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.
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.
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 CEE or MaPrimeRénov' audit.
Useful reminders for 2026: RGE compatibility, performance requirements and consistency with the energy audit
In 2026, check for suitable RGE certification for every eligible trade and performance that's consistent with the funding. If an energy audit 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.
Key figures
60–100 cm
Insulation depth
5–10 cm
Perimeter insulation thickness
0.5–1.0 W/m·K
Linear heat loss
Frequently asked questions
In a living room, aim at minimum for an SC1 a2 Ch-type compression class (often 150 kPa) to limit creep under the screed. In a garage or workshop, lean instead towards 200 to 300 kPa depending on the loads and use. Always check the insulant's technical approval/DTA and its compatibility with the floating screed.
Louis Airy
COO of Argile
