When looking to insulate the external walls of a property, the market for insulation materials is vast and often complicated. While there are many insulation brands out there that offer high performance, knowing which material is appropriate for your building type while also achieving performance is essential. We get asked a lot by our clients whether they should install Mineral Wool insulation or Expanded Polystyrene (EPS), two EWI Pro insulation materials that offer excellent insulating capabilities in tandem with their many other features and benefits. In this article we are going to discuss their key differences and hopefully help you to make the right choice!

If you would like to get a quote for any of these materials, click the picture below to try out our free materials calculator for a quick and easy full-system quotation.

EPS and Mineral Wool: what are they?

Expanded Polystyrene (EPS) is made from polymer impregnated with a foaming agent which, when exposed to steam, creates a uniform closed cell structure that is highly resistant to heat flow and moisture penetration. EPS is incredibly lightweight and easy to install while also achieving high thermal performance.

Mineral wool, made from volcanic rock that has been heated in a furnace and spun to create a cotton candy-like texture before being flattened into dual density insulation boards. It is extremely flexible, and holds in place between timber, metal frames and rafters without the need for additional fixings. This means it’s easy to install and work with. It consists of a rock mineral wool slab with a water-repellent additive. The mixture is characterised by its consistent density and high strength, which make it ideal to be used in external wall insulation systems. Thanks to its intrinsic properties, the mixture is malleable, very easy to handle and can be installed to any substrate including timber, metal frames and rafters.

Thermal Performance

The typical U-value for standard thicknesses of EPS board is 0.032 W/mK. By using 100mm of graphite EPS retrofitted on a traditional solid wall or new brickwork will take the u-value of the wall down to just 0.28w/m2k – a substantial improvement on current building regulations.

Typical U-value for standard thicknesses of Rock-470 Mineral Wool (Duo Density) – 100mm board is 0.035W/mK. By using 100mm of mineral wool retrofitted on a traditional solid wall or new brickwork will take the u-value of the wall down to just 0.30w/m2k.

Type Wall Thickness (mm) Wall U-value (w/m2k)
EPS system 100 0.28
Mineral wool system 100 0.30


Expanded Polystyrene (EPS) is one of the most economic and cost-effective options for insulating your home. For a Silicone Silicate render system with EPS of 100mm thickness the cost per pack of 6 sheets (Size: 1000mm x 500mm) is £8.33 while for mineral wool of 100mm the cost per pack of 2 sheets (Size: 1200mm x 600mm) is £14.58. The adhesive material for EPS cost £3.13 while for mineral wool cost £3.63.



RRP Price per 1m2

Mineral Wool

RRP Price per 1m2


EPS (Graphite) – 100mm




Mineral Wool – 100mm



EWI-220 Universal EPS Adhesive


EWI-225 Premium EPS Adhesive


Total Price for SiSi* Render System


Total Price for SiSi* Render System


* Silicone Silicate Render System

Breathability and Moisture Resistance

Expanded polystyrene boards are lightweight, highly resistant to moisture (but not fully water impregnable like extruded polystyrene), durable and are expected to remain effective for many years. Apart from their durability they have the advantage of being easy to work with. Refer to the data sheet link for additional information on EPS properties.

Mineral Wool is non-hygroscopic and is carefully treated to ensure maximum water repellency. Additionally, it is odourless, rot-proof, does not sustain vermin and will prevent the growth of fungi, mould or bacteria. It contains a water repellent additive to ensure that no liquid water is able to pass through the slab and reach the existing masonry during installation and construction.

Fire Proofing

Although the material itself is combustible, when properly installed, the EPS insulation is protected by the cladding or any other facing materials. Provided that there are no elements of the insulation left exposed, expanded polystyrene will have no adverse effect on either the surface spread of flame rating or the fire resistance of the wall. On applications over two storeys, it is important that Mineral Wool fire breaks are installed at each floor level.

EWI Mineral Wool insulation boards are classified as Euroclass A1 to BS EN ISO 13501-1. Moreover, this type of insulation can be used up to an operating temperature of 850°C. In the event of a fire, the Mineral Wool will emit negligible quantities of smoke and fumes. The heat emission from this product is insignificant.


Advantages of Expanded Polystyrene (EPS) over Mineral wool insulation:

  • More cost-effective option
  • More efficient by offering a superior thermal performance
  • Provides high resistant to heat flow and moisture penetration
  • Maintains its insulating properties, strength and dimensions over the years
  • Provides high resistance to compression
  • Allows a clean installation without dust or irritant fibres
  • Easy to handle and install

Advantages of Mineral wool insulation over EPS:

  • Provides superior fire resistance properties without fire stops requirements
  • More breathable material compared with EPS
  • Offers solid support for the render
  • Provides excellent acoustic insulation
  • Completely recyclable
  • UV resistant (offers the building sun protection, preventing overheating in the summer)

Both of them are long lasting and highly durable, excellent for use in any energy efficient project and able to offer a high quality insulation system to your home. They meet the minimum requirements for U-Values (ability to prevent heat leaking in our out) according to building regulations depending on their thickness. So, it is up to you to choose between expanded polystyrene (EPS-410) and mineral wool insulation (ROCK-470) depending on your design and building requirements.

To know more about the U-values you can achieve you can visit our blog: <<Getting to grips with U-Values!>>

What about Wood Fibre?

Since Wood Fibre joined the ranks of our insulation range, we’ve received many enquiries as to how Wood Fibre compares with EPS and Mineral Wool. Well, Wood Fibre is an incredibly eco-friendly insulation material that offers excellent levels of breathability (enough to compete with Mineral Wool) while also maintaining a high thermal performance. Check out our dedicated blog all about Wood Fibre and its advantages, or have a read of our other blog post ‘the best type of insulation for EWI?’ where we compare EPS, Mineral Wool and Wood Fibre side by side!

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