Compare Insulation Types: Mineral Wool vs PIR vs Multifoil

Pick what matters, then read the row

Every material below is the right answer somewhere and the wrong one somewhere else. Use the highlight buttons to surface the options usually considered for your situation. It is a starting point for a conversation with an installer, not a specification.

Material	Typical use	Thickness	Installation	Moisture	Cost band	Trade-off
Mineral wool	Loft floor to 270mm; between joists and studs	270mm loft; 100–150mm walls/rafters	Simple; DIY-friendly	Breathable; needs a ventilated loft	Lowest cost per m²	Bulky; crushes if boarded over
PIR board	Rafters, warm roofs, floors, solid walls	50–150mm	Cut and fit; needs sealing at joints	Vapour-tight face; detailing matters	Mid to high	Rigid; awkward around services
Multifoil	Rafters, loft rooms, floors where depth is tight	30–45mm plus air gaps	Fast; staple and tape; needs air gaps both sides	Vapour control built in; air gaps must be kept	Mid; higher per m² than wool	Performance depends on installed air gaps
Blown cavity fill	Cavity walls built after 1930s	50–100mm cavity	Specialist injection; half a day	Needs a dry, suitable cavity	Low per m²	Not for narrow, damp or exposed cavities
EWI / IWI systems	Solid-walled homes; whole-house retrofit	60–120mm	Specialist; disruptive	Must be designed for moisture	Highest	Cost and planning considerations

Mineral wool

Use: Loft floor to 270mm; between joists and studs
Thickness: 270mm loft; 100–150mm walls/rafters
Install: Simple; DIY-friendly
Moisture: Breathable; needs a ventilated loft
Cost: Lowest cost per m²
Trade-off: Bulky; crushes if boarded over

PIR board

Use: Rafters, warm roofs, floors, solid walls
Thickness: 50–150mm
Install: Cut and fit; needs sealing at joints
Moisture: Vapour-tight face; detailing matters
Cost: Mid to high
Trade-off: Rigid; awkward around services

Multifoil

Use: Rafters, loft rooms, floors where depth is tight
Thickness: 30–45mm plus air gaps
Install: Fast; staple and tape; needs air gaps both sides
Moisture: Vapour control built in; air gaps must be kept
Cost: Mid; higher per m² than wool
Trade-off: Performance depends on installed air gaps

Blown cavity fill

Use: Cavity walls built after 1930s
Thickness: 50–100mm cavity
Install: Specialist injection; half a day
Moisture: Needs a dry, suitable cavity
Cost: Low per m²
Trade-off: Not for narrow, damp or exposed cavities

EWI / IWI systems

Use: Solid-walled homes; whole-house retrofit
Thickness: 60–120mm
Install: Specialist; disruptive
Moisture: Must be designed for moisture
Cost: Highest
Trade-off: Cost and planning considerations

How to read thickness and U-values

A U-value describes how fast heat passes through a whole build-up — lower is better. The current edition of Approved Document L (as amended) sets limiting and target values for upgraded roofs and walls in England, subject to transitional arrangements, and installers design to those. Two materials can reach the same U-value at very different thicknesses: roughly, 270mm of mineral wool on a loft floor, 120–150mm of PIR between and under rafters, or a multifoil quilt with air gaps combined with a modest board layer. That is why thin products look attractive in loft rooms — but the air gaps a multifoil needs are part of its thickness, not extra.

The three questions that decide it

Is the loft floor or the roof slope being insulated? Loft floor: mineral wool is almost always the cheapest good answer. Roof slope: PIR or multifoil, chosen by headroom and ventilation.
Are the walls cavity or solid? Cavity: injected fill if the cavity is suitable. Solid: internal or external systems, which are a bigger project.
Is depth constrained? Loft rooms, low headroom and shallow floors push toward PIR or multifoil. Open lofts do not.

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Moisture is where insulation jobs go wrong

Insulation moves the dew point. Put a vapour-tight board on the wrong side of a timber roof, block the eaves ventilation with wool, or skip the air gap on a multifoil, and the result is condensation on timbers a year later. Any installer on our panel has to show you the ventilation path in the quote. Ask to see it.

Head-to-head comparisons

Multifoil vs PIR — the loft-room decision
Multifoil vs mineral wool — space against cost
SuperFOIL vs SuperQuilt — the two big multifoil brands
Alternatives to spray foam

Questions homeowners ask

Which insulation is best?

The one that suits the location, the construction and the space you have. For an open loft floor, mineral wool. For a roof slope with little headroom, PIR or multifoil. For cavity walls, injected fill. The comparison table above sets out the trade-offs.

Is thinner insulation worse?

Not necessarily. Thinner products usually have a lower thermal conductivity, or in the case of multifoil rely on reflective surfaces and air gaps. What matters is the U-value of the finished build-up, installed as designed.

Can I mix materials?

Yes, and installers often do — for example a multifoil under the rafters with PIR between them, or mineral wool between joists with a foil layer over it. The combination must be designed for moisture as well as heat.

Sources and when they were last checked

Energy Saving Trust — Roof and loft insulation — checked 2 September 2026
GOV.UK — Room in roof insulation best practice — checked 2 September 2026
SuperFOIL — product range — checked 2 September 2026

Compare insulation types