Insulation Materials Compared: Mineral Wool, PIR, Foam and Natural

Insulation is the single fabric measure that appears in every UK energy scheme, from loft insulation to cavity wall insulation, and the material you choose determines how thick the layer must be, how it behaves with fire and moisture, and what it costs. The main materials used in UK cavity walls alone are mineral wool (glass or rock wool), EPS polystyrene bead and polyurethane foam¹, and beyond the walls there are rigid PIR and phenolic boards, blown beads, wood fibre, sheep's wool, cellulose, hemp and cork. Official statistics put material costs in 2024 at roughly £10 per square metre for fibreglass and polystyrene, £15 per square metre for PIR board and mineral wool, £20 or more for sheep wool and polyurethane foam, and often over £100 per square metre for an aerogel blanket².

The performance spread is just as wide. Thermal conductivity, the number that tells you how well a material blocks heat, runs from about 0.007 W/mK for sealed vacuum panels and about 0.015 W/mK for aerogel, through about 0.02 to 0.025 W/mK for plastic foams, up to 0.038 to 0.054 W/mK for wool³. In practice, that means a thin board of PIR can match a much thicker layer of mineral wool: 100mm of spray foam insulation is equivalent to around 170mm of mineral wool loft insulation⁵. This page compares the families side by side, so you can see what each one does, where each one belongs, and what each one leaves your home depending on.

Trapped air: the principle behind insulation

Every insulation material does the same job by the same trick: it holds air still. Loft insulation acts as a blanket, trapping heat rising from the house below⁷. Double glazing works on the same principle, trapping air between two panes of glass to create an insulating barrier that reduces heat loss, noise and condensation⁸. Air is a poor conductor of heat, but only when it cannot move; a draughty loft loses heat fast precisely because the air is free to circulate and carry warmth away.

That is why the structure of a material matters more than its substance. Fibrous materials such as mineral wool trap air between millions of strands. Foams trap it in closed or open bubbles of gas or air⁵. Rigid boards hold it in a dense cellular structure. What the material must never do is let that air circulate, which is why compressing fibrous insulation, squashing a thick roll into a thin space, or leaving gaps around the edges all undo the performance you paid for.

It also explains why thickness and performance trade off against each other. A material that traps air more effectively per millimetre, such as a plastic foam, needs less depth to reach the same target than a fibrous one. The thickness required varies depending on the material you decide to use and the manufacturer's specification⁹, so there is no single correct depth for every product on the market.

Understanding lambda values in thermal conductivity

Thermal conductivity, written as lambda and measured in W/mK, is the number used to compare materials directly: the lower it is, the less heat passes through each millimetre. The range across the materials sold in UK homes is wide. At the top end, sealed vacuum panels achieve 0.007 W/mK and aerogel insulation about 0.015 W/mK³. Some types of plastic foam have a lower thermal conductivity of about 0.02 to 0.025 W/mK³.

For the common materials, scheme guidance for cavity wall insulation distinguishes products with a thermal conductivity below 0.028 W/mK from those in the 0.029 to 0.034 W/mK band¹⁰. Wool insulation sits at 0.038 to 0.054 W/mK⁴, and hemp varies more widely than other materials, at 0.038 to 0.060 W/mK⁴.

What this means in practice is thickness. A material at 0.022 W/mK needs roughly half the depth of one at 0.044 W/mK to hit the same U-value, which is why PIR boards dominate where space is tight and mineral wool dominates where depth is free, as in an empty loft. The trade-off is price: the thinnest materials are the most expensive per square metre, often dramatically so². For most homes, the question is not which material is best in the abstract but which one fits the space available at a cost that makes sense. Our page on U-values and R-values explains how these figures convert into the targets Building Regulations set.

Mineral wool: affordable and fire resistant

Mineral wool, in its glass and stone (rock) forms, is the material most UK homes end up with, and the reasons are practical rather than glamorous. It is described as the cheapest, easiest and most efficient option for most people for standard loft insulation¹¹, and as more cost effective than any other material available⁶. It comes in roll, batt, mattress or blown form, a form to suit any project⁶, and installation is a simple process, as a batt or blown to suit any specific project¹².

Its fire performance is its standout property: mineral wool is non-combustible and carries an A1 or A2 Euroclass fire rating⁶. No other common insulation family can match that, and it matters wherever insulation sits close to a heat source or where a building's fire strategy depends on the fabric. In a cavity wall, the risk of condensation is negligible with the correct product and installation¹², and liquid water is not transmitted to the inner leaf when the appropriate glass wool product is used with correct installation¹².

In timber frame walls, mineral wool is supplied to fit tightly between standard stud widths without the need for on-site cutting, and is self-supporting¹³. Where cutting is required, the compressible insulation can be cut slightly oversize and friction fit between the studs to prevent cold air penetrating through the joints¹³. Mineral wool boards, glass mineral wool and soundproof insulation mats can also be installed in walls as boards¹⁴, so the same family covers acoustic as well as thermal use. Its limits are equally plain: it needs more depth than foams to reach the same target, and it must stay dry and uncompressed to perform.

PIR and phenolic: slim insulation for a given U-value

Rigid boards are the answer when depth is the constraint: a room-in-roof, a flat roof, a floor, or a wall where every centimetre of internal space counts. PIR board has the best thermal efficiency of the board types, ensuring that heat stays inside the property¹⁴. The maker states that thanks to its thermal performance, thinner PIR boards can achieve the same insulation levels as thicker alternatives¹⁵.

The closed-cell structure does more than insulate. The maker states that PIR boards are highly resistant to moisture and water absorption, making them ideal for use in humid or wet environments and helping prevent mould, rot and long-term degradation, and suitable for residential and commercial buildings¹⁵. PIR also stands out for fire-resistant properties, forming a protective char layer when exposed to flames¹⁵, and independent guidance describes it as a rigid foam board that is durable and has a high fire safety value¹⁴. A char layer is not the same as non-combustibility, but it is a meaningful fire behaviour.

Within the board family there are distinctions worth knowing. PUR insulation board is lighter and cheaper than PIR, but with slightly less thermal efficiency¹⁴. For stud walls specifically, PIR boards are likely the better option¹⁴. Boards must be cut accurately and taped or sealed at joints, because a rigid board cut short of a stud leaves a gap that fibrous material would have filled by compression. Where boards meet timber, the framing itself becomes a thermal bridge, which is a subject covered in detail on our thermal bridging page.

Spray foam: strengths and shortcomings

Spray foam is the most controversial material on this list, and the controversy is not about its thermal performance. It comes in two forms, closed-cell and open-cell¹⁶, both usually made from polyurethane and filled with bubbles of gas or air⁵. Closed-cell foam is denser and insulates better; open-cell foam does not provide the same level of insulation as it is not as dense¹⁶. In thermal terms, 100mm of spray foam insulation is equivalent to around 170mm of mineral wool loft insulation⁵.

Where it fits: spray foam can be used to insulate roofs, walls and floors¹¹, and it can be used in a number of ECO-eligible insulation applications to fill voids and gaps⁵. Some types of foam insulation are waterproof but also breathable, making them suitable for homes in flood-prone areas⁵. It is applied as a retrospective measure to the underside of roofs of dwelling properties¹⁷.

Where it fails is the record of the material in occupied homes. Any timbers that are coated in foam will be more difficult to replace, and the same goes for tiles or slates if there is no roofing felt or other layer between foam and tiles¹¹. It should not be used in listed buildings or houses with thatched roofs⁵. Because the foam bonds to what it covers, it can conceal the condition of roof timbers, which is why lenders have treated it with caution; the Commons Library briefing on spray foam and mortgages covers that history, and our page on spray foam roof rot sets out the timber risks. A household considering foam should weigh the thermal gain against the fact that the roof structure becomes harder to inspect and repair.

Plant and animal fibre insulation

The natural family covers sheep's wool, cellulose, cork, wood fibre and hemp, with some recycled from waste paper or textiles³. Recycled PET insulation, made from waste plastics, comes in many forms including loose-fill, rolls and rigid board⁴. These materials appeal on environmental grounds, and in older buildings there is a technical case too: official Scottish guidance on converting traditional buildings notes that vapour permeable, capillary active and hygroscopic materials can safely absorb, transfer and desorb (release) moisture, and can be used to reduce moisture build-up risk without the need for cavities¹⁸. That breathability is why natural fibres are often discussed for traditional and pre-1919 buildings, where sealing moisture into a wall causes decay.

The performance and price spread within the family is large. Hemp's thermal conductivity rating varies more widely than other materials, at 0.038 to 0.060 W/mK⁴. Wool insulation manages 0.038 to 0.054 W/mK⁴ and costs around £18 to £25 per m2, one of the more expensive options⁴. Cellulose is one of the most competitively priced sources of eco-insulation, at around £9 to £12 per m2⁴, though it has also been described as still comparatively expensive, at up to twice the cost of mineral wool or rigid foam⁴. Cork is not widely produced in the UK, which makes it more expensive, at up to ten times the price of equivalent mineral wool products⁴.

So the natural family is not one thing: cellulose can be price-competitive with mineral wool, while cork sits at the far end of the scale. What they share is hygroscopic behaviour, the ability to buffer moisture, which rigid foams do not offer. Our page on sheep's wool insulation in older homes looks at that case in detail.

Comparing fire performance, damp and lifespan

Fire performance separates the families more sharply than any other attribute. Mineral wool is non-combustible with an A1/A2 Euroclass rating⁶. PIR forms a protective char layer when exposed to flames¹⁵ and is described as having a high fire safety value¹⁴. Foams and natural fibres are combustible to varying degrees, which is why fire safety guidance around heat-producing appliances matters: where a gas-fired appliance needs shielding from combustible fabric, an air space of at least 75mm, or a shield of non-combustible material such as insulating board with a fire-resistant surface, is required¹⁹.

Moisture behaviour divides along a different line. Closed-cell PIR is highly resistant to moisture and water absorption¹⁵. Mineral wool in a cavity does not transmit liquid water to the inner leaf when correctly installed¹², but it is not waterproof in the way a closed-cell foam is. Natural fibres are hygroscopic: they absorb and release moisture, which is a benefit in a breathable wall build-up¹⁸ and a risk if they stay wet. Some types of spray foam are waterproof but also breathable, making them suitable for homes in flood-prone areas⁵.

Durability is where spray foam diverges from everything else. Timbers coated in foam are more difficult to replace, as are tiles or slates if there is no roofing felt or other layer between foam and tiles¹¹. Every other material on this page can be lifted out, topped up or replaced without damaging the structure around it. That reversibility is worth weighing heavily in a home you intend to keep, and it is one reason cavity wall insulation problems are remediable in a way spray foam problems often are not.

Prices from about £10 per square metre to over £100

Official statistics give the clearest picture of material costs in 2024: approximately £10 per m2 for fibreglass and polystyrene, £15 per m2 for PIR board and mineral wool, £20 or more per m2 for sheep wool and polyurethane foam, and often over £100 per m2 for an aerogel blanket². Those are capital costs of materials only; installation is separate, and installed costs can range from £950 for loft insulation to over £12,000 for external solid wall insulation on a typical three-bed semi-detached house².

The independent price readings line up with the official ones at the extremes. Cellulose sits at around £9 to £12 per m2⁴, wool at around £18 to £25 per m2⁴, and cork at up to ten times the price of equivalent mineral wool products⁴. A Northern Ireland consultation on roof insulation at joist level priced the improvement to 0.12 W/m2K at £1.50 per m2 of roof plan area²⁰, which shows how cheap top-up measures can be when access is easy.

Two things follow. First, the thinnest materials cost the most per square metre, so you pay for space saving, not for better insulation, since a thicker cheap layer reaches the same U-value. Second, installation dominates the total for anything beyond a loft: the material in over £12,000 of external solid wall insulation is a fraction of that figure². Our page on insulation and glazing costs breaks the installed figures down, and grants in England, Scotland, Wales and Northern Ireland can change the arithmetic entirely, since schemes fund insulation for lofts, roofs and walls²¹.

Meeting building rules on depth

The thickness question is where material choice meets Building Regulations. The commonly quoted target for loft insulation is 270mm, and that required thickness is often higher than the joists, so boards cannot be laid on top⁵. Guidance for a pitched roof of a dwelling normally involves new or additional loft insulation of at least 250mm of mineral fibre or cellulose fibre as quilt, laid between and across ceiling joists, or loose fill or equivalent²². In Northern Ireland's Decent Homes Standard, a loft insulation thickness of 50mm is a minimum designed to trigger action on the worst housing²³.

The material determines the depth. The thickness of insulation required varies depending on the material you decide to use and the manufacturer's specification⁹, so a PIR board reaches a given U-value at perhaps half the depth of mineral wool. Specific targets depend on the application: suspended underfloor insulation under scheme rules must achieve a U-value of at least 0.25 W/m2K¹, and Approved Document L requires intermediate floor insulation of not less than 0.5 (m2·K)/W for electric underfloor heating systems and not less than 0.75 (m2·K)/W for wet systems²⁵.

Two rules apply across the UK regardless of material. The insulation material used must be suitable for the wall construction concerned, as the Building Regulations in Wales state for cavity walls²⁶, and installing insulation to a loft area requires an application for approval under the Building Regulations²⁷. Building regulations also need adequate ventilation when installing loft insulation, so check with your local building control body²⁸; our page on Building Regulations Part L covers the targets in full, and home ventilation explains why a well-sealed insulated loft needs planned airflow.

Working safely: dust and protective kit

Working with insulation is safe if you treat it as a dusty material. Mineral wool releases fibres when cut and handled, so gloves, a long-sleeved top, eye protection and a dust mask are the standard kit, and skin should be washed after contact. The material itself is non-combustible with an A1/A2 Euroclass fire rating⁶, and it is installed in millions of UK homes under government schemes without special controls beyond ordinary site precautions.

Cutting technique matters for performance as much as for safety. Mineral wool for timber frame walls is supplied to fit tightly between standard stud widths without the need for on-site cutting, and is self-supporting¹³. If cutting is required, the compressible insulation can be cut slightly oversize and friction fit between the studs to prevent cold air penetrating through the joints¹³. Cutting oversize is the rule: an undersize piece leaves a gap, and a gap is a draught path through the insulation layer.

Rigid boards present less dust but their own hazards: a knife or saw cutting PIR produces fine particles, and offcuts are sharp-edged. Spray foam is a different category entirely, since it is applied by trained operatives rather than householders, and the concerns around it are structural and financial rather than handling-related¹¹. Whatever the material, work in a ventilated space, keep food and drink out of the work area, and bag waste before it spreads through the house. Our page on insulation safety, fire and handling covers the fire-side rules in more depth.

Carbon footprint and green claims

The environmental case for insulation rests on two legs: what the material is made from, and what it saves over its life. On the first, mineral wool uses recycled material, has zero ODP and GWP, and carries a BRE Green Guide rating of A+ to B¹². PIR boards contribute to reduced carbon emissions through improved energy efficiency and can often be recycled at the end of their lifecycle, according to the maker¹⁵. Natural fibres have the strongest story of all on embodied carbon: cellulose is recycled from waste paper, and some organic materials are recycled from waste paper or textiles³.

On the second leg, the savings are measured and large. Northern Ireland's energy efficiency schemes recorded a reduction of 3,152 tonnes in lifetime carbon emissions in 2023/24²⁹, and 1,264 tC of total lifetime carbon saved in 2024/25³⁰. Those figures are scheme totals rather than per-home numbers, but they show the scale on which insulation is treated as a carbon measure, not just a comfort one.

The honest comparison is not straightforward. A material with high recycled content but higher thermal conductivity needs more depth, and more material, to reach the same target. A high-performance foam saves more carbon through thinner application but is petrochemical in origin. What no material escapes is the lifetime saving: every kilogram of carbon embodied in making insulation is outweighed many times over by the heat it keeps in the fabric over decades of use.

Insulation choices for an autonomous home

Insulation is the foundation of household energy independence, because it reduces the amount of heat a home needs in the first place, before any question of generation, storage or tariffs arises. A well-insulated loft at 270mm⁵, cavity walls filled with an appropriate material¹ and floors brought to at least 0.25 W/m2K²⁴ cut the baseline demand that every other measure then has to serve. Solid walls lose even more heat than cavity walls³¹, so for the third of the stock with solid walls the fabric work matters even more.

The material choice is where independence and reversibility intersect. Mineral wool, boards and beads can all be inspected, topped up and removed without damaging the building. Spray foam cannot: it bonds to timbers and makes them harder to replace¹¹, which is a lasting dependence on the installer's competence and on the foam's own behaviour over decades. For a household planning a whole-house retrofit, or sizing up what insulation a heat pump needs, a reversible material keeps future options open.

What remains after insulation is done is the dependence that fabric cannot remove: the heating system that tops up what the insulation cannot hold, and the ventilation needed to keep an airtight home healthy²⁸. Insulation shrinks those dependencies to their minimum. It does not eliminate them, and a home that is well insulated but badly ventilated has traded one problem for another, which is why the condensation, damp and mould risks of a sealed house belong in the same plan as the insulation itself.