Building insulation materials are usually specified on one number — thermal conductivity — and that number explains less than half of how the material will actually perform in the building. Two products with identical conductivity can behave completely differently once they are wet, compressed, exposed to fire, or installed by someone working quickly. Choosing well means understanding what each material is good at and, more usefully, what makes each one fail.
Reading the numbers correctly
Thermal conductivity, written as lambda and measured in watts per metre kelvin, describes how readily heat passes through a material. Lower is better. Across the building insulation materials in common construction use, the practical range runs from roughly 0.020 to 0.045 W/mK — a factor of a little over two between the best and the worst of the mainstream options.
That range matters less than it appears, because conductivity is a property of the material and not of the installed assembly. What the building experiences is the assembly’s U-value, which is affected by thermal bridges through fixings and framing, by air movement around or through the insulation, by moisture content, and by gaps. A high-performance board installed with 5% of its area missing loses far more than the difference between two material grades.
This is the practical rule: the gap between materials on paper is small, and the gap between good and poor installation is large. Specify the material that tolerates the installation you will actually get.
Mineral wool: stone and glass
Mineral wool is spun from molten rock or recycled glass into a fibrous mat, supplied as batts, rolls or rigid slabs. Conductivity typically sits around 0.033 to 0.045 W/mK for stone wool and around 0.030 to 0.044 for glass wool, with the denser products at the better end.
Its defining advantages are fire behaviour and acoustics. Mineral wool is non-combustible in its base form, which makes it the default where fire performance governs — around structural steel, in compartment walls, on facades with combustibility restrictions. Its open fibrous structure also absorbs sound, so it does double duty in partitions and floors where acoustic separation is required.
It is also vapour-open, which means it lets moisture pass rather than trapping it — an advantage in assemblies designed to dry, a liability in assemblies where the vapour control layer is imperfect. Its weakness is water: wet mineral wool loses most of its thermal value, and unlike closed-cell foams it holds that water. It also slumps if compressed or installed loosely in a vertical cavity, opening a gap at the top of the bay that is invisible once the wall is closed.
Expanded polystyrene
EPS is made by expanding polystyrene beads with steam and fusing them into a block that is then cut. Standard grades sit around 0.036 to 0.040 W/mK; graphite-enhanced grades, where graphite particles reflect radiant heat within the foam, reach roughly 0.030 to 0.033.
EPS is the value option and it is genuinely good at what it does. It is dimensionally stable, holds its thermal performance over time without any blowing agent to lose, and is available in a wide range of compressive strengths, including grades that carry structural loads under slabs. It is the standard core for external wall insulation systems.
Its limitations are combustibility, low resistance to solvents and some adhesives, and a semi-open cell structure that absorbs more water than XPS. It also has poor dimensional tolerance to heat, softening at temperatures that are entirely achievable behind a dark facade in strong sun. Where it is used, it needs to be protected by a render, cladding or covering rather than left exposed.
Extruded polystyrene
XPS is the same polymer processed differently — extruded as a continuous closed-cell board rather than expanded from beads. Conductivity spans roughly 0.025 to 0.042 W/mK depending on grade, blowing agent and board thickness.
The closed-cell structure is the point. XPS absorbs very little water and retains most of its thermal performance when damp, which makes it the standard choice in the wet and buried positions: below-grade walls, under slabs, inverted roofs beneath ballast, and perimeter insulation. It also has high compressive strength, so it carries load without crushing.
It is more expensive than EPS, it is combustible, and older formulations lose some performance as the blowing agent diffuses out over time. Like EPS it must be protected from ultraviolet exposure, which degrades the surface within months.
Polyurethane and polyisocyanurate
PUR and PIR are rigid closed-cell foams, supplied as faced boards or sprayed in place. They are the best thermal performers in general construction use, with conductivity broadly in the 0.020 to 0.029 W/mK range depending on facing, formulation and whether the product is board or spray-applied.
The advantage is thickness. Where space is constrained — a roof build-up limited by an existing parapet, a reveal that cannot grow, a floor zone fixed by adjoining levels — PIR delivers a given thermal resistance in noticeably less depth than mineral wool or EPS. That is often the deciding factor rather than any cost calculation.
Spray-applied PUR adds airtightness, since it expands into irregular voids and seals them, which is valuable on retrofits with complex existing geometry. It also makes future access difficult and can conceal problems in the substrate it covers, so it should not be sprayed onto anything whose condition has not been verified first.
Both are combustible and both require careful attention to fire strategy, particularly in facades and roofs. PIR performs better than PUR under fire exposure because it chars rather than melting, but neither is a substitute for a non-combustible material where the fire strategy demands one.
Choosing building insulation materials by position rather than by product
The most reliable way to specify insulation is to work from where it sits, because the position dictates which failure mode is available.
Below ground and under slabs, the insulation will get wet and will be loaded. That narrows the choice to closed-cell boards with proven compressive strength — in practice XPS, or high-grade EPS where the water exposure is controlled.
In cavity walls, the insulation must not bridge moisture across the cavity and must stay in place for the life of the wall. Full-fill and partial-fill are different products with different rules, and mixing them up is a common cause of damp penetration.
In roofs, the governing question is where the insulation sits relative to the waterproofing. Above the membrane on an inverted roof, it must be closed-cell and moisture-resistant, and the waterproofing system beneath it has to be one that tolerates permanent burial. Below the membrane on a warm roof, thickness and compressive strength usually decide. Either way, the insulation and the waterproofing layer should be designed as one system, since each constrains the other.
In facades, fire performance frequently overrides thermal performance entirely, and the applicable rules depend on building height and use. This is the one position where the material decision should start with the fire strategy and only then consider lambda.
Where the performance is actually lost
The gap between designed and delivered thermal performance is usually made up of four things, none of them related to material choice.
Thermal bridging at slab edges, balconies, lintels, columns and around openings can consume a large share of the assembly’s benefit. A continuous insulation layer that is interrupted by a structural element conducts heat straight through that element, and the detail drawing is where this is either solved or ignored.
Air movement around insulation defeats it. Air that can circulate behind a board carries heat past it, so boards need to be tight to the substrate and to each other. This is why an airtightness layer and an insulation layer are complementary rather than alternative.
Gaps and compression are the installation faults that matter. A batt cut short, a board pushed past a pipe, insulation squashed to fit a service — each creates a local cold spot that is also a condensation risk.
Moisture undoes everything else. Insulation that becomes wet and cannot dry loses performance permanently, and the assembly that traps it usually also grows mould. Vapour control on the warm side and drying capacity on the cold side are not optional refinements; they are what keeps the thermal design true over time.
Getting these right is a matter of detailing and supervision rather than procurement, which is why insulation should be inspected before it is covered. Once the finish is on, the only evidence available is the energy bill. Where insulation is being specified as part of a wider scope, coordinating it with the engineering design review and the construction works avoids most of these losses before they are built in.
Frequently asked questions
Does more insulation always mean better performance?
Thermal resistance rises with thickness, but the return diminishes: each additional layer saves less than the one before, because the total resistance of the assembly is already high. Beyond a certain point the money is better spent on eliminating thermal bridges and improving airtightness, both of which are usually cheaper per unit of benefit than adding depth
Can different insulation materials be combined in one assembly?
Yes, and hybrid build-ups are common — a non-combustible layer where fire performance is needed with a higher-performance foam elsewhere, for instance. The requirement is that the vapour profile of the combination is checked as a whole. Placing a vapour-tight material on the cold side of a vapour-open one traps moisture between them, which is exactly the condition that causes interstitial condensation
Does insulation help in hot climates as well as cold ones?
It does, and the mechanism is the same: it slows heat transfer in whichever direction the temperature difference runs. In hot conditions the priority shifts somewhat, since solar gain through roofs and glazing often dominates, and reflective surfaces plus shading contribute alongside the insulation itself. The insulation still does the work of keeping conditioned air conditioned
How long does building insulation last?
Most insulation materials, kept dry and undisturbed, outlast the finishes around them. The failures that occur are almost always environmental rather than material — water ingress, compression from later work, damage during service installation, or displacement of loose-fill material over time. Protecting insulation from moisture and from subsequent trades matters more than the material’s nominal durability
