How Much Floor Insulation Do I Need?
Quick Answer
Ground floor area in m² — typically 70–100mm PIR boards
A 50m² ground floor needs ~35 boards (2400 × 1200mm) at 100mm thickness
How We Calculated This
- Ground floor area: 50m²
- Board size: 2400 × 1200mm = 2.88m² per board
- Boards needed: 50 ÷ 2.88 = 17.4, say 18 boards
- Plus 10% cutting waste: ~20 boards
Methods by Floor Type
- Suspended timber floor: Mineral wool or PIR between joists (supported by netting or battens). 100mm minimum
- Solid concrete floor (retrofit): PIR boards over existing slab + new floating floor. 50–100mm
- New build slab: PIR below slab or below screed. 70–150mm depending on U-value target
U-Value Targets
- New build (Part L 2021): 0.13 W/m²K
- Existing building (renovation): 0.25 W/m²K
The required insulation thickness depends on the floor’s perimeter-to-area ratio. Larger floor areas lose proportionally less heat and may need less insulation. Always run a U-value calculation for your specific project.
Insulation Options
- PIR (Celotex/Kingspan): Best thermal performance per mm (λ 0.022)
- EPS (expanded polystyrene): Cost-effective, λ 0.032–0.038
- Mineral wool: Flexible, good for between joists, λ 0.035–0.040
Last updated: April 2026