EPS vs Mineral Wool for External Wall Insulation

The two boards that dominate EWI

An EWI system is a sandwich: adhesive and mechanical fixings hold an insulation board to the wall, then a reinforced basecoat and a decorative render go over the top. The board in the middle is where the two camps form.

• EPS (expanded polystyrene) — the white or grey rigid foam board. Grey (graphite-enhanced) EPS is the standard specification today. Light, cheap, easy to cut and rasp flat.
• Mineral wool — dense slabs of spun stone fibre, ROCKWOOL being the name everyone knows. Non-combustible, vapour-open, heavier and more expensive.

Installers tend to develop tribal loyalty to one or the other. We install both, as part of certified systems including Licata render build-ups, and the near 50/50 split across our quotes reflects that: the right board depends on the wall in front of you, not on habit.

Thermal performance per centimetre

Insulation is compared by thermal conductivity (lambda, W/mK) — lower is better. Grey EPS typically sits around 0.031–0.032 W/mK; EWI-grade mineral wool slabs around 0.034–0.038 W/mK. In practice that means:

The thermal gap is real but modest. Where it bites is on tight details: beside a boundary, under a shallow eaves overhang or around a bay, an extra 10–20mm of thickness to hit the same U-value can be the difference between a tidy detail and a compromise. On an open elevation it rarely decides anything.

Fire classification — the honest version

Mineral wool is non-combustible, Euroclass A1. It simply does not burn, which is why it is the default on taller buildings and the easy answer wherever fire performance is the client's first question.

EPS is a combustible plastic. That sounds alarming until you look at how a certified system handles it: the board sits behind a reinforced render skin, and the system certificate requires fire barriers — bands of non-combustible mineral wool — at the positions the certificate specifies, typically at storey lines and around openings. Installed that way on a typical two-storey house, EPS systems have an enormous, decades-long track record across Europe.

The installer detail worth knowing: fire barriers are invisible once rendered, which makes them the easiest thing for a cut-price crew to omit. Ask any EWI quote to state where the barriers go. Ours itemise them — it is one of the reasons a fixed, itemised quote protects you.

Breathability and moisture

Mineral wool is vapour-open: water vapour passes through it easily, so a wall that takes on moisture can dry outwards through the insulation and a breathable render finish. EPS is much more vapour-closed and effectively caps the wall's outward drying.

On a dry modern cavity or post-war brick wall, that rarely matters. On an older solid-brick wall — especially one with a history of damp, or soft bricks and lime mortar that rely on evaporation — it matters a lot. This is the same logic we set out in our breathable insulation guide, and it is why our surveyor checks the wall's condition and construction before recommending a board. If you are not sure which wall type you have, start with cavity wall vs solid wall.

Weight, handling and what it does to cost

EPS boards weigh almost nothing. One fitter can carry a stack up scaffolding, cut boards with a hot wire or handsaw, and rasp the whole elevation dead flat before basecoat — which is partly why EPS elevations tend to finish so true.

Mineral wool slabs are dense and heavy. They soak up more adhesive, need more mechanical fixings per square metre, blunt blades, and the fibres mean gloves and long sleeves all day. All of that is labour, and labour is money: the installed premium for mineral wool comes as much from handling as from the board price.

For context, our EWI projects have a median price of £17,300 across 170 priced quotes, with headline system rates of £130–£140/m² and finished, fully detailed jobs landing at £176–£232/m² all-in. Board choice is one of the levers that moves you within those ranges — the full breakdown is in our 2026 London price report.

Which we recommend, property by property

• Standard two-storey brick house, dry walls, budget matters: grey EPS. Best value per pound, slimmest build-up, excellent finish.
• Solid-wall Victorian or Edwardian home, any damp history: mineral wool with a breathable render. Let the wall keep drying outwards.
• Three storeys or more, or fire classification is a priority: mineral wool. A1 non-combustible ends the conversation.
• Tight boundary, shallow eaves, thickness-critical details: EPS — its better conductivity buys you slimmer boards for the same U-value.
• Noise from a busy road: mineral wool's density gives a useful acoustic side-benefit EPS cannot match.

Whichever board you choose, remember the wall is only part of the decision — our EWI vs IWI guide covers whether outside is even the right side to insulate, and what EWI does to your gutters, soffits and pipework covers the detailing either board will need.

The honest caveat: system certification beats board tribalism

Here is the part board-versus-board arguments miss. An EWI system is certified as a complete build-up — board, adhesive, fixings, basecoat, mesh, render and every accessory, tested together. A brilliant board in an uncertified mix of components is a worse wall than a modest board in a properly certified system, installed to its certificate. Most EWI failures we are asked to look at were not caused by the wrong board; they were caused by the right board installed as part of no system at all — missing fire barriers, wrong fixings, incompatible render.

So ask less “EPS or wool?” and more “whose certified system, and will you install it exactly to the certificate?” We install certified systems with both boards, with our own in-house team — TrustMark registered (2259022), PAS 2030-19 certified, 500+ installations, 10-year manufacturer warranties and a 25-year insurance-backed guarantee. If you want a recommendation for your actual wall rather than a tribe to join, get a free, fixed, itemised quote.

EPS vs mineral wool FAQs