By Topic Intermediate

Ground Floor Insulation: Crawl Space and Slab Methods

10 min read

Why Insulate the Ground Floor?

The ground floor is the building element that separates your living space from the ground or an unheated area (crawl space, earth, cellar). Without insulation, it can account for 7 to 10% of a home’s heat losses. It is also a major source of discomfort: a cold floor creates an unpleasant sensation even when the air temperature is adequate.

Under France’s RE2020 regulations, the target thermal resistance for a ground floor is at least R = 4 m².K/W, which requires substantial insulation thicknesses. It pays to choose the right technique from the outset.

💡 Tip — Ground floor insulation is one of the most cost-effective improvements in terms of cost-to-comfort ratio. Even if you need to prioritise your budget, do not skip this element.

flowchart TD A{Which configuration?} A -->|New build on grade| B[INSULATION BELOW SLAB] A -->|Existing renovation| C[INSULATION BELOW SCREED] A -->|Crawl space or cellar| D[UNDERSIDE INSULATION] B --> E{Soil moisture level?} C --> E D --> E E -->|High| F[XPS or PUR] E -->|Low| G[EPS or mineral wool] style A fill:#0F4C81,stroke:#0F4C81,color:#fff style E fill:#FDFCF9,stroke:#C67A3C,color:#0F4C81 style B fill:#56C6A9,stroke:#56C6A9,color:#fff style C fill:#F58220,stroke:#F58220,color:#fff style D fill:#6B5876,stroke:#6B5876,color:#fff style F fill:#CD212A,stroke:#CD212A,color:#fff style G fill:#FDB813,stroke:#FDB813,color:#fff

The 3 Ground Floor Configurations

Before choosing your insulation technique, identify your floor configuration:

Configuration Description Suitable insulation technique
Slab on grade The slab rests directly on the ground Insulation below the slab or below the screed
Crawl space Ventilated gap between the ground and the floor Insulation on the underside of the floor
Basement / cellar Unheated space below the floor Insulation on the underside of the floor
GROUND FLOOR INSULATION — 3 CONFIGURATIONS Comparative cross-sections BELOW SLAB (slab on grade) Drainage bed PE membrane INSULATION EPS / XPS / PUR 9-16 cm Concrete slab Floor finish Heat flow Natural ground BELOW SCREED (floating screed) Concrete slab INSULATION EPS / PUR 9-16 cm PE membrane Floating screed Floor finish Heat flow Natural ground UNDERSIDE (crawl space) Concrete floor Floor finish INSULATION Min. wool / spray PUR 10-18 cm CRAWL SPACE ventilated ~ ~ Heat flow RE2020 TARGET: R >= 4 m2.K/W Slab on grade: insulation before pouring Floating screed: insulation on existing slab Underside: insulation fixed below

Technique 1: Insulation Below the Slab (Slab on Grade)

This is the most common solution for new builds on grade. The insulation is laid before the slab is poured.

Suitable Materials

The insulation must withstand compression and moisture:

Material Conductivity (λ) Thickness for R = 4 Advantages Disadvantages
EPS (expanded polystyrene) 0.032 W/m.K 13 cm Lightweight, affordable, easy to lay High carbon impact
XPS (extruded polystyrene) 0.029 W/m.K 12 cm Excellent moisture and compression resistance More expensive than EPS
PUR (polyurethane) 0.022 W/m.K 9 cm Very high performance, low thickness Expensive, not bio-sourced
Cellular glass 0.038 W/m.K 16 cm Incompressible, rot-proof, load-bearing Very expensive, heavy

Step-by-Step Installation

  1. Excavate and level the natural ground
  2. Lay a drainage bed (20/40 mm gravel, minimum 20 cm thick)
  3. Level the gravel bed with a straightedge and compact
  4. Roll out a polyethylene membrane (vapour barrier) with 20 cm overlaps between sheets
  5. Lay the insulation boards edge to edge, staggered joints, in one or two crossed layers
  6. Install the welded mesh reinforcement (as per structural calculations)
  7. Pour the concrete slab (minimum 12 cm)
  8. Ensure insulation upstands at the perimeter to address the slab-to-wall thermal bridge

⚠️ Warning — Do not neglect the thermal bridge at the slab edge. This is the weak point of this technique. Provide a vertical insulation return (thermal break or L-shaped insulation) around the entire perimeter.

Technique 2: Insulation Below the Screed (Renovation or Existing Slab)

When the slab already exists, you can insulate on top by laying insulation then a floating screed.

Installation

  1. Check the flatness of the existing slab (apply levelling compound if needed)
  2. Lay a polyethylene membrane on the slab
  3. Lay the insulation boards (EPS, PUR, or high-density wood fibre)
  4. Roll out a separation film
  5. Pour a floating screed (minimum 5 to 6 cm) or lay dry screed boards
  6. Fit resilient perimeter strips to decouple the screed from the walls

💡 Tip — Be mindful of the floor level increase: every centimetre of insulation eats into your ceiling height. In renovation, you will typically lose 10 to 15 cm. Check that your doors and thresholds remain compatible.

Technique 3: Underside Insulation (Crawl Space or Cellar)

This is the simplest technique when an accessible space exists beneath the floor. The insulation is fixed under the floor, within the crawl space or cellar.

Suitable Materials

Material Conductivity (λ) Thickness for R = 4 Fixing method Notes
Spray PUR 0.022 W/m.K 9 cm Direct spray application Excellent continuity, no thermal bridges
Semi-rigid mineral wool 0.035 W/m.K 14 cm Adhesive + mechanical fixings Affordable, readily available
Graphite EPS 0.031 W/m.K 13 cm Adhesive or mechanical fixings Good performance/price balance
Wood fibre 0.038 W/m.K 16 cm Adhesive + mechanical fixings Bio-sourced, excellent thermal lag

Installation

  1. Clean the underside of the floor (dust, laitance)
  2. Deal with services: move or protect pipework before installation
  3. Apply adhesive in dabs or beads on the boards
  4. Press the boards firmly against the underside of the floor
  5. Supplement with hammer-in fixings (4 to 6 per m²)
  6. Seal joints between boards with mastic or expanding foam
  7. Do not block the crawl space vents — they must remain functional to prevent condensation

⚠️ Warning — In a crawl space, the insulation must be rot-proof or protected from moisture. Unprotected mineral wool without a vapour barrier degrades quickly in a damp environment. Prefer spray PUR or EPS if moisture is present.

Target Thermal Resistances by Regulation

Regulation Minimum ground floor R Comment
RT2012 R ≥ 3.0 m².K/W Sufficient at the time
RE2020 (zone H1) R ≥ 4.0 m².K/W Common recommendation
RE2020 (zone H3) R ≥ 3.5 m².K/W Milder climate
E+C- label / PassivHaus R ≥ 6.0 to 8.0 m².K/W For very high-performance projects

Common Mistakes to Avoid

  • Failing to treat the thermal bridge at the slab perimeter (slab edge)
  • Forgetting the polyethylene membrane below the insulation (rising damp)
  • Using insulation that cannot withstand compression under a slab
  • Neglecting crawl space ventilation after underside insulation
  • Under-specifying insulation thickness to save a few euros
  • Forgetting to run ducts and services before pouring the slab
  • Not cross-layering insulation boards (continuous joints = thermal bridges)
  • Using unprotected mineral wool in a damp environment (crawl space)

Key Takeaway

Ground floor insulation is an often underestimated but essential element for the comfort and energy performance of your home. Choose your technique based on your configuration (slab on grade, crawl space, cellar), aim for at least R = 4 m².K/W, and pay attention to the details — particularly the treatment of thermal bridges at the perimeter. If the budget allows, bio-sourced insulation (wood fibre) offers an excellent balance between performance, thermal lag, and low carbon impact.