EPS Insulation: The Budget Floor Board That Needs More Thickness but Saves You Hundreds

Your builder quotes 100mm PIR insulation for the floor at £20-30 per square metre. You do the maths on a 24m2 extension and it's £480-720 just for floor insulation. Then someone on a forum mentions EPS boards at half the price. You order them, lay them, pour the slab, and three weeks later building control fails the floor because you bought 100mm EPS when you needed 160mm. The insulation is now buried under concrete. Ripping it out and starting again costs thousands.

EPS is a genuinely good product for floor insulation. But it plays by different rules to PIR, and the numbers you need to know before ordering are not the ones most guides explain.

What it is and what it's for

EPS (expanded polystyrene) is the white rigid foam board you've seen in packaging. Construction-grade EPS is denser and stronger than packaging foam, but it's the same base material: tiny polystyrene beads expanded with steam and fused together, creating a closed-cell structure that's 98% trapped air and 2% polystyrene. That trapped air is what provides the insulation.

The standard lambda value (thermal conductivity, the number that tells you how well a material insulates, where lower is better) for white EPS is 0.038 W/mK. Compare that to PIR at 0.022 W/mK, and you can see EPS is significantly less thermally efficient per millimetre. That's the trade-off: EPS costs considerably less per square metre than PIR, but you need substantially more thickness to achieve the same thermal performance.

Where does that trade-off make sense? Below the floor slab. Under the slab, space is rarely a constraint. You're digging down anyway, so an extra 60mm of insulation depth costs almost nothing in labour. The material savings on a 24m2 floor are substantial. This is why the community consensus on every UK building forum is clear: use EPS below the slab, save your money, and put the savings toward thicker insulation or better materials elsewhere.

EPS boards comply with BS EN 13163 (the European product standard for factory-made expanded polystyrene). The major UK brands are Jablite (the market leader, owned by Bewi Group) and Kay-Metzeler. Both carry BBA certification (BBA 87/1796 for Jablite), which means they've been independently tested and certified for use in UK building applications. Building control inspectors recognise BBA-certified products without question.

Types, sizes, and specifications

The grade numbers on EPS boards confuse almost everyone. EPS70, EPS100, EPS150. It's natural to assume these relate to thermal performance. They don't. All standard white EPS grades share the same lambda value: 0.038 W/mK. The number refers to compressive strength at 10% compression, measured in kilopascals (kPa). It's a measure of how much weight the board can bear before deforming.

For a domestic house extension, EPS70 is the right grade. A standard domestic floor load is roughly 5 kPa (the weight of the concrete slab plus furniture, people, and normal household loading). EPS70 handles 70 kPa, fourteen times the actual load. There is no structural reason to upgrade to EPS100 for a house floor.

The exception: if you're planning a polished concrete floor or anticipate heavy point loads (a large kitchen island with stone worktop, a piano), mention it to your structural engineer. They may specify EPS100 for comfort margin. It costs only 20-30% more per board, so there's no strong reason not to upgrade if you have any doubt.

Grey graphite EPS: the performance upgrade

Standard white EPS contains reflective graphite particles in the grey variant. These particles reflect radiant heat within the foam cells, reducing the lambda value from 0.038 to 0.030 W/mK. That's a 20-25% improvement in thermal performance per millimetre.

In practical terms: where you'd need 160mm of white EPS to hit 0.18 W/m2K (the Part L extension floor target), grey EPS achieves the same with about 125mm. That 35mm saving matters if your floor build-up depth is constrained. Jablite sells grey EPS as the "HP+" range (Jabfloor HP+), and it costs roughly 20% more than the equivalent white board.

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Standard board size

EPS floor insulation boards come in 2400 x 1200mm sheets (covering 2.88m2 per board). At EPS70 density (15 kg/m3), a 100mm board weighs about 4.2 kg. You can carry one under each arm. That's a real practical advantage over concrete blocks, timber sheets, or even PIR boards (which are denser). A single person can insulate an entire floor without help.

EPS is not XPS

One common confusion. EPS (expanded polystyrene) and XPS (extruded polystyrene) are different products made from the same raw material. XPS is denser, stronger, more moisture-resistant, and much more expensive. XPS boards are typically blue, pink, or green (depending on manufacturer). EPS is white or grey. For below-slab domestic floor insulation, EPS is the standard choice. XPS is overkill unless you're building on waterlogged ground or need extreme compressive strength.

How to work with it

Cutting

EPS cuts with anything sharp. A handsaw works best for straight cuts across full boards. A bread knife (genuinely, ask any builder) works for trimming edges and shaping around pipes. For precision, a hot wire cutter produces the cleanest edge, but you won't have one on a domestic build site and don't need one.

Unlike PIR, which requires a clean saw cut if you go oversize, EPS can be rasped down with a surform. Cut 5mm oversize, offer up the board, and rasp the edge until it fits. The foam shaves easily.

Laying in a floor build-up

The correct sequence for a ground-bearing concrete floor with EPS insulation:

1. Compacted hardcore (typically 150-200mm of Type 1 or similar)
2. Sand blinding (25mm layer to create a smooth surface and prevent sharp stones puncturing the DPM)
3. DPM (1200 gauge / 300 micron polythene, lapped and taped at joints, turned up at perimeter walls)
4. EPS boards laid tight-butted with staggered joints (like brickwork, never aligned in a straight line)
5. Polythene slip layer above the EPS to prevent wet concrete seeping into board joints
6. Steel mesh reinforcement on spacers
7. Concrete slab (typically 100-150mm)
8. Screed (if underfloor heating, the UFH pipes sit in or below the screed layer)

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A detail that gets overlooked: the perimeter insulation upstand. A strip of EPS (or purpose-made perimeter strip) must run vertically from the floor insulation up to the top of the screed at every wall junction. Without it, heat short-circuits through the concrete at the edges of the room, creating a cold bridge that undermines the entire floor insulation. Building control inspectors check for this.

Protecting the DPM

Sharp stones in the hardcore can puncture the DPM when the weight of EPS, concrete, and floor loading bears down on it. The sand blinding layer prevents this. Some builders use a sacrificial 25mm layer of cheap EPS offcuts below the DPM as additional protection. It's a few pounds' worth of material that prevents a DPM failure you can never access again.

Storage on site

EPS degrades under prolonged UV exposure. The surface yellows and becomes chalky. This is cosmetic rather than structural, but boards left in direct sunlight for more than 60-90 days start to lose surface integrity. Cover stacked boards with a tarpaulin or store them indoors. If boards have been on site for weeks and look yellowed, they're still usable, just scuff off the degraded surface layer before laying.

How much do you need

Thickness for Part L compliance

Approved Document L (2021 edition, in force since June 2022) sets floor U-value targets:

These are approximate standalone insulation thicknesses. The actual required thickness depends on the full floor build-up and the floor's perimeter-to-area (P/A) ratio. A narrow rectangular extension loses more heat at the edges than a square one, so the P/A ratio affects the U-value calculation. Your structural engineer or architect should provide the specific thickness for your build. But these figures give you the right ballpark for ordering and budgeting.

Quantity calculation

Each board covers 2.88m2 (2.4m x 1.2m). For a typical 4m x 6m extension floor:

• Floor area: 24m2
• Boards needed: 24 / 2.88 = 8.3 boards
• Add 5% waste: 8.7 boards
• Round up: 9 boards

EPS cuts easily and produces minimal waste compared to PIR, so 5% wastage is usually sufficient. If your floor is an awkward L-shape with lots of cuts, allow 10%.

If your target thickness is 160mm and boards come in 100mm and 50mm, you'll need two layers (100mm + 50mm, or 80mm + 80mm). Stagger the joints between layers so no joint aligns vertically. This eliminates thermal bridging through the joints and creates a more stable base for the slab.

Cost and where to buy

EPS70 at 100mm thick costs £6-12 per square metre from online insulation specialists and builders' merchants. The wide range reflects pack sizes and supplier: bulk packs of 6 boards from specialist online retailers sit at the lower end, while single boards from general merchants sit higher.

Specific pricing from March 2026: Builder Depot lists a single Jablite Jabfloor 70 100mm board at £30.24 (£10.51/m2). Trade Insulations sells a 3-board pack for £55 (£6.36/m2). Cut Price Insulation offers 6-board packs at £113 (£6.53/m2). The message is clear: buy in packs, not singles.

For context, equivalent PIR boards (100mm Celotex or Kingspan) run £20-30/m2 from the same suppliers. On a 24m2 extension floor, choosing EPS over PIR can save roughly £200-500 even after accounting for the extra thickness you need.

Where to buy

Online insulation specialists offer the best prices. Trade Insulations, Cut Price Insulation, Insulation UK, and National Insulation Supplies all carry Jablite and Kay-Metzeler EPS boards with free delivery above modest order thresholds. Wickes stocks smaller-format EPS boards but primarily for external wall insulation (EWI) applications, not floor insulation, and at higher per-m2 prices.

Travis Perkins and Jewson stock EPS floor boards on their trade counters, but pricing varies by branch and account. If your builder is ordering through their trade account, ask what they're paying per board and compare to online prices. The gap can be significant.

How to avoid the cable trap

This is the safety note that most insulation guides don't mention, and it matters.

If electrical cables run through your insulated floor build-up (common with underfloor heating controls, zone valves, or floor-level socket feeds), they must be enclosed in conduit or physically separated from the EPS by a barrier. Your electrician needs to know that the floor insulation is EPS, not PIR, because the cable routing requirements are different.

BS 7671 (the IET Wiring Regulations, 18th Edition) Section 523.9 also addresses cable installation in thermal insulation, requiring derating factors (reducing the maximum current the cable can carry) when cables are enclosed in insulation. This affects cable sizing, which is your electrician's responsibility, but you should flag it during planning if you know the floor uses EPS.

Solvent sensitivity

EPS dissolves on contact with hydrocarbons, ketones, esters, and solvent-based adhesives. Standard PVA wood glue is fine. Polyurethane expanding foam (like Soudal or Gorilla Foam) is fine. But solvent-based contact adhesives, paint thinners, and spray paints will melt the foam on contact. If you're using any adhesive near EPS boards, check the product data sheet for "polystyrene compatible" or "solvent-free" before application.

This matters most during installation. If a builder uses the wrong adhesive to temporarily fix a board or seal a joint, the EPS dissolves and you get a gap in your insulation layer. Under a concrete slab, that gap is permanent.

Alternatives

PIR insulation (Celotex, Kingspan) is the main alternative for floor applications. Lambda 0.022 W/mK means 100mm of PIR does the same job as 160mm of EPS. PIR costs roughly double per square metre, but saves 60mm of floor build-up depth. If your finished floor level is constrained (matching an existing floor height in the house, for example), the extra thickness of EPS can be a genuine problem. PIR is the better choice where depth matters.

Mineral wool (Rockwool, Knauf Earthwool) is not suitable for below-slab floor insulation. It compresses under load and absorbs moisture. Mineral wool belongs between timber studs, joists, and rafters, not under concrete.

Grey graphite EPS splits the difference between standard EPS and PIR. At lambda 0.030 W/mK, it needs about 125mm for extension floor compliance instead of 160mm for white EPS. The price premium is roughly 20% over standard white EPS, making it a strong middle-ground option when depth is moderately constrained.

The hybrid approach

Several BuildHub forum contributors report using a hybrid floor build-up: 100-160mm of cheap EPS on the bottom, topped with 50-100mm of PIR. The EPS provides the bulk of the thermal resistance at low cost, while the PIR on top creates a firm surface for underfloor heating pipe staples (standard short staples grip PIR much better than EPS) and adds thermal performance in less depth. This is a legitimate approach and has passed building control on multiple documented self-build projects.

Fire classification

Standard EPS without fire retardant additive (FRA) is Euroclass F under BS EN 13501-1. That's the lowest fire performance rating. Most floor-grade boards sold in the UK include FRA as standard, achieving Euroclass E. Neither rating is anywhere near mineral wool's Euroclass A1 (non-combustible).

For most domestic extension applications, Euroclass E or F is acceptable under Approved Document B (fire safety), but check your specific project's ADB classification with your architect or building control officer. EPS must always be covered by a non-combustible layer (concrete slab above, hardcore below). It must never be left exposed as a finished surface, and it must not be stored near heat sources or open flames on site.

Where you'll need this

• Insulation - ground floor slab insulation where depth isn't constrained and cost is a priority

EPS insulation appears in floor build-ups across any extension, new build, or renovation project where a ground-bearing concrete slab is specified. The guidance above applies regardless of project type.

Common mistakes

Ordering the wrong thickness. The single most expensive EPS mistake. 100mm of EPS70 gives a U-value of roughly 0.28 W/m2K, which fails the 0.18 extension floor target by a wide margin. You need approximately 160mm of standard white EPS for an extension floor. Check with your architect or structural engineer, but if someone tells you 100mm of EPS is enough for a floor, they're confusing it with PIR.

Confusing grade numbers with thermal performance. EPS70, EPS100, and EPS150 all have the same lambda value: 0.038 W/mK. The number is compressive strength in kPa, not a thermal performance indicator. EPS150 isn't warmer than EPS70. It's just stronger.

Omitting the perimeter upstand. Without a continuous strip of insulation running from the floor insulation up to the top of the screed at every wall junction, heat escapes through the concrete at the room's edges. This cold bridge causes noticeable heat loss that undermines the entire floor insulation. Building control inspectors look for it specifically.

Laying EPS below the DPM. EPS goes above the DPM, not below it. The DPM protects the insulation from ground moisture. Some builders (especially on smaller jobs) get the sequence wrong. The correct order, bottom to top: hardcore, blinding, DPM, EPS, slip layer, slab.

Using solvent-based products near EPS. A quick squirt of spray paint to mark the floor, a solvent-based adhesive to stick a pipe clip. Both dissolve EPS on contact, leaving permanent voids in your insulation. Use only solvent-free products around polystyrene boards.

Leaving boards exposed to weather for months. EPS degrades under UV. Boards stored uncovered on site for more than 60-90 days develop a chalky, yellowed surface. Cover them with a tarp or use them promptly. If they've been sitting in the sun, rasp off the degraded surface before laying.