The short answer
Cavity wins on cost and disruption when you have an empty cavity. External wall insulation (EWI) wins on thermal performance and is the only route for solid-wall houses. Internal wall insulation (IWI) is the path of last resort — cheaper than EWI but eats internal floor area and requires re-skirting, re-decorating and re-fitting electrics on every external wall.
| Cavity wall | External wall (EWI) | Internal wall (IWI) | |
|---|---|---|---|
| SEAI grant (semi-D) | €1,200 | €6,000 | €3,500 |
| Typical job cost (semi-D) | €1,800–€3,500 | €16,000–€28,000 | €10,000–€18,000 |
| Net you pay (after grant) | €600–€2,300 | €10,000–€22,000 | €6,500–€14,500 |
| U-value improvement | 1.6 → 0.4 W/m²K | 1.6 → 0.18 W/m²K | 1.6 → 0.25 W/m²K |
| Annual heat-bill saving | €350–€600 | €500–€900 | €450–€800 |
| Payback (no grant) | 3–6 yrs | 20–35 yrs | 15–25 yrs |
| Payback (with grant) | 1–4 yrs | 11–22 yrs | 8–16 yrs |
| Disruption | 1 day, no internal mess | 5–10 days, scaffolding, externals re-finished | 3–5 days/room, internal re-decoration |
| Lifespan | 40+ years | 30+ years | 30+ years |
Which house gets which?
Cavity wins if
You have a cavity wall built between roughly 1940 and 2000 with an empty cavity (no foam, no bonded bead, no mineral fibre already in). The contractor will drill 25mm holes in the mortar joints, blow in bonded EPS bead or mineral wool, and re-point the holes. One day's work, no scaffolding, almost zero internal disruption. The grant economics are excellent — payback of 1–4 years, in Cork energy prices.
EWI wins if
You have a solid wall (most houses pre-1940; many older Cork city-centre terraces and many countryside cottages), or your cavity is already partially filled but underperforming, or you want the deepest possible thermal upgrade (e.g. heading toward a Passive House standard). EWI also fixes thermal bridging at junctions in a way nothing else does. Yes, it's expensive — but it's the only route that delivers Passivhaus-grade performance on a retrofit. Look at the SEAI grant table — the €6,000–€8,000 grant materially changes the maths.
IWI wins if
EWI is impossible (planning restriction, listed building, terraced wall facing the public footpath, neighbour won't agree at a party wall) and the cavity is already filled or doesn't exist. You lose 80–120mm of internal room dimension on every external wall. You need to re-fit electrics (sockets move forward), re-skirt, and redecorate. Get quotes specifically for the disruption work as well as the insulation; cheaper-looking IWI quotes often exclude the make-good.
The thing nobody tells you about cavity wall insulation in Cork
Cork's exposure to Atlantic-facing weather (driving rain from the southwest) is higher than most of inland Ireland. Cavity wall insulation in driving-rain-zone-3 areas (which covers most of West Cork, the Beara peninsula, Bantry, parts of the Sheep's Head) requires extra care to avoid moisture transferring across the cavity from outer leaf to inner leaf. Bonded bead is preferred over mineral wool in these zones; some installers won't quote mineral wool at all on the more exposed sites.
If you're in a high-exposure Cork area, ask the contractor specifically about driving-rain-zone classification and which insulation type they recommend for your wall.
Cork contractors
For cavity work in Cork, multi-measure contractors and specialist cavity installers compete keenly on price. Get 2–3 quotes — they should be tightly clustered. For EWI in Cork, dedicated specialists like EcoWrap tend to be the technically strongest. Multi-measure contractors like HPS Group can be a strong fit when you're doing several measures together (cavity + attic + windows under one contract). See the Cork insulation comparison for the named-contractor breakdown.