Supporting · Insulation materials

Insulation board types: PIR, EPS, phenolic, and wood fibre explained

The main types of insulation board used in UK homes and which to choose for your project.

Last updated: 10 May 2026·5 min read

In short

The main insulation board types: PIR (most common, good all-rounder), EPS (cheap, good for floors), phenolic (thinnest, most expensive), mineral wool (breathable, non-combustible), and wood fibre (natural, heritage-friendly). PIR is the default choice for most UK retrofits.¹

PIR (polyisocyanurate) boards

What it is: Rigid foam board with foil facings. Brands: Celotex, Kingspan, Ecotherm.

Thermal performance: 0.022 to 0.023 W/mK (good). 100mm achieves U-value of 0.22 W/m²K.

Cost: £40 to £60 per m² installed.

Best for: Roof, wall, and floor insulation. The default choice for most UK retrofits.

Pros: Good thermal performance, easy to cut and fit, widely available.

Cons: Combustible (needs fire barriers on tall buildings), loses performance if it gets wet.

EPS (expanded polystyrene) boards

What it is: White foam board (like packing foam but denser).

Thermal performance: 0.030 to 0.038 W/mK (lower than PIR). 150mm achieves similar U-value to 100mm PIR.

Cost: £20 to £35 per m² installed.

Best for: Floor insulation (good compressive strength), external wall insulation (cheaper than PIR).

Pros: Cheap, good compressive strength, doesn't lose performance when wet.

Cons: Thicker than PIR for same U-value, combustible.

Phenolic foam boards

What it is: Rigid foam similar to PIR but with better thermal performance.

Thermal performance: 0.018 to 0.020 W/mK (best). 80mm achieves same U-value as 100mm PIR.

Cost: £50 to £70 per m² installed.

Best for: Roof and wall insulation where you're tight on thickness (e.g., shallow rafters, small rooms).

Pros: Thinnest option, best thermal performance.

Cons: Most expensive, combustible.

Mineral wool (rock wool or glass wool)

What it is: Flexible batts or rigid slabs made from spun rock or glass fibers.

Thermal performance: 0.032 to 0.044 W/mK (lower than foam boards). 150mm to 200mm for same U-value as 100mm PIR.

Cost: £25 to £45 per m² installed.

Best for: Loft floor insulation, suspended floor insulation, roof rafter insulation (where you want non-combustible material).

Pros: Non-combustible, breathable, good acoustic performance, cheaper than foam boards.

Cons: Thicker, itchy to handle (wear gloves and mask), loses performance if it gets wet.

Wood fibre boards

What it is: Rigid boards made from compressed wood fibers. Natural, renewable material.

Thermal performance: 0.038 to 0.050 W/mK (similar to mineral wool). 180mm to 240mm for same U-value as 100mm PIR.

Cost: £60 to £90 per m² installed.

Best for: Heritage buildings, eco-builds, internal wall insulation on solid walls (breathable, reversible).

Pros: Breathable (lets moisture escape), natural, lower embodied carbon, good acoustic performance.

Cons: Thickest, most expensive, not widely stocked (need to order in).

Which should you choose?

Default choice: PIR boards (good all-rounder, widely available, competitive price).
Tight on space: Phenolic foam (thinnest).
Tight on budget: EPS boards (cheapest).
Fire performance required: Mineral wool (non-combustible).
Heritage building: Wood fibre (breathable, reversible).