Roof Insulation in Ireland — Part L 0.16 W/m²K, A1 0.13, and Where to Put It

Roof U-value in Ireland: Part L 2022 backstop 0.16 W/m²K, BER A1 typically 0.13, Passive House 0.10–0.13. Three places to insulate: ceiling level (cheapest, easiest to retrofit, 300–400 mm mineral wool between joists, the standard for an attic conversion or a house with a cold attic), rafter level (warm roof, 150–200 mm PIR between and over the rafters, more expensive but no cold bridge), or both (rare, expensive). Airtightness at ceiling level is critical: a 1 mm gap in the ceiling membrane is the equivalent of a 50 mm hole in the wall. The ceiling membrane must be continuous and sealed at all penetrations (downlighters, services). Loft hatches must be insulated and sealed.

TL;DR

• Roof U-value in Ireland: Part L 2022 backstop 0.16 W/m²K, BER A1 typically 0.13, Passive House 0.10–0.13.

• Three places to insulate: ceiling level (cheapest, easiest to retrofit, 300–400 mm mineral wool), rafter level (warm roof, more expensive), or both.

• Airtightness at ceiling level is critical: a 1 mm gap in the ceiling membrane is the equivalent of a 50 mm hole in the wall.

When this matters most

You're at design stage and need to specify roof insulation.

When this doesn't apply

The roof is on and insulated.

Frequently asked questions

What is the Part L 2022 roof U-value?

0.16 W/m²K is the backstop. BER A1 typically needs 0.13. Passive House 0.10–0.13. 300–400 mm of mineral wool at ceiling level hits the backstop.

What is the cheapest way to insulate a roof?

Ceiling level (cold attic). 300–400 mm of mineral wool between joists. Cheapest, easiest to retrofit. Standard for new builds with a cold attic.

What is a warm roof?

A roof where the insulation is at rafter level, not ceiling level. No cold bridge at the eaves. More expensive than a cold attic but allows the loft to be used as living space. Standard for room-in-roof designs.

Sources

Building Regulations 2023, TGD-L. https://www.gov.ie/