Wall Insulation in Ireland — PIR, EPS, Mineral Wool, and What Hits the U-value Target
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Wall insulation in Ireland comes in four main types. PIR (polyisocyanurate): 0.022 W/m²K thermal conductivity, the thinnest for a given U-value. Mineral wool: 0.035 W/m²K, vapour-permeable, good fire performance. EPS (expanded polystyrene): 0.038 W/m²K, cheap, vapour-resistant. Wood fibre: 0.040 W/m²K, vapour-open, used in vapour-open construction. Part L 2022 backstop is 0.18 W/m²K: 100 mm PIR or 150 mm mineral wool. BER A1 typically needs 0.13: 150 mm PIR or 200 mm mineral wool. The best U-value per mm is PIR; the cheapest per m² is EPS; the best for fire safety is mineral wool; the best for vapour-open construction is wood fibre. Most Irish self-builds use a combination: PIR for the cavity, mineral wool or wood fibre for the external insulation.
TL;DR
Insulation thermal conductivity (W/m²K, lower is better): PIR 0.022, mineral wool 0.035, EPS 0.038, wood fibre 0.040.
Wall U-value 0.18 W/m²K (Part L 2022 backstop) needs 100 mm PIR or 150 mm mineral wool. U-value 0.13 needs 150 mm PIR or 200 mm mineral wool.
Best U-value per mm: PIR. Cheapest per m²: EPS. Best for fire safety: mineral wool. Best for vapour-open construction: wood fibre.
When this matters most
You're specifying wall insulation.
When this doesn't apply
Walls are built and insulated.
Frequently asked questions
What is the best wall insulation in Ireland?
Depends on the wall system. PIR is the best U-value per mm; mineral wool is best for fire safety; wood fibre is best for vapour-open construction. Most self-builds use a combination.
How much wall insulation do I need for BER A1?
150 mm PIR or 200 mm mineral wool. Wall U-value 0.13 W/m²K is the typical A1 target. Part L 2022 backstop is 0.18 (100 mm PIR or 150 mm mineral wool).
What is the difference between PIR and mineral wool?
PIR is a rigid foam board with low thermal conductivity (0.022 W/m²K). Mineral wool is a fibrous insulation (0.035) that's vapour-permeable and has good fire performance. PIR is thinner; mineral wool is safer in fire.
Sources
Building Regulations 2023, TGD-L. https://www.gov.ie/
Frequently asked questions
What is the best wall insulation in Ireland?
Depends on the wall system. PIR is the best U-value per mm; mineral wool is best for fire safety; wood fibre is best for vapour-open construction. Most self-builds use a combination.
How much wall insulation do I need for BER A1?
150 mm PIR or 200 mm mineral wool. Wall U-value 0.13 W/m²K is the typical A1 target. Part L 2022 backstop is 0.18 (100 mm PIR or 150 mm mineral wool).
What is the difference between PIR and mineral wool?
PIR is a rigid foam board with low thermal conductivity (0.022 W/m²K). Mineral wool is a fibrous insulation (0.035) that's vapour-permeable and has good fire performance. PIR is thinner; mineral wool is safer in fire.