Internal Wall Insulation in Ireland — When to Use It and Why Most Self-Builders Don't
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Internal insulation: 50–100 mm on the inside of the wall. Used in retrofit; rarely in new build. Loses floor area and creates thermal bridges.
Internal wall insulation (IWI) is insulation on the inside of the wall, typically fixed to battens or directly to the wall, then plastered or dry-lined. 50–100 mm. Used in retrofit where external insulation isn't possible (heritage sites, conservation areas, apartments). Rarely used in new build because it loses floor area (50 mm of IWI on every wall of a 150 m² house loses 5–8 m² of floor area) and creates thermal bridges at the floor and roof junctions. If you must use IWI in retrofit, accept the floor-area loss and design the floor-wall and roof-wall junctions carefully to avoid thermal bridges. Most Irish self-builds use EWI instead.
When this matters most
You're retrofitting an existing house or working on a heritage site where external insulation isn't allowed.
When this doesn't apply
You're building a new house on a standard site.
Sources
SEAI, Deep Retrofit Cost Benchmarks 2024. https://www.seai.ie/
Frequently asked questions
When is internal wall insulation used in Ireland?
Mainly in retrofit where external insulation isn't possible — heritage sites, conservation areas, apartments. Rarely used in new build because it loses floor area and creates thermal bridges.
How much does internal wall insulation cost?
€60–€120/m² installed. For a 150 m² house: €9,000–€18,000. Cheaper than EWI but loses 5–8 m² of floor area and creates thermal bridges.
Is internal wall insulation as good as external?
No. External is better because it eliminates thermal bridges at floor-wall and roof-wall junctions. Internal is acceptable for retrofit where external isn't possible.
When this matters most
You're retrofitting an existing house or working on a heritage site where external insulation isn't allowed.
When this doesn't apply
You're building a new house on a standard site.
Where to go next
Next decision
The next step in your build sequence.
- Airtightness in Ireland — Part L 1–3 m³/m²·hr, Passive House ≤0.6, and the Blower Door Test →
- External Insulation in Ireland — The EWI Decision for Blockwork, BER, and Thermal Bridging →
- Lintels and Reveals in Ireland — The Detailing That Makes or Breaks Airtightness →
- Wall Insulation in Ireland — PIR, EPS, Mineral Wool, and What Hits the U-value Target →
Branch into
Adjacent decisions at the same stage.
Watch out for
Common mistakes, hidden costs, gotchas.
- Brick Cladding in Ireland — Cost, Lifespan, and When to Choose It →
- Cladding in Ireland — Render, Brick, Timber, and Zinc, Compared by Cost and Planning →
- Stone Cladding in Ireland — Real Stone vs Stone Veneer, Cost, and Heritage Use →
- Timber Cladding in Ireland — Cost, Maintenance, and Best Species for Irish Weather →
- Zinc Cladding in Ireland — Standing-Seam, Cost, and Lifespan →