Under a slab-on-grade, the insulation sits between the drainage layer and the concrete, so it has to carry the load without creeping and stay indifferent to water: in practice extruded polystyrene, high-density expanded polystyrene, PUR/PIR or cellular glass, with a compressive stress at 10% deformation measured to EN 826 of at least 150 kPa in a living room and 200 to 300 kPa in a garage or workshop. The retrofit target for a ground floor in Approved Document L is a U-value of 0.25 W/m²·K, which on a slab-on-grade means roughly 100 mm of extruded polystyrene or 80 mm of PUR/PIR. Note that the requirement is expressed as a U-value for the whole element and never as a minimum thickness, which is why asking for "the required insulation thickness under a slab" has no single answer. In a retrofit, insulating under the slab means breaking out what is there, so the work becomes a groundworks package, and what settles it on survey is not the U-value but the height available under the door thresholds.
Understanding the slab-on-grade: constraints, risks and watch points
Identifying the slab's makeup (concrete, drainage layer, vapour membrane) and the substrate's condition
A slab-on-grade often sits on a drainage layer, then a vapour membrane, then the concrete. Before adding anything, do a small probe at the perimeter or in a cupboard. Look for cracks, settlement, hollow areas, traces of efflorescence, and check that the membrane is properly continuous.
Managing rising damp and thermal bridges at the slab's perimeter
The sensitive point is rising damp. Added insulation can block drying and damage the screed, adhesives or parquet. If needed, first address drainage, ventilation and vapour-barrier continuity. At the edge of the slab, the junction with the wall creates a thermal bridge. A vertical upstand insulation at the skirting or a thermal break limits the cold floor effect.
Checking finished levels, service runs and tolerances before adding insulation
Insulation plus a screed quickly adds up to 6 to 12 cm. Check door thresholds, stair steps, ceiling height and services. Take laser level readings, identify the service runs, then validate flatness before pouring to avoid bad surprises. Those figures are worth more than a check: Argile's AI turns the on-site survey into the areas, heights and dimensions that feed the pricing.
Choosing the right insulation under or over the slab: solutions and selection criteria
Insulation under the slab (new build or major rework): load-bearing panels and compressive strength
In new build, or when you're redoing everything, the insulation goes under the slab. Load-bearing panels (XPS, high-density EPS, PU, cellular glass) able to carry the loads without crushing are preferred. Look at the compressive strength (often expressed in kPa) and long-term behaviour. Also consider the vapour membrane and perimeter upstands to limit moisture and thermal bridges.
The four families usable under a slab separate on three criteria only: lambda, which sets the thickness, compressive stress, which sets the admissible use, and behaviour in water, which decides whether the product survives contact with a damp drainage layer.
| Insulant under slab | λ (W/m·K) | Thickness for R = 3 m².K/W | Compressive stress at 10% (EN 826) | Behaviour in water |
|---|---|---|---|---|
| Extruded polystyrene (XPS) | 0.032 to 0.036 | 100 to 110 mm | 200 to 500 kPa depending on grade | very low absorption, accepted in ground contact |
| High-density expanded polystyrene | 0.031 to 0.038 | 95 to 115 mm | 100 to 200 kPa | low absorption, to be kept clear of free water |
| Faced PUR/PIR | 0.022 to 0.028 | 70 to 85 mm | 120 to 175 kPa | facing must stay intact, joints to be treated |
| Cellular glass | 0.040 to 0.050 | 120 to 150 mm | above 500 kPa | rot-proof, impervious, accepted under heavy load |
The thickness you settle on is not only a matter of R. XPS at 100 mm and PUR/PIR at 80 mm give the same declared performance, but those 20 mm change the threshold level and sometimes the number of steps to rebuild. Price the variant that preserves ceiling height, and put the finished floor level on the quotation rather than the insulation thickness alone.
Insulation over an existing slab: rigid panels plus a screed, or ready-to-install assemblies
On an existing slab, the most common solution is rigid panels then a floating screed. A quicker alternative is ready-to-install assemblies such as insulated floor panels, useful when the installation needs to stay clean and dry. Check flatness, hard points, and compatibility with underfloor heating and floor coverings.
Weighing use, loads, moisture and available height on the slab
The right choice depends on use (living room, workshop, garage), loads, residual moisture, and the height available on the slab (thresholds, doors, stairs). At a shallow thickness, a higher-performing insulant can avoid "eating into" ceiling height. In a damp zone, secure the waterproofing and treat rising damp before closing up. To compare the options, also see insulating the ground floor from below or above.
Preparing the job: diagnosis, compatibilities and interface management
Diagnosing the existing slab: flatness, cracks, moisture, efflorescence and cold spots
Before laying insulation or a screed, check that the sound slab is flat enough. Identify active cracks, previous patches and powdery areas. Check moisture (rising damp, condensation), efflorescence and cold spots that reveal a thermal bridge. If the substrate isn't stable or is too damp, treat that first, or the job will need to be redone.
Treating the perimeter: thermal break, insulation upstands and wall-slab connection
The perimeter is often where watts are lost. Install a perimeter strip and plan for continuous insulation upstands to break the thermal bridge between wall and slab. Take care with thresholds, load-bearing partitions, posts and openings. The goal is simple: an envelope with no gaps, and a screed that can expand without pushing against the walls.
Coordinating services and underfloor heating: routing, encasement and protection
Plan service routing in advance to avoid drilling into the slab at the last minute. Sleeve and mark every reservation, protect the pipes, and respect the minimum encasement depth for underfloor heating circuits. A clear layout, protection at crossing points, and photo documentation before pouring secure what comes next. To go further on sizing and pipe spacing, see our article on low-temperature underfloor heating.
Installation on slab-on-grade: practical steps for a durable, high-performing floor
Laying the insulation on the slab: layout, tight joints, vapour barrier and airtightness
On a clean, flat slab, plan the layout and lay the panels with staggered joints, tightly fitted, with no gaps. Treat the upstands at the perimeter. Add a vapour barrier or a membrane suited to the substrate, with taped overlaps, then connect it to the walls for continuous airtightness.
Laying the screed over the insulated slab: mesh, thicknesses, drying time
Over the insulation, lay a floating screed, with a separation film if needed. Respect the minimum thicknesses required by best practice. Add mesh reinforcement if specified, and protect the services before pouring. Wait for complete drying before laying floor coverings, often several weeks.
Getting the details right that make the difference: thresholds, hatches, wet rooms and garages
Defects creep in at the junctions. At thresholds, avoid thermal bridges with an insulation return. Around hatches, plan an insulated, sealed frame. In wet rooms, secure the waterproofing under the tiling. On the garage side, treat the separation to maintain thermal continuity.
Quality, checks and rules in 2026: aiming for performance without bad surprises
Checks during the job: insulation continuity, moisture, slab flatness
On site, check the installation as you go. The insulation must stay continuous, with no gaps, and well-treated junctions. Before closing up, check residual moisture and the slab's flatness to avoid settlement, rising water and cold spots. A 2 m straightedge and a visual check are often enough.
Best practices to limit disputes: photos, product data sheets, reports and traceability
To limit disputes, keep clear evidence. Dated photos before and after, product data sheets and certificates, handover report. Note references, surface areas, thicknesses and the people involved. This traceability also helps in case of a funding audit.
Useful reminders for 2026: registration, performance requirements and consistency with the retrofit assessment
In 2026, check for suitable registration for every eligible measure and performance that's consistent with the funding. If a retrofit assessment is driving the project, align materials, ventilation and systems with the chosen scenario. Reinforced insulation without proper moisture management often ends up costing more than expected.
