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

Quick Answer: PIR (polyisocyanurate) rigid foam offers the best thermal performance per millimetre (λ = 0.022–0.023 W/mK), making it the preferred choice for thin build-ups such as solid floor insulation and flat roofs. Mineral wool (glass or rock) is the most cost-effective and fire-safe choice for loft, stud wall, and cavity applications (λ = 0.035–0.044 W/mK). Phenolic foam achieves even better performance than PIR (λ = 0.018–0.021 W/mK) but at higher cost. All materials must meet Approved Document L and, for certain applications, Approved Document B fire performance requirements.

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

Quick Reference Table

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Material λ (W/mK) Fire Class Vapour Permeability Typical Application Relative Cost
Glass wool (e.g. Knauf Earthwool) 0.035–0.038 A1 High (permeable) Loft, cavity, stud wall Low
Rock wool (e.g. Rockwool) 0.033–0.044 A1 High (permeable) Loft, acoustic partitions, fire-rated Low–Medium
PIR board (e.g. Kingspan, Celotex) 0.022–0.023 B–C Low (impermeable foil face) Floor, flat roof, internal wall Medium
EPS bead (blown cavity) 0.030–0.038 E Low–medium Cavity wall fill Low
EPS board 0.030–0.038 E Medium External insulation systems, floor Low–Medium
XPS board (e.g. Styrofoam) 0.030–0.033 E Very low Below DPC, inverted roof, wet areas Medium
Phenolic board (e.g. Kingspan Kooltherm) 0.018–0.021 B Low Thin section walls, internal insulation High
Sheep wool (e.g. Thermafleece) 0.035–0.040 E High (moisture buffering) Loft, stud wall, timber frame Medium–High
Wood fibre board (e.g. Pavatex) 0.038–0.050 E–F High Breathable wall systems, timber frame High
Aerogel blanket 0.014–0.018 A1–A2 Variable Listed buildings, very thin sections Very high

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:

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 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:

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

Application Recommended Material Why
Loft (between joists) Mineral wool semi-rigid rolls Cost-effective, friction fit, fire-safe
Loft (over joists, cross-layer) Mineral wool rolls Same; cross-laid to cover joists
Cavity wall (new build, partial fill) Mineral wool cavity batts Factory-controlled, non-combustible
Cavity wall (existing, full fill) EPS beads or mineral wool granules Injected to fill irregular cavities
Ground floor slab PIR or EPS boards Rigid under screed; PIR more efficient
Internal wall insulation (IWI) PIR or phenolic boards Minimum thickness loss
External wall insulation (EWI, below 18m) EPS or mineral wool Cost (EPS) or fire performance (mineral wool)
External wall insulation (EWI, above 18m) Mineral wool A1-rated only Approved Document B/safety regulations
Warm flat roof PIR boards Above deck; high performance
Inverted flat roof XPS boards Below ballast, above membrane; waterproof
Party wall / acoustic High-density rock wool Fire and acoustic performance
Listed building / thin section Aerogel or phenolic Maximum performance per mm

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