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What U-value do replacement windows need?

Part L sets a maximum U-value for replacement windows in existing dwellings, and modern double or triple glazed units easily meet it. SEAI grant-supported window upgrades apply a tighter performance requirement than the regulatory backstop.

The U-value quoted should be the whole-window figure including the frame, not the centre-pane glass figure, which always looks better. Ask for the certified whole-window value.

Installation quality matters as much as the unit. Poor sealing at reveals and uninsulated cavity closers can create thermal bridges and condensation at the frame even with a high performance window.

What to ask a supplier for

  • Certified whole-window U-value
  • Warm-edge spacer and gas fill specification
  • Details for insulating and sealing the reveal
  • Guarantee terms on seals and hardware

Sources

Last reviewed 28 August 2026. General information only — not legal or planning advice.

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