What external wall insulation is, and what it does to a house
External wall insulation is rigid board fixed to the outside of the house and covered in render. The walls get thicker, the house changes colour, and nothing inside is touched.
What it is
Scaffolding goes up. Rigid insulation boards are fixed to every outside wall, a mesh is embedded in them for strength, and a render goes over the lot to keep the weather out. SEAI list those four steps in that order, and they take weeks rather than days — render needs time.
You keep living in the house the whole time. Nothing comes off an inside wall, no room gets smaller, and there's no dust indoors. What you get instead is scaffolding on the drive, a house that changes colour, and a run of jobs around the edges: gutters and downpipes off and back on, sills extended, and possibly a call to ESB Networks if the cables and the meter box are in the way. SEAI say to check that one before the work starts, and there may be a fee.
It is the most expensive of the three wall jobs, and the one with the largest grant behind it.
Compared with the other two ways of doing a wall
| External (the wrap) | Internal (dry lining) | Cavity | |
|---|---|---|---|
| Typical cost, semi | €23,247 | €10,420 | €1,550 |
| Grant, semi | €6,000 | see the internal page | see the cavity page |
| What you live through | scaffolding outside, weeks | rooms cleared, dust | a day, nobody comes in |
| Room size | unchanged | a little smaller everywhere | unchanged |
| The look of the house | new render, new colour | unchanged | unchanged |
| Cold corners | covered, including the awkward ones | easy to leave a gap at a junction | limited to the cavity |
| Planning permission | sometimes — ask the council | never | never |
| Works on a solid wall | yes | yes | no cavity, nothing to fill |
Grant €3,000 to €8,000 by house type, €6,000 semi-detached. SEAI median cost €23,247 (semi), n=47,067 upgrades in the 2025 medians. U target for grant-aided external insulation 0.21 W/m²K. Whole-surface solution required. 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
Wrapping the outside puts the insulation on the cold side of the wall, which leaves the whole mass of block or stone on the warm side. That mass then works for you: it takes heat in slowly while the heating is on and gives it back for hours after it goes off, so the house coasts instead of cooling. It is the opposite feel to dry lining, which heats a room fast and lets it go fast.
The other gain is that the wrap is continuous. Heat escapes fastest at the awkward places — the corner where a chimney breast meets the front wall, the line where the upstairs floor sits into the wall — and those are exactly the spots that internal work has to break around. A wrap goes over them without a join.
Modelled wall heat loss for that group of homes: U 0.60 → 0.37 W/m²K. 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
- Do I need planning permission for external insulation?
- Sometimes. SEAI say to check with your contractor or the local planning authority before you start, because it depends on the house and where it is. A protected structure or a house in an architectural conservation area is the usual reason for a yes. The other pre-start call is to ESB Networks, if the cables and meter box on the wall have to be moved.
- Will it look like a rendered box?
- It will look like a rendered house, which is what many Irish houses already are. The render comes in a range of colours and finishes, and detail can be kept — deeper reveals at the windows, a plinth at the bottom, sills extended out over the new thickness. What it cannot do is stay looking like brick or stone. If the front of the house is the reason you bought it, dry line the inside instead.
- Can I do it if the cavity is already filled?
- Yes. SEAI's rules on a second wall job say cavity fill can be followed by internal or external insulation, and internal can be followed by external. External is the end of the road — there is no third wall measure after it.
Where to next
Where this comes from
Last checked 2026-09-08.