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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!>>


33 thoughts on “Mineral Wool Insulation vs Expanded Polystyrene

  1. How much extra would installers charge for the mineral wool installation and not the polystyrene? I have approx. 25m2 wall at the front of the property, and the big wall is the side wall of approx. 68m2. The back I wouldn’t like to insulate because I have a one and half storey glasshouse at the back, so don’t have much space. Appreciate any help here.

    1. Hi Eva,
      You are looking at about £35 per m2 for materials for Mineral wool where as it is £22 per m2 for Expanded polystyrene. Obviously that doesn’t take into account labour, but this will marginally higher for mineral wool just because it is a bit harder to work with (heavier and itchy!).

  2. I was just reading through this Mineral Wool Insulation vs Expanded Polystyrene article and my question is regards to weight. The mineral wool I assume is much heavier (if the wool you put in the loft is anything to go by). Does this have any detrimental impact on the structure of the property as I assume the walls are being ‘dragged down’. Any thoughts?

  3. If you are adding any substance to the envelope of your property they need to be breathable. Your home breaths – that is water crosses all surfaces, through the roof, through the floor and through the walls. If you put a water impermeable barrier then it will lead to issues down the line. Therefore we never recommend installing EPS insulation. We always recommend our clients install mineral wool. The wool is a slightly more expensive product, but the breathability is key to your home!!

  4. We had EPS installed on our house and my wifes brother reversed into it and it has made a big dent on the insulation. Does mineral wool have higher compressive strength?

  5. Please outline how you deduce expanded polystyrene boards are breathable, because in my experience old houses clad in this material become wetter and wetter and bar ripping off the cladding, the only solution is to run a dehumidifier which rather defeats the energy saving potential.
    Of course its easy to get a good exterior finish on polystyrene which is one of the reasons its so popular, and is the material of choice for the handyman builders . In fact, in Eastern Europe they use it for almost everything that needs to be hidden. I agree with Shengan breathabilty is key.

  6. I had EPS installed on my property 3 years ago under the green deal scheme. It has now cracked in a couple of places and we need to get it fixed. how do we do this?

  7. We had mineral wool installed on our new build and having moved there from an old cottage where it used to get very cold in the winter, the difference now is absolutely incredible. We barely have to put the heating on!

    1. Hi Meagan, thanks for your comment. I had 100mm of EPS insulation put on the back of my house and then covered with a silicone based render and it has made a real difference too. This was an old property (1930), but the u-value of the back wall is now less than 0.28 where as it was in the region of 2.5+ so the heat loss is just so much less!

  8. Presumably you are seeing considerably more interest in mineral wool since that fire on the tower block in London? I am interested in insulating my home, but just not sure I can face having 10cm plus added to my external walls. The windows are fairly deep set as it is and so they will be miles back by the time the insulation is added. Could you use Aerogel on external wall insulation? I am keen to get the energy savings and my energy bills are very large, just don’t want to lose light coming into my home.

  9. Very useful article. We are in the process of getting quotes for EWI and were wondering which one to go for. Since we aren’t installing it ourselves we will definitely go for the mineral wool mainly because it is non combustible.

  10. We had solid wall insulation installed on our house under the green deal home improvement fund 3 years ago. I have been desperately searching for some decent information on the insulation they used on our house since that awful fire on the Grenfall tower. From what I understand though, the system they used had a flammable cover on it and a big gap which acted like a chimney. I know that our solid wall insulation has a mineral render on top but they did use expanded polystyrene. Is there a risk here? I can see the systems used in Grenfall and my house are very different, but would be good to get some expert feedback.


  11. We had 90mm EPS attached to the back of our home and then covered with a Silicone top coat. It works very well and the house is now much much warmer in the winter months. I do have a question though, we need to put a sky dish on the wall which I guess will need to sit on the insulation. Will this be strong enough to support the weight of the sky dish or will the fixing need to anchor into the wall underneath. Is there a fixing you sell that you can suggest to help with this.

    1. Hi Lisa, glad the insulation is working for you. Expanded polystyrene is a great way to increase the thermal comfort of your home and also hopefully the finish on the silicone render has made your home look great. In terms of fixing a satellite dish to the existing wall, I would strongly suggest anchoring this through to the wall – using anchor bolts or long screws that go into the brick itself. Some people use timber attached to the wall in the place of the insulation material and then attach the dish to that, but wood expands and contracts disproportionately to the render with both temperature and moisture content, which again can lead to micro cracking and water ingress behind the EWI system.

  12. What is the exact thickness of Rockwool needed to conform with building regulations when the insulation is attached to 9 inch brick?


    1. 110mm of Rockwool dual density slabs will hit a u-value of 0.29, so that will do the job! The Rockwool mineral wool insulation has a Lamda (thermal efficiency) of 0.036W/mK

  13. We had pebbledash on our property and the plasterer is currently here removing the it, but apparently it is pulling away the front face of the underlying brickwork. I am therefore worried that the mineral wool boards that are due to go on will be too heavy for the wall support. Is this something you have come across. Would we be better off with the cheaper expanded polystyrene insulation?

    1. Hi Ivana131083, I would certainly look at the EPS as it is far lighter in weight, but the mechanical fixings should anchor the insulation material very well to the wall to be honest.

    1. Hi Oriobiscuit, the rock is heated to a very high temperature and spun (very much like candy floss). The trapped air pockets created by this process are what makes the wool such a great insulator.

  14. What has the response to the fire been in London? Just wondering if more people are now buying mineral wool insulation?

  15. Great Article. Just wanted to know what thickness of EPS and Rockwool meets the minimum requirements for U-Values according to building regulations depending on their thickness. Me and my partner are considering getting external wall insulation for our house in the summer.

  16. Hi all,

    great topic

    I am bilding an extension to the back of my house, using an 100mm sfs screen(filled with 50 or 100mm insulation) with a 10mm cp board on top. I have to use a pebble dash render finish.

    What would be a recomended thickness of insulation idel for reduce cost and reduced overall thickness?
    Do I need to use a vapor barrier membrane? if yes where in the system ?

    Thank you all

  17. Hi all. Grea article. I’ve stumbled across it as I’ve got a 70s ex council home that has brick/cavity on ground floor but just some kind of terracotta cladding on the first floor and I’d like to get rid of the cladding and add some better insulation. Could we remove the clad and do EPS but keep the brick or would we have to do the whole wall?

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