What internal wall insulation is, and what it's like to live through
Internal wall insulation is insulated board fixed to the inside of every outside wall, then plastered and painted. The house looks the same from the road and warms up faster inside.
What it is
The plaster comes off the outside walls, insulated boards go on, and a vapour control layer, plasterboard, a skim and paint go over them. Skirtings, sockets and radiators come off first and go back on last. It is done one room at a time, so you can usually stay in the house — but the room being worked on is out of use, and SEAI describe the work as disruptive and dusty.
It's the answer for a wall with no cavity to fill, and for a house where wrapping the outside is out of the question: a protected structure, a terrace where your wall isn't yours to change, or a front elevation nobody wants rendered over.
The gain is fast heat. A dry-lined room warms in minutes, because you are only heating the air and the boards, not the whole mass of stone or block behind them.
Compared with wrapping the house outside
| Internal (dry lining) | External (the wrap) | |
|---|---|---|
| Typical cost, semi | €10,420 | €23,247 |
| Grant, semi | €3,500 | see the external page |
| The look of the house | unchanged outside | new render, new colour |
| Room size | a little smaller in every room | unchanged |
| Living through it | rooms cleared one at a time, dust | you stay put; scaffolding outside |
| How the room feels | heats quickly, cools quickly | slow and steady, holds heat for hours |
| Planning permission | never needed | sometimes — ask the council |
| Chimney breasts and party walls | awkward corners, easy to leave a gap | not an issue |
Grant €1,500 to €4,500 by house type, €3,500 semi-detached. SEAI median cost €10,420 (semi), against €23,247 external. U target for grant-aided dry lining 0.27 W/m²K. Whole-surface solution required — every external wall. Planning permission is not required for internal insulation. Amounts verified 2026-09-07.
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
Where the saving comes from
A solid wall loses heat straight through it, and the thicker and denser the wall the more heat it holds on the way. Insulation on the inside face puts a barrier between the room and that mass. Heat from the radiator now stays in the air and in the surfaces you touch, instead of soaking into the block and away.
The trade-off is that the wall behind is colder than it used to be. That's why the vapour layer matters, and why the fiddly corners — where a chimney breast, an internal wall or a floor meets an outside wall — are where a bad job goes wrong. Warm damp air reaching a cold surface makes mould. A good contractor insulates the returns at those junctions for exactly that reason.
Modelled wall heat loss for that group of homes: U 0.60 → 0.37 W/m²K. Of the 212 kinds of home we publish a number for, 101 have dry lining in the cheapest grant-eligible route to a B; external wall insulation appears in 0 of them. That is a cost ranking inside the model, not advice about any one wall. 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 much room do I lose?
- SEAI put it as a small amount of space in each room, and that's fair — it is the thickness of the board plus the plasterboard over it. You notice it at window reveals and in a small bathroom more than in a sitting room. Deep window boards and a slightly narrower hall are the visible cost of the job.
- What has to come off the walls?
- Skirting boards, sockets and switches, radiators, and anything screwed to an outside wall — shelves, a wall-hung television, kitchen units. SEAI describe the work as disruptive and dusty and say the rooms have to be cleared. Curtain poles and pictures go back on afterwards, but not in the same holes.
- Is it worth doing if I'm redecorating anyway?
- That's the best time to do it. The plaster is coming off and the room is being painted either way, so the extra work is the boards and the second skim. SEAI make the same point. If the room is fine and you'd be wrecking it for no other reason, wrapping the house outside may suit you better.
Where to next
Where this comes from
Last checked 2026-09-08.