The short answer
There is no single best insulation for solid walls — the right material depends on the age and construction of the wall and whether you insulate inside or out. For modern, less moisture-sensitive walls, rigid foam boards (PIR/PUR) give the best thermal performance per millimetre, which suits internal jobs where space is tight. Mineral wool is widely used, non-combustible and a good all-rounder, especially in external systems. For older, pre-1919 brick and stone walls that need to breathe, wood-fibre insulation with lime finishes is usually the safest choice because it is vapour-open and lets the wall dry. So 'best' means best-matched: high performance for space-limited modern walls, breathable for period properties, and the right fire and moisture properties for the location.
Asking for the 'best' insulation usually gets a product name, but the genuinely correct answer is whichever material suits your wall's moisture behaviour and your method. Here is how the main options compare.
Choosing by wall type
- Tightest space (internal)Rigid PIR/PUR board
- Fire performance / all-roundMineral wool
- Older breathable wallsWood-fibre + lime
- Key questionDoes the wall need to breathe?
- Best overall ruleMatch material to the building
Rigid foam boards (PIR / PUR)
Rigid polyisocyanurate or polyurethane boards have a low thermal conductivity, so they reach a given U-value in less thickness than most alternatives. That makes them popular for internal insulation, where every centimetre comes off the room, and they are often supplied as insulated plasterboard laminates for a quick fit. Their weakness is breathability: they are largely vapour-closed, so on an older wall that needs to dry through its inner face they can trap moisture if not detailed perfectly. They suit modern or less moisture-sensitive walls, or jobs with a correctly designed and sealed vapour control strategy.
Mineral wool and external systems
Mineral wool (rock or glass) is a versatile, widely used insulation that is non-combustible and more vapour-open than rigid foam. It needs more thickness than PIR for the same U-value, so it is often favoured in external systems, where space is not constrained, and where its fire performance is valued on taller or terraced buildings. Many external wall insulation systems are built around mineral wool or expanded polystyrene (EPS) boards finished with reinforced render. EPS is cheaper and lighter but combustible, so material choice on EWI is partly a fire-safety decision driven by building height and type as well as thermal performance.
| Material | Strength | Watch-out |
|---|---|---|
| Rigid PIR/PUR board | Best performance per mm | Vapour-closed; detailing critical |
| Mineral wool | Non-combustible, vapour-open | Thicker for same U-value |
| EPS (external) | Lower cost, lightweight | Combustible; check fire rules |
| Wood-fibre + lime | Breathable, suits old walls | Thicker; specialist finish |
Indicative comparison for guidance. Sources: Energy Saving Trust; Historic England.
Breathable systems for older walls
For pre-1919 solid brick and stone walls, the most important property is usually not raw thermal performance but breathability. These walls were built without modern damp barriers and rely on absorbing and releasing moisture, so the insulation should be vapour-open and able to handle moisture without trapping it. Wood-fibre insulation board finished with lime plaster (internally) or lime render (externally) is the established choice: it insulates, buffers moisture and lets the wall keep drying. It needs more thickness than rigid foam and a specialist finish, but on a period property that compatibility is what prevents the damp problems that stiff, sealed materials can cause.
How to choose for your home
Decide in this order. First, establish the wall's age and moisture behaviour — older, lime-built walls point to breathable wood-fibre and lime; newer, less sensitive walls open up rigid-board options. Second, choose your method — internal jobs favour thinner high-performance boards to limit space loss, external jobs can use thicker mineral wool or EPS. Third, consider fire and building type, which can rule out combustible materials on some properties. The most reliable route is a PAS 2035 retrofit assessment by a TrustMark-registered installer, which weighs all three and specifies a system matched to your wall rather than a one-size product.
Properties beyond thermal performance
It is easy to judge insulation only on thermal conductivity, but several other properties matter as much in a solid wall. Vapour permeability decides whether the wall can keep drying — critical on older buildings. Fire behaviour matters especially for external systems on terraced or taller buildings, where non-combustible mineral wool is often required over combustible options. Moisture buffering — the ability of a material such as wood-fibre to absorb and release humidity — helps smooth out condensation risk. And on external systems, impact resistance and the render's durability affect how the finish copes with knocks and weather over the years.
Cost and availability play in too, but they should not lead the decision. A material that is cheaper per square metre but wrong for the wall — a sealed board on a breathable wall, or a combustible system where fire rules forbid it — is a false economy that can cause damp or fail a building control inspection. The better mindset is to define what the wall needs first (breathable or not, combustible or not, how much thickness is available), then choose the best-value product that meets all of those needs, rather than starting from a favourite brand and forcing it to fit.
Why 'best' is the wrong question on its own
The reason there is no universal best insulation is that the materials are not competing on a single scale. Rigid foam wins on thermal performance per millimetre but loses on breathability and fire. Mineral wool wins on fire safety and vapour-openness but needs more depth. Wood-fibre wins on compatibility with old walls and moisture buffering but is thicker and needs a specialist lime finish. Each is genuinely the best at something, so asking 'which is best' without saying 'best for what wall, fitted which way, on what building' produces a misleading answer — usually whichever product the asker happened to read about first.
A more useful frame is to treat the wall as setting the requirements and the material as meeting them. A draughty, cold pre-1919 cottage and a 1950s solid-walled house with cement render have completely different needs, and the right product for one would be a mistake on the other. Once the wall's age, moisture behaviour, the chosen method and any fire constraints are written down, the 'best' material is simply the one that satisfies all of them at the lowest sensible cost — a question with a clear answer, rather than a matter of opinion or marketing.
A simple decision path
If you want a clear way to narrow the field, work through three questions in turn. Is the wall a traditional, pre-1919, lime-built one that needs to breathe? If yes, you are looking at vapour-open materials — wood-fibre with lime — and most rigid foam options drop away. Are you insulating internally, where space is tight, or externally, where it is not? Internal jobs lean toward thinner, higher-performance boards (subject to the breathability answer); external jobs can use thicker mineral wool or EPS. Does fire safety or building type restrict the material? On some buildings, non-combustible insulation is effectively required, which rules out combustible boards regardless of their thermal numbers.
Answer those three and the shortlist usually becomes obvious, often down to one or two suitable systems rather than a confusing field of products. The remaining choice — between similar compliant options — is where cost, warranty, installer familiarity and certification come in. Approaching it this way keeps you from the common trap of buying the best-marketed product and discovering too late that it was the wrong fit for your wall, your method or your building's fire requirements.
Frequently asked questions
Is PIR or mineral wool better for solid walls?
Neither is universally better. PIR (rigid foam) gives the best thermal performance per millimetre, so it suits internal jobs where space is tight, but it is vapour-closed. Mineral wool is more breathable and non-combustible but needs more thickness. The right one depends on the wall's moisture behaviour and your method.
What insulation is best for a period or listed home?
Usually a breathable, vapour-open system — most often wood-fibre insulation with lime plaster or render — because older walls rely on being able to absorb and release moisture. Sealed, vapour-closed materials risk trapping damp. Listed buildings also need consent, so specialist advice is worth getting first.
Does the best insulation depend on internal or external fitting?
Yes. Internal insulation favours thinner, high-performance boards to limit lost room space, while external insulation can use thicker mineral wool or EPS since space is not a constraint. The fitting method is part of choosing the material, alongside the wall's age and fire requirements.
Sources & further reading
Figures on this page are typical UK ranges drawn from published sources and depend on your specific property. They are guidance, not a quotation.