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Mineral Wool Insulation vs Expanded Polystyrene

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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    Rockwool insulation

    Welcome to the EWI-Pro solid wall materials calculator

    Ordering the correct amount of materials for a solid wall insulation or rendering job is not always as easy as it might seem. In order to make this easier for our customers we have developed an online tool that is incredibly simple to use and produces a PDF of all the required materials.

    You can start using the EWI Store calculator by pressing the start calculating button below:

    How to use the solid wall insulation calculator

    Please press “Start Calculating”

    Question 1 – Type of Render

    – Choose the render system you are interested in; there are three options, premium, standard and eco, these relate to the top-coat/finish you want on your property.

    Premium – EWI Pro Silicone Silicate Render is the most popular finish (top-coat) especially when using the EPS or mineral wool external solid wall insulation systems. It is available in 100’s of colours (extra cost), and is breathable and hydrophobic meaning it will self-clean and is water repelling. It is a sympathetic render which is highly durable and is recommended for all older buildings

    Standard – EWI Pro Acrylic Render is a slightly cheaper finish especially when using the EPS External Solid Wall Insulation. Available in 100’s of colours (extra cost), this render will hold vibrant colours longer (without fading), however although tough against variable weather, it can crack over time and does need additional maintenance.

    Eco – EWI Pro Mineral Render is the preferred choice for installers looking to carry out work in harsher conditions (wet / cold) since it dries very quickly and is fantastic at withstanding impacts. A silicon paint is added after application of the mineral render – and it is for this reason, we recommend this render for those who like to change colour of their property on a regular basis. For installers, although mineral render is cheaper to buy, the fact it needs to be painted once the render has been applied – the increased labour costs make mineral render and SiSi render similiar in price.

    Question 2 – Amount of m2 required

    Enter the amount of system required (so the coverage in m2); this is simply calculated by multiplying the height by the width of each wall you are looking to insulated. Do not subtract doors or windows as it will give you an incorrect reading since our calculator builds in wastage into the final calculation.

    Question 3 – Type of insulation

    Next you need to specify whether you want to use mineral wool or EPS (expanded polystyrene). EPS is cheaper and offers greater thermal efficiency (when comparing like for like thicknesses) however mineral wool is fully breathable (provided mineral or SiSi renders are used). Mineral wool is also a flame retardent.

    Question 4 – Type of mechanical Fixings

    Enter the type of mechanical fixings you want, we offer two main types – metal or plastic pin. If you are using EPS insulation you can also use the premium EcoDrive mechanical fixings – these are simply screwed in, so fixing is quicker. Both the metal and plastic pin mechanical fixings require hammering. All the mechancial fixings can be bought with EPS caps to help minimise thermal bridging, however the EcoDrive have these built in.

    Question 5 – Thickness of insulation

    Thickness of insulation is up next – obviously the thicker the EPS or mineral wool the better the insulating properties. For our EPS (Expanded Polystyrene) there are three choices – 50mm,70mm,100mm. The depth of EPS is to do with u-values, new build properties currently need to be less than 0.3 W/m2k. 100mm give gives a u-value of 0.27 (the lower the better), thus saving on heat loss and more importantly bills. 100mm is always advised except where insulation has already been added to the inside of the property or external space is in short supply.

    For mineral wool we offer 2 thicknesses – 100mm and 120mm. For this type of insulation 120mm is required to achieve a u-value of 0.3W/m2k

    If buying a building a property to rent out or converting commercial buildings, councils insist on 100mm.

    Question 6 –  Number of Corner beads

    Calculate how many linear metres of corner bead you need. These are required around window and door reveals as well as the corners of the building. They provide a straight corner edge and also provide strength.

    Question 7 – Number of Stop Beads

    Stop beads are similar to corner beads providing a clean finish when butting up to doors and windows, when a point of termination is required. They help to prevent cold bridging.

    Question 8 – Number and type of starter track

    ; Measure the linear meters around the property – remember our starter track comes in 2m lengths, therefore divide the total linear metres and divide by 2 to calculate the number to put in this box. uPVC starter tracks need no spacers, no mesh and allows the wall to be uneven, due to the flexibility of the material – it also minimises cold bridging. It is for this reason that the uPVC starter track is considered the premium starter track.

    Hit Next to produce a materials shopping list.

    This can then be downloaded as a PDF and we can quickly process these orders if you are interested in getting the materials dispatched.

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      Thermal Bridges

      Thermal bridges are very common in old houses or houses with insufficient insulation. Thermal bridges cause the affected part of the construction to have a significantly higher heat transfer than the surrounding materials. This results in an overall reduction in the thermal performance of the construction. Partially insulated building elements are responsible for up to 35% of thermal losses and increased condensation that leads to mould growth.

      Thermal Bridges: 3 Types

      Thermal bridges can be classified into three main types (Leeds Beckett University: Low Carbon Housing Learning Zone):

      1. Repeating or quasi-homogeneous thermal bridges usually follow a regular pattern and are evenly distributed over an area of the thermal envelope
      2. Non-repeating or linear thermal bridges are often caused by discontinuities in the thermal envelope and occur at a specific point in the construction
      3. Geometrical thermal bridges are a result of the geometry or shape of the thermal envelope

      Στιγμιότυπο 2015-11-17, 12.41.45

      The most common parts of construction where the thermal bridges can occur are:

      • The parts of the building envelope that are exposed to the external environment and have a different thermal conductivity
      • The parts where a change in the thickness of the fabric occurs
      • The wall-floor-ceiling junctions of the building where there is a difference between internal and external areas




      To avoid thermal bridging you should ensure that your house is properly insulated in the affected areas:

      • Concrete balconies that extend the floor slab through the building envelope.
      • Areas around glazing, window frames and rooflights
      • Metal ties in masonry cavity external walls
      • Ceiling joists in cold pitched roofs
      • Ground floor joists in an insulated suspended timber ground floor
      • Timber studwork and I-beams in timber frame constructions
      • Mortar joints in an insulating block inner leaf
      • Areas round loft hatches
      • Wall/ roof junctions or wall/ floor junctions
      • At the corner of an external wall
      • Junctions between adjacent walls

      Strategies For Avoiding Thermal Bridges:

      • Apply a continuous layer of insulation (such as EPS or Mineral Wool around the external walls of your house
      • Install insulation at the outside corner of your building where the walls and floor are connected

      By reducing thermal bridges you will have saved a great amount of heat losses and energy needs of your home resulting in reduced energy bills and increased building lifespan.

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        7 Reasons to insulate your home

        Heat energy can be transferred out of your house by three basic mechanisms – conduction, convection and radiation. Un-insulated external walls are responsible for 35% of your home’s heat losses. Air leakage is also common phenomenon in un-insulated roofs with a 25% heat of losses. Most common insulation materials work by slowing conductive and convective heat flow. So, there is an imperative need to insulate your house in order to reduce the energy escaping from your property. External wall and loft insulation has a number of benefits as follows.

        1. EWI Improves energy efficiency of your home

        Heat flows from warmer to cooler spaces until there is no longer a temperature difference. Insulation helps to reduce heat losses in cold weather and heat gains in warm weather. It provides resistance to heat flow and works as a barrier that protects the indoor environment from the outside weather conditions. By installing wall and loft insulation the air leakage rate of your house will be reduced dramatically and your heating and cooling bills will be cut by up to half.


        Solid wall insulation – external walls
        Type of property Installation cost Savings per year
        Detached house (four bedrooms) Between £5,000 and £18,000 £455
        Semi-detached house (three bedrooms) £260
        Mid-terrace house (three bedrooms) £175
        Detached bungalow (two bedrooms) £180
        Mid-floor flat (two bedrooms) £145
        The figures are based on a typical gas-heated home. All data has been obtained from the Energy Saving Trust (EST).


        2. EWI Improves the thermal comfort

        Insulation helps keeping your home warm in winter and cool in summer. It improves thermal comfort by reducing the cold drafts and heat losses through the walls and roof. Your house will maintain a more stable temperature throughout the year providing a pleasant indoor environment to live in.


        3. EWI Reduces Thermal Bridges

        Thermal bridges occur in parts of the building envelope where there is insufficient amount of insulation and are responsible for up to 35% of thermal losses and increased condensation that leads to mold growth


        4. EWI Reduces the Risk of Condensation and Mould growth

        Condensation is caused by the temperature difference between the external and internal surfaces of the walls and windows of your home. EWI helps to reduce the risk of condensation and damp of walls. The top coat renders available for external wall insulation offer an 100% waterproof and hydrophobic protective layer.

        Mold growth in domestic houses is also connected with the air temperature and the percentage of humidity inside the house. By providing more insulation to your home the surface temperature of your walls will be higher preventing the risk of mold growth. Mold growth not only deteriorates your house’s condition but is also connected with indoor air quality and health being.


        5. EWI makes the construction more durable

        There is a number of different insulating renders and finishes available that are especially designed to provide a long lasting, protective barrier on your external walls. External wall insulation (EWI) fills cracks and gaps in the walls and prevents damp of the building envelope offering protection from continuous exposure to cold and rain weather such as in UK climate. Due to the highly durable materials EWI increases resistance against the elements and protects the brickwork remaining the construction at its first condition for more years.


        6. EWI makes your house look better

        External wall insulation can significantly improve the outside appearance of your home by using a wide range of renders and silicone paints, renders, bricks or tiles. Either your home is old or new, and either it has a Victorian or modern design, the external wall insulation could match excellent to you property maintaining its style and design.


        7. EWI Minimizes environmental impact and prevent climate change

        UK domestic homes are responsible for 27% of the UK’s GHG emissions, 80% of this is for space and water heating. Therefore, minimising the energy consumed for domestic heating can reduce CO2 emissions and help to prevent climate change. Less fossil fuel consumption will ensure a more sustainable environment for the next generations and less fuel poverty in the future.


        To know more about the cost of external wall insulation and the types of insulation materials you can visit our blogs:

        >>> How much does EWI cost? <<< 

        >>> Mineral Wool Insulation vs Expanded Polystyrene <<<