Subfloor Insulation in Ireland — PIR, EPS, or Mineral Wool, and What Part L 2022 Demands

Subfloor insulation in Ireland needs to achieve a floor U-value of 0.13 W/m²K to meet Part L 2022 backstop (BER A1 typically 0.10–0.13). 100–150 mm of PIR (polyisocyanurate) under a slab achieves 0.13. 200 mm of EPS (expanded polystyrene) achieves 0.13. Mineral wool is less common under slabs but is used in suspended timber floors. Edge insulation — running 1 m of insulation vertically at the slab edge — is the most under-detailed part of a subfloor. A 1 m run of uninsulated edge costs 5–10% of the floor's heat loss. Insist on a continuous edge insulation detail in the engineer's drawings. Radon barrier is mandatory on every ground-bearing floor in Ireland (1,200-gauge polythene).

TL;DR

• Floor U-value 0.13 W/m²K is the Part L 2022 backstop. BER A1 typically needs 0.10–0.13.

• 100–150 mm PIR under a slab achieves 0.13. 200 mm EPS achieves 0.13. Mineral wool less common under slabs but used in suspended floors.

• Edge insulation is the most under-detailed part of a subfloor. A 1 m run of uninsulated edge costs 5–10% of the floor's heat loss.

When this matters most

You're at design stage and specifying the floor build-up.

When this doesn't apply

Floor is poured.

Frequently asked questions

What is the Part L 2022 floor U-value?

0.13 W/m²K is the backstop. BER A1 typically needs 0.10–0.13. Airtightness 1–3 m³/m²·hr at 50 Pa.

What insulation do I use under a concrete slab?

100–150 mm PIR or 200 mm EPS. Both achieve 0.13 W/m²K. PIR is thinner and more expensive; EPS is thicker and cheaper.

Do I need a radon barrier under the slab?

Yes, on every new build in Ireland since 1998. 1,200-gauge polythene membrane. In high-radon areas (Carlow, Wexford), add a radon sump and extract fan.

Sources

Building Regulations 2023, TGD Part L (Conservation of fuel and energy — Dwellings). https://www.gov.ie/