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What is a heat loss indicator?

The heat loss indicator is a measure of a dwelling's heat loss per square metre per degree of temperature difference. SEAI uses it to judge whether a home is suitable for a heat pump, with a threshold applied for grant eligibility.

The HLI combines fabric heat loss through walls, roof, floor and windows with ventilation and infiltration losses, divided by floor area. A lower figure means a house that holds heat well and can be warmed at a low flow temperature.

Where the assessment returns a figure above the threshold, the assessor identifies the specific upgrades that would bring it within range, which becomes the specification for the fabric works.

What drives a poor HLI

  • Uninsulated or poorly insulated walls
  • Single glazing or early double glazing
  • Uninsulated ground floor
  • High air leakage around openings and services

Sources

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

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