Upgrade · Fabric · the walls · BER file 26 Sep 2026 · checked 8 Sep 2026
External wall insulation: 32% of a 1970s semi’s heat goes through its walls
External wall insulation is rigid board fixed to the outside of the house and covered in render. Walls let out 32% of the heat a 1970s semi loses (SEAI BER file, 8 Sep 2026); windows 18%. Insulating the walls moves the typical home's rating 46.7 points, more than windows or the attic. SEAI's typical price for a semi-detached house is €23,219 and the grant pays €6,000.
Bungalows in Hull, England, being wrapped in outside insulation. The walls get thicker; the rooms stay as they are.
Garrick Close, Kingston upon Hull: bungalow renovations, September 2023 · Hullian111 · CC BY-SA 4.0
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32%
Of the heat a semi-detached house built 1971–80 loses goes out through its walls
BER file 8 Sep 2026 · n=4,498
BER assessment46.7
Points on the BER scale it adds, for the typical home it applies to
kWh/m²/yr · research lane, BER file 26 Sep 2026
Our model63%
Older A- and B-rated houses with insulated walls. Rated C to E: 25%
Walls U ≤ 0.37 · built before 2011 · BER file 26 Sep 2026
BER assessment26%
Of SEAI's typical semi-D price paid by the grant
€6,000 of €23,219 · SEAI H1 2026
Published figureUpdated · grants checked · BER file · prices SEAI H1 2026 · How we know · Corrections
Where this sits · the heat chain
Most heat leaves through the walls, roof, floor and windows. Fix these first and the heating has less to replace.
- Fabricwalls, roof, floor, windows
- Airdraughts and ventilation
- Heatheat pump or boiler, radiators, controls
- Usebills, comfort, the BER
- The idea behind itFollow the heat
- You are hereExternal wall insulation
- Next questionHow much does SEAI pay towards external wall insulation?
A third of the heat a semi-detached house built 1971–80 loses goes through its walls
Where a typical semi-detached house built 1971–80 loses its heat. This job acts on the walls, pin 4. BER assessment
A third of the heat a semi-detached house built 1971–80 loses goes through its walls
Walls 32%, windows 18%, air 17%, floor 11%, roof 10%, junctions 10%, doors 2%.
A section through a semi-detached house built 1971–80, front to back. Each arrow's width is that element's share of the heat the house loses for every degree it is warmer inside than out, from the insulation values and areas on 4,498 BER certificates for homes sold or let, assessed 2022–26. The median wall lets through 1.10 watts per square metre per degree, the roof 0.40 and the windows 2.80; the median heat loss indicator is 3.15, over the heat pump line of 2.3. Air (draughts, vents and open chimneys) isn't measured on a BER: it is what's left once the six measured elements are subtracted, so it carries the most uncertainty. The numbered pins show where each of the jobs acts, in the order they're usually done.
EstimateAir is estimated as the remainder (heat loss indicator × floor area, less walls, roof, floor, windows, doors and thermal bridging). Shares are rounded to sum to 100.
Sources: SEAI BER Research Tool, 8 Sep 2026 · Certificates for a sale or private letting assessed 2022–26 with a heat loss indicator: semi-detached house built 1971–80, 4,498 certificates
The numbers
| Element | Share of heat loss | Median |
|---|---|---|
| Walls | 32.0% | 1.10 W per m² per degree, 89 m² |
| Windows | 17.6% | 2.80 W per m² per degree, 21 m² |
| Air (estimated) | 16.8% | the remainder |
| Floor | 10.7% | 0.64 W per m² per degree, 60 m² |
| Roof | 10.5% | 0.40 W per m² per degree, 61 m² |
| Junctions | 10.2% | cold bridges at joins |
| Doors | 2.2% | 2.95 W per m² per degree, 2 m² |
On its own, external wall insulation moves a rating 46.7 points; a heat pump, 93
What one job on its own does to the typical home's rating, where the job applies. One letter on the new scale is 50 points. Our model
External wall insulation: 46.7 points on its own, where it applies
Where the job applies, the median gain is 93 kilowatt-hours per square metre a year for a heat pump, 60.7 for solar panels, 46.7 for wall insulation and 10.4 for new windows. One letter on the new scale is 50.
Each bar is the median improvement to a home's BER from that one job on its own, among homes sold or let where the job applies, worked out on each certificate's own figures. A heat pump moves the rating nearly two letters in the D to F range; attic insulation, draught-stripping and blocking chimneys barely move the rating on their own. The bracket shows one letter's width (50, for D, E and F). The bars are not additive.
EstimateOur model: the research lane's one-job recalculation on each certificate's own figures (BER file of 26 Sep 2026, not the site's 8 Sep file). It matches SEAI’s published heat loss to a median ratio of 1.00 and uses the electricity factor in force on each certificate’s date.
Sources: fixmyber research lane, research/ber/s4_thresholds.py, 26 Sep 2026
The numbers
| Job | Median gain, kWh per m² a year |
|---|---|
| Heat pump | 93 |
| Solar panels | 60.7 |
| Wall insulation | 46.7 |
| New windows | 10.4 |
| Attic insulation | 6.1 |
| Blocking chimneys | 2.4 |
| Draught-stripping | 0.4 |
Older A- and B-rated houses are 2 times as likely to have insulated walls
What older houses rated A or B have, against those rated C to E. BER assessment
63% of older A- and B-rated houses have insulated walls; 25% of those rated C to E
The biggest gap is insulated walls: 63% against 25%.
- Insulated walls. Rated A or B: 63%. Rated C to E: 25%.
- Solar panels. Rated A or B: 47%. Rated C to E: 10%.
- An insulated floor. Rated A or B: 74%. Rated C to E: 40%.
- Attic at the grant standard. Rated A or B: 33%. Rated C to E: 19%.
- High-performance windows. Rated A or B: 26%. Rated C to E: 3.8%.
- A heat pump. Rated A or B: 21%. Rated C to E: 0.2%.
- Solar water heating. Rated A or B: 7.8%. Rated C to E: 2.2%.
- Mechanical ventilation. Rated A or B: 7.2%. Rated C to E: 0.9%.
Each pair of bars is the share of houses built before 2011 that have the upgrade, among those rated A or B (dark) and those rated C to E (grey), from every certificate issued 2019-2026. Insulated walls: 62.8% against 25.4%. It shows what the better-rated houses have, not proof of what got them there: newer houses are more often A or B and were built with some of these.
Sources: SEAI BER Research Tool, every existing-home certificate 2019–26 for houses built before 2011 (440,689 certificates), 26 Sep 2026 · Insulated walls: Walls U ≤ 0.37 · Solar panels: Solar PV · An insulated floor: Floor U ≤ 0.45 · Attic at the grant standard: Roof U ≤ 0.16 · High-performance windows: Windows U ≤ 1.6 · A heat pump: Heat pump · Solar water heating: Solar water · Mechanical ventilation: Mechanical ventilation
The numbers
| Upgrade | Rated A or B | Rated C to E |
|---|---|---|
| Insulated walls | 62.8% | 25.4% |
| Solar panels | 46.9% | 9.7% |
| An insulated floor | 73.6% | 39.9% |
| Attic at the grant standard | 33.3% | 19.3% |
| High-performance windows | 26.1% | 3.8% |
| A heat pump | 20.9% | 0.2% |
| Solar water heating | 7.8% | 2.2% |
| Mechanical ventilation | 7.2% | 0.9% |
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 | €23,219 | €9,843 | €1,585 |
| 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 |
What external wall insulation costs, by house type and per square metre
Grant €3,000 to €8,000 by house type, €6,000 semi-detached. SEAI median cost €23,219 (semi), n=23,412 upgrades in the Jan–Jun 2026 medians. U target for grant-aided external insulation 0.21 W/m²K. Whole-surface solution required. Amounts verified 2026-10-03.
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 €36,848, grant €22,300, you pay p25–p75 €7,300–€21,700, saving €1,797/yr, payback 4.7–12.4 yrs, n=23,707, confidence 0.9
Questions people ask
- 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.
How we know
- Ratings: From SEAI's BER Research Tool (files of 8 Sep 2026 and 26 Sep 2026): certificates for homes sold or let, and every certificate for houses built before 2011. A BER is a standard calculation, not a meter reading.
- One job at a time: Our model reworks each certificate with one job done, on the BER file of 26 Sep 2026. Its heat loss matches SEAI's own figure to a median ratio of 1.00. A group figure, not an assessment of your house.
- Grants and prices: SEAI's grant pages, read on the dates shown, and SEAI's median costs of individual upgrades for 2025. A median is a middle price, not a quote.
Sources
seai.ie · insulation · seai.ie · wall insulation · seai.ie · Median costs of individual energy upgrades
Found a mistake? Every fix is dated on Corrections. Method in full: How we know.
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Updated · grants checked · BER file · prices SEAI H1 2026 · How we know · Corrections