereas Kingspan has products made from both PIR and phenolic foam. 

Brief Overview

Before we jump into more detail, we’ve put together a quick overview comparing the two brands.
 

Understanding the materials

Kingspan and Rockwool are made from different materials. 

Kingspan

Kingspan has two types of insulation boards: those made from PIR foam and those made from phenolic foam.
Those made from PIR foam have a closed-cell structure that’s highly effective at preventing heat transfer. These boards typically feature a foil facing on one side to improve thermal performance and can act as a vapour barrier (although not all the time). 

Kingspan Kooltherm

Kingspan’s Kooltherm product range is made with a phenolic core instead, which offers the lowest (and best) thermal conductivity for insulation boards. These also produce less smoke and toxic gas when exposed to fire compared to other rigid foam boards. 

Rockwool

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 an insulation product that’s fire-resistant, sound-absorbent and water-repellent.

As it’s made from volcanic rock, it is heavier and denser than Kingspan, but this is what gives Rockwool its superior acoustic and fire protection properties. 

Thermal Performance

When it comes to thermal performance, Kingspan offers much greater thermal efficiency, with a significantly lower Lambda value compared to Rockwool. This allows you to achieve target U-values with much thinner boards. 

• Kingspan PIR thermal conductivity rating: 0.022-0.027 W/mK
• Kingspan phenolic thermal conductivity rating: 0.019W/mK
• Rockwool thermal conductivity rating: 0.035 - 0.039W/mK

This is why we recommend Kingspan, or other insulation brands like Celotex, Ecotherm or Recticel, particularly in tight spaces such as loft conversions and internal walls, over Rockwool if thermal efficiency is the only criterion you require, as you would require a significantly thicker layer of Rockwool to achieve the same insulation value as a Kingspan board. 

If you want to achieve your target U-values for energy efficiency, whilst also improving the fire safety, sound insulation, and durability, you can use both in conjunction with each other. 
 

Fire Safety

Every Rockwool product is suitable for fire safety, but only some Kingspan products are suitable for fire safety. 

Rockwool

Rockwool is naturally non-combustible and achieves a Euroclass A1/A2 fire rating - the highest possible rating. It 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. 

Kingspan

Kingspan has many product ranges, with the majority made from PIR foam and achieving a Euroclass rating of F (the worst fire rating possible). However, there are some products which achieve a higher rating than F. 

• Kooltherm:
  K15: C fire rating
  K103: C fire rating
  K118: B fire rating
• GreenGuard: F fire rating
• K-Roc: A2 fire rating
• ThermaRoof: F fire rating
• ThermaWall: F fire rating
• ThermaBate: F fire rating
• Alpacore: A2 fire rating
• Optim-R: E fire rating

A1/A2 is non-combustible / very limited contribution to fire
B-F will contribute towards a fire, increasing the worse the grade becomes

Kingspan boards achieving less than an A2 fire rating are not suitable for fire safety. 

Those achieving a Euroclass rating of F are made from foam and will contribute towards the spread of fire. 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. 

The Kooltherm range, which is made from phenolic foam, produces less smoke and toxic gas when exposed to fire compared to other rigid foams.

The boards from Kingspan achieving A2 rating don’t use PIR or Phenolic cores, K-Roc uses a stone based fibre (similar to Rockwool) and the Alpacore uses a silica-based core which allows them to achieve higher fire ratings.
 

Acoustic Performance

Rockwool insulation is excellent for improving sound insulation and reducing noise transmission. Its dense, fibrous structure effectively absorbs sound waves, helping to minimise airborne noises such as voices, music, and impact sounds. This makes it a great choice for partition walls, floors, and ceilings where acoustic comfort and noise control are a priority.

Kingspan insulation, on the other hand, is not designed for acoustic performance. While it provides outstanding thermal efficiency, the rigid foam cores used in its PIR and phenolic boards do little to absorb or block sound. This is why we do not recommend Kingspan products if sound insulation is your main consideration.

Moisture & Vapour Performance

Rockwool

Rockwool insulation performs exceptionally well when it comes to managing moisture and vapour within a building. Its dense yet breathable structure is highly vapour permeable, allowing water vapour to pass through and escape rather than becoming trapped inside. 

At the same time, its water-repellent fibres prevent liquid moisture from being absorbed, making it an excellent choice for areas prone to humidity or condensation.

Please note that while Rockwool performs exceptionally well with moisture, it will still require a vapour barrier. 

Kingspan

Kingspan insulation, while highly thermally efficient, is generally far less breathable than mineral wool.

Most of its rigid foam boards, such as those in the Therma and Kooltherm ranges, have a closed-cell structure that provides strong resistance to water vapour — effectively acting as a vapour barrier. 

However, this means that moisture cannot easily escape through the material. Without an additional vapour control layer and proper ventilation, condensation can build up within the wall or roof assembly. 

The exception is Kooltherm K118, which features an integrated vapour control layer and internal dry lining, providing built-in moisture protection. Although Kingspan boards are resistant to water vapour, they should not be allowed to get wet either in storage or in use.

Installation

Rockwool insulation is supplied in large rolls or slabs, which both require a spacious cavity to fit into. They can be easily cut down and fitted into irregular spaces, but this should be done carefully using a sharp knife and with protective clothing to avoid irritation from the fibres.

Kingspan insulation is typically supplied as rigid boards that are lightweight, easy to handle, and simple to cut to size. Their slim profile makes them ideal for fitting neatly into straight, flat cavities or between framing. However, because of their rigidity, they are less adaptable to uneven surfaces or awkward spaces, where achieving a precise fit may be more difficult to achieve.
 

Which is cheaper?

While cost is always an important factor, the choice between Kingspan and Rockwool shouldn’t come down to price alone, as they inherently serve different purposes. 

If your main priority is achieving high thermal performance whilst saving as much space as possible, Kingspan will be the better option. Its excellent thermal conductivity and thin profile mean you can reach your target U-values using less material and without needing a lot of space. 

However, if you’re insulating for acoustic performance or fire safety, Rockwool is the better investment. It offers outstanding sound absorption and is naturally non-combustible, making it the safer and more durable choice in these applications.

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.