Internal Wall Insulation in Ireland — When to Use It and Why Most Self-Builders Don't
On this page
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.
TL;DR
Internal wall insulation (IWI): 50–100 mm on the inside of the wall. Used in retrofit where external insulation isn't possible.
Rarely used in new build because it loses floor area and creates thermal bridges at the floor and roof junctions.
If you must use IWI, accept the floor-area loss and design the floor-wall and roof-wall junctions carefully to avoid thermal bridges.
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.
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.
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.