When it comes to insulating the external walls of your property, choosing the right insulation material is crucial. The two most popular options in the UK are Mineral Wool and Expanded Polystyrene (EPS). Both offer excellent performance, but each comes with its own set of advantages depending on your project needs, budget and building type.
In this updated guide, we’ll break down the key differences between EPS and Mineral Wool insulation to help you make an informed choice.
What is Expanded Polystyrene (EPS)?
EPS is a lightweight, rigid insulation material made from polymer and expanded with steam to form closed cells. This structure gives it:
-
Excellent thermal insulation
-
High moisture resistance
-
Ease of handling and installation
It’s commonly used in External Wall Insulation (EWI) systems due to its affordability and strong performance.
What is Mineral Wool?
Mineral Wool (also known as Rockwool) is manufactured by heating volcanic rock and spinning it into fibres, creating a wool-like texture. These fibres are then compressed into rigid slabs. Mineral Wool offers:
-
Superior fire resistance
-
Excellent acoustic performance
-
High vapour permeability (breathability)
-
Strong support for renders
It is ideal for projects where fire safety or breathability is a priority.
Thermal Performance: U-Values Compared
| Material | Thickness | Typical U-Value (W/m²K) | Resulting Wall U-Value |
|---|---|---|---|
| EPS (Graphite) | 100mm | 0.032 | 0.28 |
| Mineral Wool | 100mm | 0.035 | 0.30 |
EPS offers slightly better thermal performance per thickness, making it a more efficient insulator in terms of heat retention.
Cost Comparison: EPS vs Mineral Wool
EPS is a more cost-effective option overall. Here's a breakdown based on a 100mm thick EWI-075 Silicone Render System:
| Component | Price & Coverage | EPS Cost/m² | Mineral Wool Cost/m² |
|---|---|---|---|
| 100mm Insulation Board | EPS: £9.98/m² Mineral Wool: £22.87/m² |
£9.98 | £22.87 |
| EWI-225 Premium Adhesive | £19.36 (25kg) ~5m² coverage |
£3.87 | £3.87 |
| Fibreglass Mesh | £47.90 (50m² roll) = £0.96/m² | £0.96 | £0.96 |
| EWI-333 Silicone Primer | £69.10 (20kg) ~60–70m² = ~£1.00/m² |
£1.00 | £1.00 |
| 075 Silicone Render | £69.99 (25kg) ~9–10m² = ~£7.00/m² |
£7.00 | £7.00 |
| Fixings (155mm) | £19.99 (100 pcs) EPS ~6/m², MW ~8/m² = £0.20–£0.25 ea |
£1.20 | £2.00 |
| Total System Cost per m² | £23.01 | £37.70 |
Note: Prices are indicative and may vary. For an instant quote, try our materials calculator.
Breathability & Moisture Resistance
-
EPS is resistant to moisture and does not absorb water, but is not fully vapour permeable. It's suitable for most masonry walls.
-
Mineral Wool is breathable and water-repellent. It allows water vapour to escape, reducing the risk of condensation – ideal for older or more breathable building fabrics.
Fire Safety
-
EPS must be properly protected behind render or cladding and requires fire breaks above two storeys.
-
Mineral Wool is Euroclass A1 fire-rated (non-combustible) and can withstand temperatures up to 850°C without releasing toxic smoke or fumes.
If your property is over 18m high or fire safety is a major concern, Mineral Wool is the preferred option.
Other Benefits
EPS Advantages:
-
Lower cost
-
Excellent thermal efficiency
-
Lightweight and easy to handle
-
Clean installation – no dust or irritants
-
Long-term performance and durability
Mineral Wool Advantages:
-
Superior fire protection (no fire breaks needed)
-
Excellent sound insulation
-
Vapour permeable for breathability
-
UV and temperature resistant
-
Fully recyclable and environmentally friendly
Final Thoughts: Which Insulation Should You Choose?
Both EPS and Mineral Wool are high-quality choices for EWI systems, and the decision comes down to:
-
Budget – EPS is more economical
-
Thermal efficiency – EPS offers slightly better U-values
-
Fire resistance – Mineral Wool is A1 rated
-
Breathability – Mineral Wool performs better
-
Acoustic insulation – Mineral Wool again comes out on top
Each project is unique. If you’re unsure, contact our technical team or use our materials calculator for a full system breakdown and quotation.