What is the best type of loft insulation?

There is no universal best — the right material depends on whether you are insulating a cold loft or a warm room, how easy the space is to work in, and what you want to pay. Here is how the main options compare.

The main materials, side by side

Loft insulation comes in a few distinct forms, each with a job it does best:

• Mineral wool rolls (glass or rock wool): the everyday standard. Cheap, widely available, fire-resistant and simple to lay in two cross-laid layers to 270mm. The default for an unused cold loft.
• Blown (loose-fill) insulation: shredded mineral fibre or cellulose blown in by machine. Ideal for lofts that are cramped, irregular or hard to reach by hand, as it fills gaps that rolls struggle with.
• Natural fibre (sheep's wool, hemp, wood fibre): breathable, pleasant to handle and lower-irritation, but noticeably more expensive per square metre than mineral wool.
• Rigid foam boards (PIR / PUR): high performance per millimetre, used at rafter level for a warm loft where there isn't room for 270mm of wool. More costly and a more involved fit.
• Multifoil / reflective products: thin foil layers, sometimes used in tight spaces, but generally as a supplement rather than a standalone solution for the recommended performance.

Indicative comparison for guidance; cost depends on area, depth and access. Source: Energy Saving Trust roof and loft insulation guidance.

Why mineral wool is the default for cold lofts

For a typical unused loft used only as a cold storage space, mineral wool to 270mm is the route most installers and the Energy Saving Trust point to. It hits the target U-value of around 0.16 W/m²K, costs the least per square metre, is non-combustible, and can be topped up over existing insulation without removing what is already there. It is also reversible — it doesn't conceal the roof structure the way spray foam does — so it never creates the mortgage and survey problems associated with foamed roofs. The main downsides are that it can be itchy to handle and must not be compressed, since squashing it under boarding destroys much of its value.

The choice between the two common mineral wools — glass wool and rock wool — rarely matters for an ordinary cold-loft top-up. Glass wool is lighter and usually a little cheaper, which is why it dominates the roll market; rock (stone) wool is denser, slightly better at resisting fire and sound, and holds its shape well, so it appears more often in board and slab form. Both reach the same U-value target at a similar depth and both are non-combustible, so for warming the rooms below either is a sound pick. Pick the product on price, availability and how pleasant it is to handle rather than agonising over the fibre type, and make sure whichever you choose carries recognised certification.

When a different material is the right call

Mineral wool isn't always the answer. If your loft is cramped, irregular or hard to reach — low headroom, lots of obstructions, awkward corners — blown insulation often does a better, more even job because it flows into gaps that rolls bridge over. If you want a warm, usable loft room, the insulation belongs at rafter level using rigid PIR boards, not wool above the ceiling, and that is a different specification with ventilation and any vapour control designed in. And if breathability or handling comfort matters to you, natural-fibre products are a genuine alternative, accepting the higher price.

It is also worth separating the two numbers people conflate when they compare materials: thermal conductivity (the lambda value, λ) and installed depth. Mineral wool and most natural fibres sit at a fairly similar lambda, around 0.040–0.044 W/mK, so they need a similar depth — roughly 270mm — to hit the target. Rigid PIR is far lower, around 0.022 W/mK, which is why it reaches the same U-value in roughly half the thickness, and that thinness is the whole reason it earns its higher price at rafter level where headroom is scarce. Above an open ceiling, where depth costs nothing, paying for a low-lambda board makes little sense — cheap wool laid thick does the same job. This is the practical logic that decides the material: where depth is free, buy depth; where depth is precious, buy performance per millimetre. Seen that way, the apparent contradiction — wool for cold lofts, board for warm ones — isn't a contradiction at all but the same cost calculation answered for two different spaces.

The honest summary is that the loft's purpose and shape decide the material more than any league table does. For the common case — an unused cold loft you simply want warm below — mineral wool to 270mm offers the best balance of cost, performance and simplicity. For everything else, match the material to the situation rather than assuming one product fits all.

The one type to avoid for most homes: spray foam

One product deserves a specific warning rather than a place in the comparison: spray foam applied to the underside of the roof. It is heavily marketed and does insulate, but it brings a serious drawback that the soft and rigid options above do not. Because it bonds to the rafters and the underside of the tiles, it conceals the roof timbers so a surveyor cannot inspect them, and it can interfere with the roof's ventilation. As a result, a large share of UK mortgage lenders will not lend, or will lend only with conditions, on a home with spray foam in the roof, and RICS surveyors frequently flag it. That can make a property hard to sell or remortgage until the foam is professionally removed.

There are two broad types — open-cell and closed-cell — and the distinction matters less than the shared problem: both obscure the structure and both trigger the same lender caution. For the overwhelming majority of homes, the sensible choice is a reversible, lender-friendly material: mineral wool at ceiling level for a cold loft, or rigid boards at rafter level for a warm one. These insulate just as effectively for the purpose, leave the roof inspectable, and never turn a value-adding upgrade into a value-destroying one. If a property you are buying already has spray foam, treat it as a survey-and-lender question before committing, not a minor detail.

The wider lesson behind the spray-foam warning is that the best material is not simply the one with the lowest lambda or the warmest claim on the brochure — it is the one that fits the building, the budget and the future sale without storing up trouble. A genuinely good loft material does its thermal job, leaves the roof inspectable and the timbers ventilated, can be topped up or removed later, and carries recognised certification a surveyor will accept. Mineral wool and rigid board both pass that test for their respective spaces; spray foam fails it on inspectability regardless of how well it insulates. So when weighing options, it is worth asking not only “how warm will this make the loft?” but “what will a lender, a surveyor or the next owner make of it?” — because a material that quietly damages saleability has a hidden cost no U-value can offset. For nearly every UK home the answer to both questions points the same way: standard, certified, reversible insulation, sized to the loft's purpose, fitted so the roof can still breathe and be seen.

Frequently asked questions

Sources & further reading

• Energy Saving Trust — roof and loft insulation
• British Board of Agrément (BBA) — product certification

Figures on this page are typical UK ranges drawn from published sources and depend on your specific loft. They are guidance, not a quotation.