External Insulation in Ireland — The EWI Decision for Blockwork, BER, and Thermal Bridging
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External insulation (EWI): 50–150 mm on the outside of the wall. Eliminates thermal bridges. Adds 5–15% to wall cost.
External insulation (EWI) is insulation on the outside of the wall, typically fixed with adhesive and mechanical fixings, then rendered or clad. 50–150 mm. Most Irish self-builds on blockwork use 50–100 mm of EWI as part of the wall build-up. 100 mm hits 0.13 W/m²K. EWI eliminates thermal bridges at the floor and roof junctions (the points where the wall meets the floor or roof, which can be 30–50% of the wall's heat loss if uninsulated). Adds 5–15% to the wall cost. Saves 10–20% on heating demand. Eliminates cold bridges that can cause condensation and mould. The render or cladding is fixed to the EWI; the EWI is fixed to the wall. Standard build-up: 100 mm block inner leaf, 50 mm cavity, 100 mm block outer leaf, 100 mm EWI, render finish.
When this matters most
You're specifying the wall build-up.
When this doesn't apply
Walls are built and insulated.
Sources
Building Regulations 2023, TGD-L. https://www.gov.ie/
Frequently asked questions
How much does external insulation cost in Ireland?
€80–€140/m² installed (EWI plus render). For a 150 m² house with ~200 m² of wall: €16,000–€28,000. Adds 5–15% to the wall cost but eliminates thermal bridges.
What is the U-value of external insulation?
Depends on the thickness. 50 mm of EPS on a block wall achieves 0.30 W/m²K (poor). 100 mm of PIR achieves 0.13 W/m²K (BER A1). 150 mm of EPS achieves 0.15 W/m²K.
What is a thermal bridge?
A point in the building envelope where heat passes through more easily than the surrounding area. Common at floor-wall junctions, roof-wall junctions, and around windows. Uninsulated thermal bridges can be 30–50% of the wall's heat loss.
When this matters most
You're specifying the wall build-up.
When this doesn't apply
Walls are built and insulated.
Where to go next
Next decision
The next step in your build sequence.
Branch into
Adjacent decisions at the same stage.
Watch out for
Common mistakes, hidden costs, gotchas.
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