Upgrade · Fabric · the walls · BER file 26 Sep 2026 · checked 8 Sep 2026
Cavity wall insulation: 12% of homes sold or let still have an empty cavity
Most Irish estate houses have two skins of block with a gap in between. 12% of homes sold or let whose wall type is known still have an empty cavity (SEAI BER file, 26 Sep 2026). 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 €1,585 and the grant pays €1,300.
An insulation hose plugged into a hole drilled in a brick wall. The beads or wool fill the gap behind it.
See your grants for wall insulation, and what you’d pay
About 10 seconds. See your numbers first, then get quotes if you want them.
12%
Of homes sold or let with a known wall type still have an empty cavity
168,097 certificates with the wall type · BER file 26 Sep 2026
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 assessment82%
Of SEAI's typical semi-D price paid by the grant
€1,300 of €1,585 · 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 hereCavity wall insulation
- Next questionHow much does SEAI pay towards cavity 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 2. 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, cavity 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
Cavity 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 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.
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 | €1,585 | €9,843 | €23,219 |
| 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 €1,585 (semi). Whole-surface solution required; planning permission not required for cavity or internal insulation. Amounts verified 2026-10-03.
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 €36,848, 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
- 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.
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.
See your grants and what you’d pay
Four questions about your home, then your numbers. Get quotes from registered contractors if you want them.
Updated · grants checked · BER file · prices SEAI H1 2026 · How we know · Corrections