Guide
Is cavity wall insulation worth it?
For an empty cavity, yes, and quickly. Savings and payback by home type from SEAI's own costs, grants and fuel prices, with the cases where it isn't worth doing.
Last checked against seai.ie and SEAI's own cost and fuel figures on 2 November 2026.
cavitywallinsulation.ie is not SEAI or a government body and does not award grants.
- €1,800 detached, €850 mid-terrace, €700 apartment
- About €285 left to pay on a semi-D
- One SEAI-registered contractor rings you
Yes, for a true cavity wall with an empty cavity. A semi-D costs about €285 after the SEAI grant, and SEAI's own case-study home saved over €300 a year, so the work pays for itself in about a year. Our heat-loss estimate for an empty cavity in a gas-heated semi-D is €508 a year.
It's worth less, or not worth doing, if the cavity is already part-filled, if the wall is hollow block or timber frame, or if the outer wall lets in rain. Those homes need a different answer, and the survey is what finds it.
How much does cavity wall insulation save?
SEAI says a home loses 20 to 30% of its heat through its external walls if they aren't properly insulated, and its guide gives one case study: a cavity wall upgrade that saved a household over €300 a year on heating (SEAI homeowner's guide to wall insulation, read 2 November 2026). Electric Ireland, quoting SEAI, says wall insulation can cut a heating bill by up to 30% (Electric Ireland, read 2 November 2026).
We also worked it out for each home type from SEAI's own figures, in the table below. It's the heat an empty cavity wall stops losing once it's insulated to the U-value the grant requires.
Cavity wall insulation payback by home type
| Home type | Cavity wall area (median) | Heat saved, kWh a year | Worth on gas | Worth on oil | Cost after grant | Payback |
|---|---|---|---|---|---|---|
| Detached house | 106 m² | 3,782 | €612 | €521 | €550 | 0.9 to 1.8 years |
| Semi-D or end of terrace | 88 m² | 3,137 | €508 | €433 | €285 | 0.6 to 0.9 years |
| Mid-terrace | 49 m² | 1,751 | €284 | €241 | €400 | 1.3 to 1.4 years |
| Apartment | 44 m² | 1,578 | €255 | €218 | €336 | 1.1 to 1.3 years |
Last checked 2 November 2026. Cost after grant: SEAI's median costs PDF (median cost, January to June 2026, 23,412 upgrades) less the grant on seai.ie. Payback: the cost after grant divided by our gas estimate and by SEAI's case-study saving of €300 a year, shortest first. Method below.
The quickest payback is the semi-D, because the grant covers most of the cost. A mid-terrace has the least wall to lose heat through, so it saves the least, but it still pays back inside two years on these figures.
When is cavity wall insulation not worth it?
- The cavity is already part-filled. On SEAI's BER register, 28% of certificates for homes built before 2011 record a part-insulated cavity, against 4.5% recorded as an empty one (SEAI's BER register). A part-filled wall saves less than the table shows.
- The wall isn't a cavity wall. Solid and hollow block walls can't be filled (Electric Ireland). See hollow block wall insulation.
- The outer wall lets in rain. Cracked render, leaking gutters or a badly exposed gable have to be fixed first. See cavity wall insulation and damp.
- The home was built in 2011 or later. It has no grant, and its walls were built to a newer standard, so there is far less to save.
Does cavity wall insulation improve your BER?
Usually, because the walls lose less heat. The grant requires the insulated wall to reach a U-value of 0.27 W/m²K or lower (SEAI homeowner's guide to wall insulation, read 2 November 2026), and SEAI requires a post-works BER after the work (seai.ie, 2 November 2026). That new BER shows the change for your own home.
How we worked out the savings
Method
- Heat saved = the drop in U-value × the cavity wall area × heating degree days × 24 ÷ 1,000. Our calculation.
- U-value drop: from 1.0 W/m²K, the line SEAI's BER file draws for an empty cavity, to 0.27 W/m²K, the grant's requirement (SEAI homeowner's guide to wall insulation, read 2 November 2026). Many empty cavities start worse than 1.0, so for them this is the low end.
- Cavity wall area: the median cavity wall area by home type on SEAI's BER register certificates for existing homes built before 2011 (public file, downloaded 29 October 2026).
- Degree days: 2,028, the middle of our 26 county figures from Met Éireann's 1991–2020 grid (base 15.5°C). Colder counties save more, milder ones less.
- Fuel: gas 13.00c and kerosene 11.72c per kWh including VAT, from SEAI Domestic Fuel Cost Comparison (1 July 2026, read 2 November 2026). Boiler efficiency: the median on the same BER certificates, 80% gas and 85% oil.
- It's the heat a home heated through the season stops losing, not a promised cut in your bill. A home heated less, or only some rooms, saves less. That's why we show SEAI's case-study saving alongside it.
FAQ
Questions people ask
Is cavity wall insulation worth it?
Yes, for a true cavity wall with an empty cavity. A semi-D costs about €285 after the SEAI grant, and SEAI's own case-study home saved over €300 a year, so the work pays for itself in about a year. Our heat-loss estimate for an empty cavity in a gas-heated semi-D is €508 a year.
How much will cavity wall insulation save me a year?
SEAI's case-study home saved over €300 a year. Our heat-loss estimate for an empty cavity is €508 a year for a gas-heated semi-D and €612 for a detached house. A part-filled wall, or a home heated less, saves less.
How long does cavity wall insulation take to pay back?
About a year for a semi-D with an empty cavity: €285 after the grant against SEAI's case-study saving of over €300 a year. Up to about two years for a mid-terrace or a detached house.
What are the disadvantages of cavity wall insulation?
It only suits a true cavity wall that stays dry. Filling the wrong wall, or a wall that lets in rain, can bring damp inside, and getting it out again is a specialist job. See cavity wall insulation problems.
Does cavity wall insulation add value to a house?
It improves the BER in most homes, and a BER is on every sale and letting advert. We found no Irish figure for the effect on sale price, so we don't give one.
Your grant
Your cavity wall grant in 3 taps
Tap your home type, when it was built and any earlier wall grant. Your SEAI grant shows as you go. Then leave your number and one SEAI-registered contractor who covers your area rings you with a price.
Where these numbers come from (6 public sources, checked 29 October 2026)
Sources (1)
Prices and facts on this page were checked against these published pages. We are not connected to these businesses.
- Electric Ireland: electricireland.ie