What is the difference between internal and external wall insulation for Irish homes?
Insulating walls in Ireland usually means choosing between External Wall Insulation (EWI), Internal Wall Insulation (IWI) or Cavity Wall Insulation (CWI). EWI wraps the outside of the building, IWI is applied to the inside face of external walls, and CWI fills the gap between the two leaves of a cavity wall. They differ in cost, disruption, how much they improve the wall’s U-value, and how much grant support they attract. Here is how they compare so you can choose with confidence.
Cavity vs internal vs external wall insulation: SEAI grants compared
| Property type | Cavity fill | Internal (dry lining) | External (EWI) |
|---|---|---|---|
| Detached | €1,800 | €4,500 | €8,000 |
| Semi-detached / end of terrace | €1,300 | €3,500 | €6,000 |
| Mid-terrace | €850 | €2,000 | €3,500 |
| Apartment | €700 | €1,500 | €3,000 |
Cavity fill grants rise to €2,300, €1,700, €1,100 and €900 respectively where the applicant is in receipt of qualifying welfare payments. Figures are the current SEAI individual upgrade grant values, September 2026.
For eligibility criteria, application steps and the full picture on funding, see our guide to grants and support for external wall insulation in Ireland.
External Wall Insulation (EWI)
External wall insulation is a system rather than a product: insulation boards are fixed to the outside face of the wall, then covered with a reinforced basecoat and mesh, and finished with a render or decorative topcoat. Because the insulation wraps the building, it is the only one of the three options that treats the wall and its thermal bridges as a single continuous layer.
Best for: Solid walls, mixed fabrics, whole-house retrofits, and projects where the facade is being refreshed anyway.
Insulation options
- EPS: the standard choice for most Irish retrofits. Best cost-to-performance ratio.
- Mineral wool: non-combustible and vapour open. Used where fire performance or breathability governs.
- Phenolic: the best thermal performance per millimetre, where overhang at eaves and reveals is tight.
- Wood fibre: vapour open and hygroscopic. Suited to older solid masonry that needs to keep breathing.
Finishes
The topcoat determines appearance, self-cleaning behaviour and long-term maintenance. Silicone, silicate, acrylic, mineral and lime finishes each behave differently on weathering and vapour transfer. Compare the Baumit facade finishes.
What to consider before specifying EWI
- Eaves, verge and roof overhang need to accommodate the added thickness
- Window and door reveals, cills, thresholds and rainwater goods all need detailing
- Boundary lines and party walls can restrict the build-up on terraces and semis
- Planning permission may apply where the external appearance is materially altered, and always needs checking for protected structures and Architectural Conservation Areas
- Scaffolding and access make it the most expensive of the three measures, offset by the largest SEAI grant
External wall insulation pros and cons
Pros: largest U-value improvement, treats thermal bridging continuously, no loss of internal floor area, minimal disruption to occupants, renews the facade, protects the existing wall from weathering, keeps the wall’s thermal mass inside the insulated envelope.
Cons: highest cost, changes external appearance, needs detailing at every junction and opening, unsuitable where the facade must be preserved.
Internal Wall Insulation (IWI)
Internal wall insulation is applied to the inside face of external walls, either as insulated plasterboard bonded directly to the wall, or as a battened system with insulation between the battens and a board finish over. It is the option of choice when the outside of the building cannot change.
Best for: Protected structures, Architectural Conservation Areas, terraced streetscapes where the front elevation must stay as built, single apartments within a block, and phased room-by-room upgrades.
Vapour open vs vapour closed systems
This is the decision that matters most on older buildings. A vapour closed build-up, typically a foil-backed PIR or phenolic laminate with a sealed vapour control layer, performs well thermally but relies on that layer being continuous and undamaged. A vapour open build-up, such as wood fibre board with a lime plaster finish, allows the wall to dry inwards and is the safer route on solid masonry that was never designed to be sealed.
Where internal insulation goes wrong
- Interstitial condensation where the vapour control layer is broken by sockets, pipes and fixings
- Cold bridges at floor junctions, party walls, window reveals and ceiling perimeters, which are not covered by the insulated area
- Embedded timber joist ends in solid walls, which become colder and damper once the wall is insulated internally
- Loss of usable floor area, typically 60 to 100mm off every external wall
- Existing damp or rain penetration must be fixed first, since internal insulation hides it rather than solving it
Internal wall insulation pros and cons
Pros: no change to the exterior, no planning implications in most cases, lower cost than EWI, can be done one room at a time, works where external access is impossible.
Cons: reduces room size, leaves thermal bridges untreated, requires careful vapour control and junction detailing, disruptive to occupants, means removing and refitting skirting, sockets, radiators and window boards.
Cavity Wall Insulation (CWI)
Cavity wall insulation fills the gap between the inner and outer leaf of a cavity wall. It is the least disruptive of the three options because the work is done from outside through small drilled holes, with no scaffolding of the whole facade and no loss of internal floor area. It is also the cheapest, and it carries the smallest SEAI grant, because it delivers the smallest improvement in wall U-value.
Best for: Sound cavity walls in low-to-moderate exposure zones.
Types of cavity wall insulation
- Bonded EPS bead: polystyrene beads blown in with an adhesive binder. The most common retrofit method in Ireland. Flows well around wall ties and irregular cavities.
- Blown mineral wool fibre: loose fibre injected under pressure. Non-combustible and vapour open, but more sensitive to water tracking in exposed locations.
- Injected polyurethane foam: used where the cavity is irregular or where some structural bonding is wanted. Less common and harder to remove later.
Full fill vs partial fill
Partial fill means insulation board fixed to the inner leaf during construction, leaving a clear residual cavity as a drainage and ventilation path. Full fill means the entire cavity is insulated. Almost all retrofit cavity work in Ireland is full fill, because the existing cavity is filled in place. The distinction matters for exposure: a full filled cavity has no clear void to drain driving rain, so the condition of the outer leaf becomes critical.
Does cavity wall insulation work?
Yes, where the wall is suitable. In a sound, clean cavity of adequate width in a low to moderate exposure zone, it is the fastest payback of the three measures. What it cannot do is fix a wall that is performing badly for another reason. It does not address thermal bridges at reveals, jambs and floor junctions, it does nothing for a solid wall, and it will not lift a poorly performing dwelling to a B2 rating on its own.
When cavity wall insulation is not the right answer
- Solid walls, or walls with no consistent, accessible cavity
- Cavities that are too narrow or irregular, or bridged by mortar droppings and debris
- High exposure zones subject to persistent driving rain
- Cracked render, defective pointing, failed wall ties or any existing damp, all of which must be repaired before filling
- Cavities that have already been filled
Where a survey rules out cavity fill, external wall insulation is usually the next step. It suits solid walls and mixed fabrics, it treats the thermal bridges cavity fill cannot reach, and it carries the largest SEAI grant. See the Baumit EWI systems or talk to us about a survey.
Cavity wall insulation pros and cons
Pros: lowest cost, quickest install, no internal disruption, no loss of room size, no change to the external appearance, no planning implications.
Cons: only suitable for certain cavities, smallest U-value improvement, does not treat thermal bridging, requires the outer leaf to be in good repair, and carries moisture risk if specified in the wrong exposure conditions.
How to decide
- Survey and moisture assessment: Identify wall types and exposure. Book a survey.
- BER target: Map the path to B2 or better with combined fabric and services upgrades. Learn more about sustainability.
- Detailing plan: Treat reveals, thresholds, vents and drainage correctly. See facade finishes for compatible systems.
Need advice on the right solution for your home? Get in touch with our team or explore training and technical guides for installers.