Floor insulation is often the most cost-effective single upgrade in a UK dwelling: a poorly insulated ground floor loses 15–25% of a home’s total heat, yet the insulation itself is cheap per unit of thermal resistance compared with walls or roofs. The challenge is that the floor build-up must be thin (headroom is fixed), strong enough to carry loads without long-term creep, and compatible with the floor finish and any underfloor heating. The right choice of insulation material depends on where it sits in the build-up — above or below the concrete slab, or between suspended timber joists.
The Three Main Materials
PIR (Polyisocyanurate) Rigid Board
PIR board is a closed-cell foam insulation faced with aluminium foil. It has the highest thermal performance per unit thickness of any mainstream floor insulation:
- Lambda (λ): 0.022–0.023 W/mK (varies by manufacturer and density)
- Compressive strength: 120–200 kPa (standard domestic grade), 200–400 kPa (heavy-duty)
- Long-term thermal resistance: PIR can suffer lambda drift over decades as blowing agent permeates; some manufacturers publish aged lambda values — check the technical data sheet
- Typical thicknesses: 50, 60, 70, 80, 100, 120 mm
- Cost (material): £12–£20/m² for 70–80 mm
PIR is used above and below concrete slabs and as roof-slope insulation (a different product but same chemistry). Under a slab, 25 mm PIR is used as a secondary layer beneath the main layer to reduce thermal bridging at the slab perimeter.
EPS (Expanded Polystyrene) Boards
EPS is available in several grades, characterised by compressive strength. EPS 70 is the minimum grade suitable under a floor slab (compressive stress at 10% compression = 70 kPa), while EPS 150 or EPS 200 is required under heavily loaded slabs or in commercial settings.
- Lambda (λ): 0.032–0.038 W/mK (grade-dependent; EPS 70 is typically 0.036)
- Compressive strength: 70–250 kPa depending on grade
- Cost (material): £6–£12/m² for 100 mm EPS 70
- Key advantage: dimensionally stable, absorbs no water, easy to cut, widely available, no lambda drift
EPS is commonly used for full-fill ground-bearing slab insulation where cost matters more than minimising thickness. It is also used as the insulating layer in insulated concrete formwork (ICF) construction.
Mineral Wool (Glass Wool / Rock Wool)
Mineral wool — either glass wool rolls/batts or rock wool slabs — is primarily used in suspended timber floors between joists. It is not suitable under a concrete slab because it will compress under load and has poor moisture resistance when confined.
- Lambda (λ): 0.035–0.044 W/mK (product-dependent; rock wool slabs typically 0.035–0.038, glass wool rolls 0.040–0.044)
- Compressive resistance: low; do not use under slabs
- Cost (material): £4–£8/m² for 100 mm glass wool roll
- Key advantage: excellent fire performance, vapour-permeable (allows sub-floor drying), flexible to cut around services
Thickness for Target U-Values
The floor U-value is calculated using the combined resistances of all layers (slab or deck, insulation, floor finish) and the ground thermal resistance. Building Regs Approved Document L (ADL) requires:
- New dwellings: ground floor U ≤ 0.25 W/m²K (Part L1A, 2021 England)
- Extensions (controlled fittings): U ≤ 0.25 W/m²K for new floor area
- Retrofit/renovation: U ≤ 0.25 W/m²K where technically feasible
Approximate insulation thicknesses to achieve U = 0.25 W/m²K in a ground-bearing concrete floor (150 mm slab, 50 mm screed, tiles, typical UK ground thermal resistance):
| Insulation material | Lambda (W/mK) | Required thickness | Material cost/m² |
|---|---|---|---|
| PIR (standard) | 0.022 | 75–85 mm | £14–£18 |
| EPS 70 | 0.036 | 125–135 mm | £8–£12 |
| EPS 100 | 0.034 | 115–125 mm | £10–£14 |
| Phenolic foam | 0.018 | 65–70 mm | £18–£28 |
Note: the actual thickness required varies with floor plan dimensions (the larger the floor, the lower the ground heat loss, so less insulation is needed for large slabs). The BRE IP 3/90 or SAP worksheet should be used for accurate U-value calculation. The figures above are for a floor area of approximately 50 m²; larger slabs may achieve the target with 10–15 mm less insulation.
Above vs Below the Slab
Below the slab (most common in new-build): insulation is laid directly on the hardcore/blinded formation, the DPM laps over the insulation edges, and the concrete slab is poured on top. This approach gives the slab a thermal lag benefit: the slab mass is within the insulated envelope and acts as thermal storage. PIR or EPS boards in two staggered layers are used to avoid cold bridges at board joints.
Above the slab (screed-over insulation): used where the slab is already poured (renovation) or where thermal mass below the insulation is not desired. The insulation sits on top of the cured slab, and the screed goes on top of the insulation. Because the screed must carry foot and furniture loads without the board creeping, use PIR with a compressive strength of at least 120 kPa, or EPS 150+.
In a suspended timber floor: mineral wool or rigid PIR/EPS cut between joists, as covered in the suspended timber floor article. Rigid boards give higher performance and don’t sag, but are harder to install. Mineral wool is easier to retrofit through the sub-floor.
UFH Build-ups
When a wet underfloor heating system is embedded in the floor, the insulation below the pipes becomes critical: insufficient insulation causes downward heat loss into the ground, reducing system efficiency and increasing running costs. For heat-pump systems operating at low flow temperatures (35–45°C), a higher proportion of heat can be lost downward than for boiler-fed systems.
Minimum insulation below UFH pipes:
| Application | Minimum R-value below UFH | Typical PIR thickness | Typical EPS 150 thickness |
|---|---|---|---|
| Ground floor new-build | R ≥ 1.5 m²K/W | 35 mm PIR (0.022) | 55 mm EPS |
| Ground floor high-performance | R ≥ 2.5 m²K/W | 55 mm PIR | 90 mm EPS |
| First floor (zone isolation) | R ≥ 0.75 m²K/W | 20 mm PIR | 30 mm EPS |
The MCS heat pump installer standard (MCS 007) references minimum insulation levels for ground floors served by heat pumps to prevent excessive ground losses. Always confirm the specific build-up with your heat-pump designer.
A typical high-performance UFH build-up from bottom to top:
- Hardcore and blinding
- DPM (0.3 mm polythene, lapped and taped)
- 100 mm PIR insulation (two 50 mm layers, joints staggered)
- UFH pipe on clip rail system or insulation board with pre-formed channels
- 65–75 mm sand-cement screed or 35–50 mm anhydrite screed
- Floor finish (tile, LVT, engineered timber)
Vapour Control and DPM
All ground-floor insulation sits on or beneath the damp-proof membrane (DPM). The DPM must be continuous and linked to the DPC in the walls (Building Regs Part C). Standard practice is 1,200-gauge (0.3 mm) polythene laid on blinded hardcore. The insulation board goes on top; the slab or screed is poured on top of the insulation. If PIR foil-faced boards are used above the slab, check whether an additional vapour control layer is required for the floor finish — some adhesives require a dry substrate.
[!tip] Thermal bridging at the perimeter Heat loss at the slab edge (the perimeter cold bridge) can account for 15–30% of total floor heat loss in a well-insulated slab. Fitting 25–50 mm PIR around the inside face of the foundation wall, from underside of slab down to below frost depth, significantly reduces this. This is required under Part L Accredited Construction Details (ACDs) for new dwellings.
Cost Per m² Installed (2026, England)
| Build-up type | Insulation material | Installed cost (£/m²) |
|---|---|---|
| Ground-bearing slab (new-build), below slab | 100 mm EPS 70 | £10–£18 |
| Ground-bearing slab (new-build), below slab | 80 mm PIR | £18–£28 |
| Screed-over retrofit, above existing slab | 70 mm PIR + new screed | £30–£50 |
| Suspended timber floor retrofit | 100 mm mineral wool | £12–£20 |
| Suspended timber floor (rigid boards) | 75 mm PIR between joists | £18–£28 |
Costs include material and labour but exclude screeds, floor finishes, or preparation works.
UK Standards Summary
- BS EN 13162 — factory-made mineral wool insulation products
- BS EN 13163 — factory-made EPS insulation products
- BS EN 13165 — factory-made PIR insulation products
- Building Regs Part C — DPM and moisture protection
- Building Regs Part L (ADL1A, ADL1B, ADL2A, ADL2B) — thermal performance targets
- SAP 10.2 — Standard Assessment Procedure for U-value calculations
- CIBSE Guide A — thermal properties of building materials (lambda reference values)
