Guide
EPS or mineral wool: which EWI board?
EPS is lighter, cheaper and slightly thinner for the same U-value, and is the usual choice on low-rise houses. Mineral wool is non-combustible, vapour-open and better for noise, so it's specified on flats, taller buildings and solid walls that need to breathe.
Comparison
| EPS | Mineral wool | |
|---|---|---|
| Conductivity | ≈0.031–0.038 W/mK | ≈0.034–0.040 W/mK |
| Fire | Combustible board (low-rise) | Non-combustible |
| Vapour | Fairly closed | Open |
| Weight | Very light | Heavy |
| Noise | Little benefit | Good |
| Guide cost | £105–£145 per m² | £125–£170 per m² |
When the rules decide for you
For buildings with a storey 11 metres or more above ground, guidance points to non-combustible external wall materials, and the external walls of relevant buildings 18 metres and over in England can't contain combustible materials at all. Scotland restricts combustible cladding above 11 metres. In those cases mineral wool is the answer.
When it's your choice
On a typical two-storey house, choose EPS for cost and slimness, or mineral wool for breathability and acoustics. Grey graphite EPS is worth considering where eaves are shallow.
Questions
Is graphite EPS better than white?
It insulates roughly 15–20% better for the same thickness, at a small extra cost.
Does mineral wool need more fixings?
Yes, it's heavier, so the system specifies more mechanical fixings.
What about phenolic?
Phenolic is the thinnest option for a given U-value, useful where space is tight, but it's combustible and costs more.