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Mineral Wool vs EPS vs Kingspan K5 vs Wood Fibre in Solid Wall Insulation Systems

With external wall insulation systems becoming more and more popular, in this blog we are going to discuss the 4 main types of insulation you can use within the EWI System, along with the advantages and disadvantages of each.

Mineral Wool (Rockwool) in Solid Wall Insulation Systems

Most people are familiar with Mineral Wool in the context of loft insulation – the big rolls of insulation you see in your local DIY store. The Mineral Wool used externally is a little different, it comes in batts rather than rolls, and the wool fibres themselves are very dense so the boards can be applied vertically without drooping.

We recommend the Rockwool Dual Density boards when using Mineral Wool in solid wall insulation applications – this Mineral Wool is made from molten rock and has lots of features that make it a great choice for your insulation project:

  • Rockwool is an architect’s favourite, mainly due to its breathability, which allows moisture to travel across the wall and helps dissipate dampness from in the house.
  • Rockwool is non-combustible. The European Reaction to Fire Classification (Euroclass) system is the common standard for assessing the qualities of building materials in the event of a fire and the Rockwool Dual Density insulation boards used are CE-marked to achieve a Euroclass rating of A1 – the highest rating available.
  • Rockwool is renowned for its excellent soundproofing properties. The insulations open wool structure makes it ideal for absorbing and regulating noise in both residential and commercial buildings, lasting for the lifetime of the property. 
  • Rockwool has fantastic thermal insulating properties. When attached to the exterior of a property, 110mm of our dual-density slabs will take the U-value down to 0.3, which means the building will then conform to building regulations. For the occupants, it means improved thermal comfort since the property will stay at more even temperatures and also lower energy bills.
Considerations when choosing mineral wool

There are a couple of things you might want to consider when deciding if Mineral Wool insulation is for you! Firstly, the boards are heavier than the other types of insulation that we are going to discuss, so it means a bit more work moving them around the site, and also a stronger adhesive EWI-225 needs to be used to stick these to the substrate. Also despite being a good insulator, both Kingspan K5 and EPS have better thermal efficiencies, so you need more Rockwool to achieve the same u-values.

We would always recommend using gloves when installing the Rockwool boards as the wool fibres can be itchy.

  • Premium Basecoat EWI-225
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    Premium Basecoat (EWI-225) – 25kg

    £19.36 Excl. VAT
  • EPS Insulation (1 board = 0.72m²)

    From £2.17 Excl. VAT
  • K5 Kingspan External Wall insulation Board (0.72m²)

    From £9.49 Excl. VAT

Expanded Polystyrene (EPS) in Solid Wall Insulation Systems

EPS is our most popular insulation product because it is easy to use, has good value, and offers great thermal improvements to the property. EPS itself is 98% captured air within a 2% cellular matrix, and it’s this trapped air that gives the product its fantastic insulating properties.

EPS on wall

Our EPS is also graphite enhanced (which gives it a grey colour), which further enhances its thermal performance. The insulation is available in a range of thicknesses (10mm increments from 20mm), however, if installing external wall insulation on an existing property, 90mm of insulation will normally suffice in bringing the U-value of the wall to 0.3. This is a significant improvement over the Mineral Wool.

Of all the types of insulation we recommend for external wall insulation, EPS is the most cost-effective. This is probably one of the reasons it is so popular with homeowners, but the boards are lightweight and easy to cut and fit so are popular with installers too.

The downside of EPS is its reaction to fire – according to EN 13501-1, its fire resistance is class E. This means the EPS insulation boards are combustible, although based on the fact polystyrene is a hydrocarbon, this is hardly surprising. The good news is that when used in external wall insulation systems, the EPS is completely embedded within the cement-based adhesive, meaning that the risk of the EPS coming into contact with air is minimal, which helps prevent it from combusting.

Kingspan K5 in Solid Wall Insulation Systems

Kingspan is a market-leading manufacturer of optimum, premium and high-performance rigid insulation products. The Kingspan Kooltherm K5 is their external wall insulation board, specially designed to be used with masonry walls.

The K5 insulation board contains a fibre-free rigid thermoset phenolic core which gives it unrivalled thermal performance.

Whereas 90mm of EPS is required to hit a u-value of 0.3 (required value when retrofitting to existing buildings), only 60mm of Kingspan Kooltherm K5 is required.

Therefore, for jobs where space is a premium (i.e. walkways between buildings) or the customer wants to minimise the depth of the reveal/insulation off the wall, then K5 is a great choice. Likewise, if you are trying to go above and beyond building regulations requirements, using like-for-like thicknesses of K5 (instead of EPS) will see the u-value reduce to less than 0.2, resulting in even greater energy savings within the building.

As a result of the K5 having superior thermal performance, the price for this type of insulation board is higher than you would pay for EPS at the same thickness. When using Kooltherm K5 you are also required to use the stronger adhesive to attach the boards to the substrate, as well as the base coat.

Wood Fibre in Solid Wall Insulation Systems

Wood Fibre insulation systems are the most sustainable way to insulate externally. The Pavatex is made from pine – a sustainable raw material – in the form of waste chippings, shavings and sawdust from sawmills.

Due to its unparalleled breathability, Insulation is perfect when used for a timber-framed property because it is a vapour-open building material. It’s essential for a timber building to be able to breathe to prevent dampness and maintain structural integrity. Wood Fibre Insulation will regulate the passage of water through the building structure, allowing for the diffusion of moisture and preventing any detrimental moisture build-up within the property and the EWI system.

In terms of thermal performance compared to EPS, Kooltherm K5 and Rockwool, the Pavatex does lag slightly behind. 120mm of Pavatex is required to hit the u-value of 0.3 when applying the insulation to a solid wall property, however, for some the eco-benefits of this type of insulation outweigh this.

In conclusion – comparing Mineral Wool vs EPS vs Kingspan K5 vs Wood Fibre in solid wall insulation systems

If you are looking for a good value insulation product, we would suggest looking no further than the EPS. This is what the majority of people will use (although it is limited to buildings that are 18m high).

If you are worried about fire safety, then Rockwool mineral wool is the insulation product to go for, although it is slightly more difficult to install than the other products.

If you are worried about space or have a project where every centimetre counts, then go for the Kingspan Kooltherm K5 insulation products, with their unrivalled thermal performance.

For the most sustainable insulation material, Pavatex wood fibre insulation is the answer!


7 thoughts on “Mineral Wool vs EPS vs Kingspan K5 vs Wood Fibre in Solid Wall Insulation Systems

  1. Is it possible to use foiled Celotex style boards as EWI on walls? (it is on warm deck roofs) I’m guessing the adhesive would need to be strong? Thanks

    1. No… so you can’t use foiled backed Celotex. Kingspan K5 is the only PUR board approved to be used externally as part of an EWI system. If you want to have a look at what materials you might need for installing Kingspan K5 you can take a look at our handy Buyer’s guide.


    1. Hi Jo, we’d usually recommend just sticking to one type of insulation to maintain the same thermal performance throughout. You will need to use Rockwool around things like pipes but other than that, mixing insulation types could result in cold bridges.

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