Subfloor Insulation in Ireland — PIR, EPS, or Mineral Wool, and What Part L 2022 Demands
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Floor U-value 0.13 W/m²K is the Part L 2022 backstop. 100–150 mm PIR or 200 mm EPS achieves it. Don't skimp on the edges.
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).
When this matters most
You're at design stage and specifying the floor build-up.
When this doesn't apply
Floor is poured.
Sources
Building Regulations 2023, TGD Part L (Conservation of fuel and energy — Dwellings). https://www.gov.ie/
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.
When this matters most
You're at design stage and specifying the floor build-up.
When this doesn't apply
Floor is poured.
Where to go next
Next decision
The next step in your build sequence.
- Foundation Depth in Ireland — 900 mm, 1.2 m, or Below Frost Line →
- Foundation for a Timber Frame House in Ireland — Raft Is the Standard →
- Piled Raft Foundation in Ireland — For Peat, Made Ground, and Soft Clay Sites →
- Raft Foundation in Ireland — The All-Rounder for Class 3–4 Ground and Subfloor Insulation →
Branch into
Adjacent decisions at the same stage.
Watch out for
Common mistakes, hidden costs, gotchas.
- Foundation Cost by Soil Class in Ireland — The €/m² by Class 1 to Class 5+ →
- Foundation Gotchas in Ireland — The Six Things That Catch Self-Builders Off Guard →
- Radon Barrier in Ireland — Where It's Needed, What It Costs, and the Carlow-Wexford High-Risk Map →
- Strip Foundation in Ireland — When It Works, What It Costs, and What the Engineer Will Check →
Compare with
Alternative approaches worth knowing.