Insulation Materials Compared: Mineral Wool, PIR, EPS, Phenolic & Sheep Wool

Summary

Choosing the right insulation material for a specific application requires balancing thermal performance, thickness constraints, fire performance, moisture behaviour, cost, and installation method. No single material is best for all applications — the right choice depends heavily on where the insulation is going and what constraints apply.

The thermal conductivity (lambda, λ) value is the headline specification — lower is better. But this figure alone does not determine which product is correct. PIR's superior thermal performance comes with lower fire resistance and sensitivity to certain building practices. Mineral wool's lower thermal performance is offset by its excellent fire resistance, acoustic performance, and compatibility with almost any construction type. Natural and bio-based insulants (sheep wool, cellulose, wood fibre) have thermal performance comparable to mineral wool but offer different vapour and moisture management characteristics.

Moisture behaviour is particularly important in building fabric applications. In walls and roofs, insulation must be positioned correctly relative to the dew point to avoid interstitial condensation. This is why the choice of insulation type is linked to the construction type, and why mixing insulant types within a build-up requires careful design.

Key Facts

• Mineral wool (glass wool) — λ = 0.035–0.038 W/mK; Euroclass A1 (non-combustible); suitable for cavities, lofts, stud walls, floor between joists
• Mineral wool (rock wool / stone wool) — λ = 0.033–0.044 W/mK; Euroclass A1 (non-combustible); higher density versions provide acoustic performance
• PIR (polyisocyanurate) rigid foam — λ = 0.022–0.023 W/mK; Euroclass B or C; commonly used in flat roofs, floors, and internal wall insulation
• EPS (expanded polystyrene) — λ = 0.030–0.038 W/mK; Euroclass E; used in cavity walls (bead blown), solid floors, external insulation systems
• XPS (extruded polystyrene) — λ = 0.030–0.038 W/mK; Euroclass E; moisture-resistant; used below DPC, inverted roofs, ground contact applications
• Phenolic foam — λ = 0.018–0.021 W/mK; Euroclass B; the highest thermal performance rigid board; used where space is at a premium
• Sheep wool — λ = 0.035–0.040 W/mK; Euroclass E; naturally moisture-buffering; used in lofts, stud walls, and timber frame
• Cellulose (blown) — λ = 0.037–0.042 W/mK; fire-retardant treated; recycled newspaper; blown into cavities and lofts
• Wood fibre board — λ = 0.038–0.050 W/mK; Euroclass E–F; used in timber frame and breathable wall systems
• Aerogel — λ = 0.015 W/mK; ultra-thin; very high cost; niche applications such as listed buildings where thickness is critical
• Building Regulations Euroclass — façade materials and materials on high-rise buildings (over 18m) must meet Euroclass A1 or A2; affects which insulants can be used in EWI systems

Quick Reference Table

Detailed Guidance

Mineral Wool (Glass and Rock Wool)

Mineral wool is the workhorse of UK building insulation. Its non-combustible classification (Euroclass A1) makes it the only acceptable insulation in many fire-rated construction applications. It is also acoustically effective, widely available, easy to cut and fit, and relatively low cost.

Glass wool (e.g. Knauf Earthwool, Isover) is lighter and easier to handle. It is the standard choice for loft insulation (semi-rigid rolls for between joists, flexible rolls for over-joist layer) and for friction-fit in timber stud walls.

Rock wool / stone wool (e.g. Rockwool, Supafil) is denser, heavier, and provides better acoustic performance. High-density slabs (30–100 kg/m³) are used in party walls, acoustic separating floors, and external insulation systems requiring fire performance. Rock wool slabs can also act as cavity barriers — their non-combustible nature prevents fire spread through the cavity in multi-storey buildings.

Limitations: Mineral wool has the highest λ values of the main insulant types, meaning it requires more thickness to achieve equivalent performance. A 270mm mineral wool loft installation achieves a U-value of approximately 0.12 W/m²K; achieving the same U-value with PIR would require only 100–120mm.

PIR (Polyisocyanurate) Rigid Foam

PIR is the most thermally efficient mainstream rigid insulation board. Brands include Kingspan Thermawall, Celotex, and Recticel. The foil facing on most PIR boards acts as a vapour control layer — important in wall and roof applications where the board is on the warm side of the construction.

Key applications:

• Flat warm roofs: PIR boards above the roof deck, below the waterproofing membrane
• Ground floors: Above the DPC, below the screed; PIR allows thinner floor build-up for given U-value
• Internal wall insulation (IWI): PIR boards (often 25–75mm) on wall face inside a room, overboarded with plasterboard
• Pitched roof (warm roof rafter-level): Between and below rafters or above rafters

Fire performance: PIR is Euroclass B or C — combustible, but with limited fire spread. Under Building Regulations Approved Document B and the post-Grenfell regulations, PIR cannot be used on the external face of buildings over 18m. It remains acceptable in most domestic applications.

Handling: Cut with a fine-tooth saw or sharp knife and straight edge. The foil facing can be cut first to define the line. Joints between boards should be taped with aluminium foil tape to maintain the vapour control layer. Allow for expansion: leave a small gap at edges.

EPS (Expanded Polystyrene)

EPS is the familiar white polystyrene, available in two forms:

• EPS beads (blown cavity fill): Injected into existing cavities by specialist contractors; achieves full-fill of irregular cavities better than batts
• EPS boards: Used in external wall insulation (EWI) systems, under ground slabs, and behind render

EPS is lower-performing than PIR but cheaper and has better long-term stability. It does not absorb moisture significantly. In EWI systems, graphite-enhanced EPS (Greyboard/Platinum EPS, λ ≈ 0.030 W/mK) offers performance close to standard PIR at significantly lower cost.

XPS (Extruded Polystyrene)

XPS has a closed-cell structure that makes it highly moisture-resistant — suitable for applications where other insulants would degrade with water exposure. Key uses:

• Inverted flat roofs: XPS above the waterproofing membrane, below ballast or paving (the membrane is protected by the insulation)
• Below-DPC applications: Under floor slabs in contact with ground where water ingress risk is higher
• Cold stores and wet rooms: Where sustained moisture exposure is expected

XPS achieves similar λ values to EPS (0.030–0.033 W/mK) but is more expensive due to the manufacturing process.

Phenolic Foam

Phenolic foam (Kingspan Kooltherm, Recticel Eureka) achieves the best thermal performance of any mainstream board insulant (λ = 0.018–0.021 W/mK). This makes it valuable where space is at an absolute premium — internal wall insulation in a period house where every millimetre of room depth matters, or thin-profile doors and window reveals.

The trade-off is cost: phenolic boards are 50–100% more expensive than PIR per m². The benefit must be weighed against whether the additional performance per millimetre is worth the cost in the specific application.

Natural and Bio-Based Insulants

Sheep wool (Thermafleece, Black Mountain Natureflex) has a unique advantage: it can absorb and release moisture vapour without losing thermal performance, and without being damaged by the process. This makes it suitable for use in traditional buildings (pre-1919 solid wall construction) where vapour permeability of the fabric is critical to moisture management. It also performs better than mineral wool at low temperatures (as moisture condenses in the wool, it actually generates a small amount of heat through hydrogen bonding — the Mollier effect).

Cellulose (blown) is recycled newspaper treated with borate fire retardant. It can be blown into roof voids and prefilled panels. Environmental credentials are good (high recycled content), but it requires specialist installation equipment.

Wood fibre board (Pavatex, Gutex) is used in breathable wall systems, particularly in timber frame construction. It has lower thermal performance than PIR but is vapour-permeable and compatible with lime and breathable render systems.

Selecting the Right Material for Each Application

Frequently Asked Questions

Can I use PIR in a cavity wall?

PIR partial-fill cavity batts exist but are less common than mineral wool for this application. The main concern with rigid boards in cavities is the difficulty of achieving a good fit around wall ties and in irregular cavities. Mineral wool batts are typically preferred for cavity wall partial fill because they are more tolerant of imperfect installation. For full-fill cavities, blown insulation (EPS beads or cellulose) is installed by specialist contractors.

Why can't I use PIR foam in an EWI system on a high-rise building?

Post-Grenfell, the Building (Amendment) Regulations 2018 and Approved Document B require that insulation products and materials on the external wall of residential buildings over 18m must be of limited combustibility (Euroclass A1 or A2). PIR is Euroclass B–C and therefore prohibited on the external face of these buildings. Mineral wool A1-rated boards are the compliant alternative for EWI on buildings over 18m.

Is sheep wool insulation a fire risk?

Sheep wool is treated with borate salts (most UK products) which act as a fire retardant. It does not ignite easily and tends to char rather than burn. Its Euroclass rating is typically E, which is the same as EPS. It is not classified as non-combustible (A1/A2). For applications requiring non-combustible insulation (party walls, above-18m EWI), mineral wool remains the only mainstream option.

What's the difference between the declared lambda and the design lambda?

The declared lambda (λD) is the manufacturer's claimed thermal conductivity under standard test conditions. The design lambda (λ) is the value recommended for use in calculations, accounting for real-world factors such as moisture uptake and installation quality — it is always equal to or slightly higher (worse) than λD. When calculating U-values for compliance or SAP assessments, always use the design lambda. Using the declared value gives an optimistically low U-value that may not be achieved in practice.

Does insulation type affect whether I need a vapour control layer?

Yes. Highly permeable insulants (mineral wool, sheep wool, wood fibre) allow moisture vapour to pass through freely. In humid applications (bathrooms, kitchens, below heated space), a separate vapour control layer (VCL) is typically required on the warm side to prevent interstitial condensation. PIR boards with foil facings act as their own VCL when properly taped at joints. XPS has very low vapour permeability. The need for a separate VCL depends on the material's vapour resistance, the location of the insulation in the build-up, and a condensation risk assessment per BS EN ISO 13788.

Regulations & Standards

• Approved Document L — Energy efficiency targets; U-value requirements that determine insulation thickness

• Approved Document B — Fire safety; Euroclass requirements for insulation in wall and roof systems

• Building (Amendment) Regulations 2018 — Post-Grenfell; prohibits combustible cladding and insulation on residential buildings over 18m

• BS EN 13162–13171 — European standards for various factory-made insulation products (mineral wool, EPS, XPS, PIR, phenolic, etc.)

• BS EN ISO 6946 — Thermal resistance calculation; where lambda values are used

• NHBC Technical Standards — Requirements for insulation in new-build construction

• Kingspan Technical Documentation — PIR and phenolic product specifications and U-value calculators

• Knauf Insulation — Glass wool specification data and installation guidance

• Rockwool UK — Rock wool product range and fire performance data

• Zero Carbon Hub / NHBC Foundation — Guidance on insulation in new-build and retrofit

• Historic England — Insulation in traditional buildings — Guidance on breathable insulants for pre-1919 buildings

• loft insulation — Loft insulation installation guidance

• cavity wall — Cavity wall insulation methods and suitability

• solid wall — IWI and EWI systems for solid walls

• u value — How to calculate U-values using lambda values

• thermal bridging — How different insulant types affect bridging calculations