What cavity wall insulation is, and how it's pumped in
Most Irish estate houses have two skins of block with a gap in between. Cavity insulation fills that gap with beads or wool, blown in through small holes drilled from outside.
What it is
A crew arrives, drills a line of holes the size of a euro coin through the outer skin of the wall, and pumps insulation into the gap behind it until it's full. Then they make the holes good so you can barely find them again on a pebble-dash wall. Nobody comes inside. It is usually done within a day.
The filling is either polystyrene bead with a light glue, or blown mineral wool. Which one depends on the wall, and that's the contractor's call.
This only works if there's a gap to fill. Estate houses from the 1960s onwards generally have one; older solid walls do not, and no amount of drilling will make one. That's why the first thing a contractor does is drill a test hole and look.
Is it right for a house like yours?
Your cert may say B2 — that's a B on the new scale.
A heat pump runs on electricity. Pick "Heat pump" if you have one.
A 1970s–80s detached house on oil, rated DEnergy rating D
- Grants you’d get
- €19,000
- It costs about
- €36,400
- You pay
- €9,000 – €16,300
- Saves you each year
- about €1,790
- Pays for itself in
- about 9 years
The commonest home on file — 23,707 like it. Change an answer above for yours.
Modelled, not observed payments. Middle case €16,329; a quarter of these homes are over €23,200. Repaid in 9.1–13.6 years. Confidence 0.9. One job in this list has no SEAI median price, so its cost is an estimate.
Show what’s included
This is the job list that suits most homes like yours (2 in 10). The money above is the middle of every route we priced, so it will not add up to this list exactly.
Attic insulation — a thicker blanket in the roof space. Half a day, nobody moves out, you lose some storage height.
Roof U-value → 0.16 W/m²K (≈300 mm). Cost €2,000 · grant €1,500 · you pay €500.
Costs about €2,500 · grant €2,000 · you pay €520
Wall insulation — the walls get a warm coat: pumped into the cavity (a morning), fitted inside (rooms shrink about 10 cm, redecorate, one to two weeks) or wrapped outside (scaffolding, new render, two to four weeks). We pick what your walls allow.
Cavity → 0.55, €1,550 · grant €1,300 · pay €250. Internal → 0.27, €10,420 · €3,500 · €6,920. External → 0.21, €23,247 · €6,000 · €17,247. Pre-1970 solid walls can't take cavity fill.
Costs about €9,000 · grant €4,500 · you pay €4,500
New windows and doors — double or triple glazed, fitted a room at a time over two to three days. You need not do every window, but the grant is paid once.
Whole-window U → 1.4 W/m²K. Estimate: €490/m² of window (no SEAI median), about €9,085 on a semi-detached house. Grant €3,000 + €800/door (max 2).
Estimated at €13,700 · grant €4,000 · you pay €9,700
Is the house warm enough to hold the heat? — SEAI's test before they'll fund a heat pump. The insulation steps are usually what gets you through.
Heat Loss Indicator ≤ 2.3 W/K·m² after fabric works. 1% of homes like yours already pass. Assessment about €276, grant €350 through a One-Stop-Shop.
Costs about €280 · grant €350 · the grant covers it
Heat pump — replaces the boiler, runs on electricity, pulls heat from the outside air. An outdoor unit the size of a large suitcase, bigger radiators in some rooms, three to five days. Runs cooler for longer — the house feels even, not blasted.
Air-to-water, SPF 3.5 heating / 2.5 hot water. Cost €16,832 · grant €6,500 + €4,000 (replacing oil/gas/coal) + €2,000 pipework + €700 controls = up to €13,200 · you pay from €3,632. 9.8c per kWh of heat vs gas 14.4c / oil 18.0c.
Costs about €16,500 · grant €6,500 · you pay €10,000
The paperwork — assessment first, grant approval, then the work, then a new energy cert, then the grant is paid. A One-Stop-Shop does all of it and takes the grant off the bill.
Order is mandatory — work started before approval is not eligible. Energy cert €276, assessment grant €350 through a One-Stop-Shop. Typically 3–6 months end to end (estimate).
Costs about €280 · grant €350 · the grant covers it
Compared with the other two ways of doing a wall
| Cavity | Internal (dry lining) | External (the wrap) | |
|---|---|---|---|
| Where it goes | in the gap in the middle | boards on the inside face | boards and render on the outside |
| Typical cost, semi | €1,550 | €10,420 | €23,247 |
| Grant, semi | €1,300 | €3,500 | €6,000 |
| Time | about a day | longer — the rooms have to be cleared | longest — scaffolding, then render |
| Disruption | none inside | dusty; sockets and skirtings come off | none inside; the house changes colour |
| Room size | unchanged | you lose a little in every room | unchanged |
| Planning permission | never | never | sometimes — ask the council |
SEAI call cavity fill the most cost-effective way to cut heat loss through a wall, if the cavity is empty. It is also the cheapest wall job by a distance, which is why it's worth finding out whether you have one before you price anything else.
Grant €700 to €1,800 by house type; €1,300 semi-detached, or €1,700 on a qualifying welfare payment. SEAI median cost €1,550 (semi). Whole-surface solution required; planning permission not required for cavity or internal insulation. Amounts verified 2026-09-07.
Where the saving comes from
An empty cavity is a chimney lying on its side. Air in the gap warms against the inner block, rises, cools against the outer block and drops again, and that circulation carries your heat out to the weather. Filling the gap stops the air moving, and still air barely conducts heat at all. The wall stops being a convector and starts being a blanket.
You feel it as the end of the cold-wall effect — the chill you get sitting against an outside wall in a room the thermostat says is warm.
Modelled wall heat loss for that group of homes: U 0.60 → 0.37 W/m²K. SEAI's standard for grant-aided cavity fill is 0.55 W/m²K or better. detached|1970-1989|D|oil: cost p50 €36,393, grant €19,000, net p25–p75 €9,019–€23,200, saving €1,791/yr, payback 5.8–13.6 yrs, n=23,707, confidence 0.9
Common questions
- How do I know if my house has a cavity?
- A contractor drills one small test hole and looks. Estate houses built from the 1960s on usually have a cavity; older stone and solid-block walls usually don't. Nobody should quote you for cavity fill without checking first, and the rating certificate on the national register is not proof either way.
- Will the beads cause damp?
- They can, in the wrong wall. A cavity that is bridged with mortar droppings, or a gable that takes driving rain off the Atlantic, can carry water across a filled cavity when it would not across an empty one. That is what the survey before the job is for. A registered contractor who says your wall is not suitable is doing the job properly.
- Can I do one wall?
- Not on the grant. SEAI pay for a whole-surface job — every external wall of the house — because insulating one wall and leaving the rest just moves the cold spot. Partial work is not eligible.
Where to next
Where this comes from
Last checked 2026-09-08.