Insulation Board

What is an Insulation Board

An insulation board is a prefabricated panel whose job is to resist the conduction, convection, and radiation of heat through the building envelope. It does this by trapping still air or a low-conductivity gas inside a cellular foam or a fibrous matrix, because still air is itself a good insulator (around 0.025 W/m·K) and the solid skeleton of the board adds only a small parallel heat path. The performance figure that matters is thermal conductivity, the lambda value, expressed in watts per metre kelvin (W/m·K). A board with a lower lambda lets less heat through per millimetre, so it can be thinner for the same thermal resistance.

Boards are supplied in standard panel sizes, most commonly 1200 by 600 mm and 2400 by 1200 mm (roughly 4 by 2 ft and 8 by 4 ft), in thicknesses from 20 mm up to 200 mm or more, and with square, shiplap, or tongue-and-groove edges to limit heat loss at joints. Unlike a loose insulation quilt, a board holds its shape and a defined compressive strength, which is why boards, not quilts, are used under screeds, on flat roofs, and behind rendered facades where the insulation must carry load or stay dimensionally stable.

The four engineering properties that decide where a board can be used are thermal conductivity (lambda), reaction to fire (the Euroclass under EN 13501-1, or the flame-spread and smoke index under ASTM E84 in North America), compressive strength at 10 percent deformation, and moisture behaviour (water absorption and water-vapour resistance). No single board is best at all four: the plastic foams win on lambda, the mineral fibres win on fire, and the right answer changes with the position in the wall, roof, or floor.

The scale of the category is large. Building insulation is the main lever for reducing the operational carbon of buildings, which is why the EN 1316x harmonised product standards (EN 13162 through EN 13171) and the ASTM specifications C578, C612, and C1289 exist to make the declared values comparable and CE or product-certification compliant. Tightening energy codes, the EU Energy Performance of Buildings Directive, and national programmes such as the UK Future Homes Standard have driven specifiers toward thinner, higher-performance boards (PIR and phenolic) on space-constrained projects, and toward non-combustible mineral wool on high-rise facades after post-Grenfell fire-safety reforms.

Because the four key properties pull in different directions, the essence of insulation-board selection is not finding the cheapest board, but mapping the position in the construction and the regulatory constraints (fire class, U-value target, imposed load, exposure to water) onto a specific board family and grade. The rest of this guide builds that map.

Material Families and Classification

Insulation boards split into two chemistry families: organic plastic foams and inorganic mineral fibres. Within the foams, four products dominate: expanded polystyrene (EPS), extruded polystyrene (XPS), polyisocyanurate (PIR), and phenolic foam. Within the mineral fibres, two products dominate: stone wool (also called rock wool) and glass wool. Wood-fibre and cellular-glass boards occupy specialist niches. The table below compares the six mainstream boards on the properties that drive selection.

Expanded polystyrene (EPS) is made by expanding and fusing polystyrene beads with steam. It is the lowest-cost board, light, easy to cut, and dimensionally stable, with lambda around 0.036 W/m·K for standard white EPS and 0.030 to 0.032 W/m·K for graphite-enhanced grey EPS, where embedded graphite particles reduce radiative heat transfer. EPS is graded by compressive strength under EN 13163: EPS70, EPS100, EPS150, EPS200, and EPS300 correspond to roughly 70, 100, 150, 200, and 300 kPa at 10 percent deformation, with nominal densities near 15, 20, 25, 30, and 40 kg/m³. EPS is the default for rendered external wall systems, cavity boards, and screed-grade floor insulation.

Extruded polystyrene (XPS) is produced by a continuous extrusion process that gives a uniform closed-cell structure with a tough surface skin. Its defining property is very low water absorption (well under 0.7 percent by volume in the long term), which makes it the standard board for below-grade walls, perimeter and ground-floor insulation, and inverted (protected-membrane) roofs, where the insulation sits above the waterproofing and is rained on directly. Lambda is typically 0.029 to 0.036 W/m·K and compressive strengths run from about 200 kPa up to 700 kPa for the heavy-load grades.

PIR and phenolic are rigid thermoset foams with the lowest lambda of all mainstream boards, which lets them hit a target U-value at the smallest thickness. PIR is typically 0.022 to 0.028 W/m·K; phenolic reaches 0.018 to 0.023 W/m·K, the best per-millimetre performance available. Both are usually supplied faced (aluminium foil or glass tissue) and are widely used in pitched and flat roofs, framed walls, and insulated plasterboard. Stone wool and glass wool are spun mineral fibres bound into boards. Their lambda (0.033 to 0.041 W/m·K) is higher than the foams, but they are non-combustible (Euroclass A1 or A2), sound-absorbing, and vapour-open, which makes them the mandated choice on tall facades and in fire-rated and acoustic separating constructions.

How the Boards Are Made

The manufacturing route fixes a board's cell structure, and the cell structure fixes its thermal, moisture, and mechanical behaviour. Understanding the four production principles below explains why two boards of similar density can perform very differently, and why you cannot substitute one family for another on lambda alone.

Polystyrene foams (EPS and XPS) share the same polymer but differ entirely in process. EPS starts as small polystyrene beads pre-expanded with pentane, which are then steam-fused inside a mould; the result is a mass of closed-cell beads with small interstitial voids between them, which is why EPS can wick and hold some water. XPS is melted, mixed with a blowing gas, and forced through a die in a continuous sheet that expands as it leaves the die, producing a uniform fine closed-cell matrix with a continuous surface skin and almost no inter-cell voids, hence its very low water absorption and higher compressive strength at equal density.

PIR and phenolic are reaction foams. In PIR, a diisocyanate reacts with a polyol on a moving conveyor (a laminator) while facers are unrolled top and bottom; the foam rises, bonds to the facers, and cures into a rigid closed-cell board. The high proportion of stable isocyanurate linkages gives PIR better high-temperature and fire behaviour than ordinary polyurethane (PUR). Phenolic foam is made by foaming and curing a phenol-formaldehyde resin between facers; its exceptionally fine, more than 95 percent closed-cell structure is what delivers the lowest lambda of any common board. Both thermoset foams are blown with a low-conductivity gas that gives the headline lambda but slowly exchanges with air over years, which is why their long-term (aged) thermal resistance is declared separately.

Mineral wool is made by melting rock (basalt and slag for stone wool, recycled glass and sand for glass wool) at roughly 1400 to 1500 degrees Celsius, then spinning or blowing the melt into fine fibres. The fibres are sprayed with a thermosetting binder, collected into a mat, compressed to the target density, and cured in an oven. The open, randomly oriented fibre structure traps air for insulation while leaving the material vapour-open and non-combustible, since the fibres are themselves stone and re-melt only above 1000 degrees Celsius. Board density is set by how much the mat is compressed, which is why mineral wool boards span a wide range from soft cavity boards to dense, rigid facade and roof boards.

Standards, Facings, and Fire Class

Every insulation board sold for construction is covered by a harmonised product standard that defines how its declared properties are measured and verified. In Europe the EN 1316x series provides one standard per material; in North America the ASTM C-series does the same. Specifying the standard and the declared value, not just the material name, is what makes two quotes comparable. The table below maps each board to its principal product standards.

Reaction to fire is governed in Europe by EN 13501-1, which assigns a Euroclass from A1 (non-combustible) through A2, B, C, D, E to F (no performance determined), plus a smoke sub-class (s1 best to s3) and a flaming-droplet sub-class (d0 best to d2). A typical mineral wool board is A1 or A2-s1,d0; PIR and phenolic typically reach B-s1,d0 or C; raw EPS and XPS are class E and must be protected behind a lining or render. North American codes use ASTM E84, which reports a flame-spread index and a smoke-developed index. After high-rise facade fires, many jurisdictions now require A2-s1,d0 or better on external walls above a height threshold (18 metres in England), which in practice forces mineral wool on tall facades regardless of its higher lambda.

Facings change a board's vapour, fire, and bonding behaviour. Aluminium-foil facers on PIR and phenolic act as a vapour barrier and add a low-emissivity reflective surface behind an air cavity, but they must sit on the warm side of a wall so they do not trap interstitial condensation. Glass-tissue or mineral-coated facers improve fire performance and give a key for plaster, render, or bitumen. EPS and XPS are usually unfaced but can be supplied bonded to a cement particle board or magnesium-oxide board for rendered external wall insulation systems. For below-grade and inverted roofs, choose unfaced XPS so the closed-cell board itself is the moisture-resistant element.

Two further declared properties round out the standard. Water-vapour diffusion resistance (the μ factor, dimensionless) tells you how vapour-tight the board is: XPS and faced PIR have a high μ (vapour-resistant), while mineral wool has a low μ (vapour-open), which is why mineral wool suits walls designed to dry outward. Dimensional stability under temperature and humidity is declared under EN 1604, and matters most for boards laid over a warm flat-roof deck, where thermal movement can open butt joints. Always design the wall's vapour path and the board's facer together, not in isolation.

Key Specification Parameters

Reading a board datasheet is the core skill for a specifier. A datasheet may list a dozen properties, but seven drive the selection decision: thermal conductivity, thermal resistance, reaction to fire, compressive strength, water absorption, water-vapour resistance, and dimensional stability. Each is explained below, with the units and the trap to watch.

Thermal conductivity (lambda, W/m·K) is the material property and the first number to compare. It is declared as a single value (the lambda-D, declared after any ageing) and is independent of thickness. Lower is better. For the thermoset foams, always confirm whether the figure is the initial or the aged lambda, because PIR and phenolic rise slightly as their cell gas diffuses; design with the aged value.

Thermal resistance (R-value, m²·K/W) is the property of the specific board: R equals thickness divided by lambda. A 100 mm PIR board at 0.022 W/m·K gives R of about 4.5 m²·K/W; the same thickness of mineral wool at 0.035 W/m·K gives only about 2.9 m²·K/W. R-value is what you sum across the wall layers to get the assembly U-value (W/m²·K) that the building code limits. The table below shows the approximate thickness each board needs to reach a typical wall target of R 5.5 m²·K/W.

Reaction to fire is the Euroclass (EN 13501-1) or the ASTM E84 flame-spread and smoke indices. This is a pass-or-fail gate, not a trade-off: if the application demands A2 or better, no amount of thermal advantage rescues a class B foam. Compressive strength is declared at 10 percent deformation in kPa and must exceed the imposed load with margin. Floor and roof boards need it; cavity and timber-frame boards barely load the insulation. Note that mineral wool boards, even rigid ones, have far lower compressive strength than the foams, so they are not used as load-bearing screed boards.

Water absorption (EN 12087 long-term immersion, percent by volume) and water-vapour resistance (the μ factor) together describe moisture behaviour. A wet board loses thermal performance, so positions exposed to ground water or rain demand the low-absorption XPS. Dimensional stability (EN 1604) and reaction under load (EN 1606, for boards under permanent compression) close out the list. The common selection error is to compare two boards on lambda and price while ignoring fire class and moisture position, the two parameters that actually decide whether a board is even permitted in that location.

Selection Decision Factors

To turn the preceding five chapters into a specific board and grade, follow the decision sequence below. Most selection mistakes come not from a single wrong number, but from deciding on material before checking the fire and moisture constraints that should rule the choice. These seven steps work as a fixed specification template.

1. Position in the construction: First fix where the board sits, because position dictates almost everything else. Below-grade and inverted roof means closed-cell XPS; rendered external wall means EPS or mineral wool; pitched or flat warm roof means PIR or mineral wool; loaded floor means high-density EPS, XPS, or PIR floor grade; a fire-rated or acoustic separating element means mineral wool.
2. Reaction-to-fire requirement: Check the building code for the required Euroclass (or ASTM E84 indices) at that height and use. If the wall demands A2-s1,d0 or better, the choice is mineral wool, full stop. This gate is applied before, not after, the thermal comparison.
3. Thermal target and thickness: Work from the required U-value to the insulation R-value, then divide by each candidate board's aged lambda to get the thickness. Where build depth is tight (internal wall lining, slim flat roof) the low-lambda PIR or phenolic wins; where depth is free, cheaper EPS or mineral wool at greater thickness is more economical.
4. Moisture and vapour: Confirm the board's water absorption suits the exposure and that its vapour resistance (μ) matches the wall's drying direction. Vapour-open mineral wool for walls drying outward; vapour-tight faced PIR or XPS where a vapour barrier is wanted on the warm side.
5. Compressive strength: Match the declared strength at 10 percent deformation to the imposed area load and any point loads from feet, wheels, or pedestals. Specify EPS or XPS grade by kPa (for example EPS150, XPS 500 kPa), not by material alone.
6. Facing, edge profile, and board size: Choose the facer for the vapour strategy and the substrate (foil, glass tissue, cement board, or unfaced), the edge (square, shiplap, or tongue-and-groove) to limit joint heat loss, and a panel size that minimises cutting waste on the module.
7. Certification, embodied carbon, and total cost: Verify the CE or UKCA marking and the declared values via a DoP (Declaration of Performance), check the Environmental Product Declaration if a carbon target applies, and compare installed cost (board plus fixings plus labour for the thickness needed), not the per-board price, since a thinner low-lambda board can offset its higher unit price.

One last dimension that is easy to overlook is serviceability and supply: panel availability in the thickness and grade you specified, lead time, compatibility with the chosen render, membrane, or fixing system, and the manufacturer's technical support for U-value calculations and fire-test evidence. Established board makers, including Kingspan (Kooltherm phenolic and TR/TP PIR ranges), ROCKWOOL (stone wool boards such as Comfortboard), Knauf Insulation and Saint-Gobain Isover (glass and stone wool), Owens Corning (FOAMULAR XPS), DuPont (Styrofoam XPS), Recticel and Celotex (PIR), and EPS makers such as Jablite, publish full declared-value datasheets, EPDs, and system-test evidence, which is what large projects rely on when a specification has to survive building-control and warranty scrutiny.