Guide
Hollow block wall insulation
The 9-inch block wall that looks like a cavity wall and can't be pumped. How to tell, the two ways to insulate it, and the SEAI grant for each.
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
Hollow block walls can't take cavity wall insulation. There's no continuous cavity to pump, so the options are internal insulation (dry lining) or external insulation (the wrap). SEAI pays €3,500 for internal and €6,000 for external on a semi-D.
Hollow block is the 9-inch single block with hollow cores, used for much of the housing built around Dublin in the forty years to 2009 (Passive House Plus, 17 March 2009). It looks like a cavity wall from outside, so check before anyone quotes you to pump it.
Can you get cavity wall insulation in hollow block walls?
No. Passive House Plus puts it plainly: "With no continuous internal cavity, pump-filling isn't an option" (Passive House Plus, read 2 November 2026). Electric Ireland says the same: houses with solid walls or hollow block walls can't be insulated with cavity fill (Electric Ireland, read 2 November 2026). SEAI's technical standards say cavity hollow block walls are not suitable for cavity wall insulation (SEAI Domestic Technical Standards and Specifications, v4.0, July 2026, clause 6.1, read 29 October 2026).
Bead pumped into hollow block sits in some cores and not others. It insulates little, and it can hold water. If yours was pumped, see cavity wall insulation removal.
How to tell if your walls are hollow block
- Measure the wall at a door or window opening. A 9-inch block is about 225 mm, so a hollow block wall with plaster and render is usually well under 300 mm. SEAI's BER file records cavity walls as 300 or 425 mm.
- Look at the build year and area. Hollow block is most common in Meath, Kildare, Wicklow and Dublin (table below).
- Check the BER. Your BER certificate's advisory report names the wall type the assessor recorded.
- Ask for a borescope look. A small drilled hole and a camera show at once whether there's a continuous cavity or block cores.
SEAI's guide says that if you don't know, a building contractor or architect can tell you what type of wall your home has (SEAI homeowner's guide to wall insulation, read 2 November 2026). Is my house suitable? has the full checks.
Where hollow block is most common
On SEAI's BER register, 4.2% of certificates for homes built before 2011 record hollow block as the main wall. It's concentrated in the east: Meath and Kildare lead, and it's almost absent in the west and north-west.
| County | Hollow block, main wall | Of those, still uninsulated |
|---|---|---|
| Meath | 11.0% | 25% |
| Kildare | 10.9% | 24% |
| Wicklow | 7.5% | 35% |
| Dublin | 7.2% | 36% |
| Laois | 6.4% | 12% |
| Westmeath | 5.5% | 23% |
| Offaly | 3.5% | 28% |
| Carlow | 3.4% | 48% |
| Ireland | 4.2% | 35% |
SEAI BER Research Tool, public file downloaded 29 October 2026: certificates for existing homes built before 2011, not counting BERs done for grant claims. "Still uninsulated": a hollow block wall with a U-value of 1.0 W/m²K or more. Our count. All 26 counties.
Internal or external: the two options for hollow block
Internal insulation (dry lining) puts insulated boards or a studded, insulated lining on the inside of the outside walls. The outside stays as it is, but rooms lose some floor space and kitchens and sockets on those walls have to come off and go back (SEAI homeowner's guide to wall insulation, read 2 November 2026). In the Passive House Plus project, a Dublin hollow block house was lined with 100 mm of sheep's wool behind a vapour membrane, and the walls reached a U-value of 0.259 W/m²K (Passive House Plus).
External insulation (the wrap) fixes insulation to the outside and renders over it. Rooms stay the same size, and it also deals with rain getting through the wall. It costs more, may need planning permission, and can take width off a narrow side passage, which is why the Passive House Plus house went internal (SEAI homeowner's guide to wall insulation, read 2 November 2026; Passive House Plus). Our sister site housewrap.ie covers external insulation in full.
Hollow block wall insulation grants and costs
| Home type | Internal grant | Internal median cost | External grant | External median cost |
|---|---|---|---|---|
| Detached house | €4,500 | €8,600 | €8,000 | €26,500 |
| Semi-D or end of terrace | €3,500 | €9,843 | €6,000 | €23,219 |
| Mid-terrace | €2,000 | €9,804 | €3,500 | €14,000 |
| Apartment | €1,500 | €5,736 | €3,000 | €11,300 |
Grants: seai.ie, read 2 November 2026. Costs: SEAI's median costs PDF, January to June 2026, 23,412 upgrades, read 2 November 2026; the apartment external figure is from five homes only.
The same rules as the cavity grant apply: a home built and occupied before 31 December 2010, an SEAI-registered contractor, the grant offer before work starts, and every external wall insulated (seai.ie). If the home already had a cavity grant, internal or external is still open as the second wall measure.
The damp risk with internal insulation
SEAI's guide warns that internal insulation carries a condensation risk behind the insulation, especially at joints and edges (SEAI homeowner's guide to wall insulation, read 2 November 2026). Passive House Plus warns that a non-breathable lining "traps water vapour inside the building, potentially leading to condensation and mould" (Passive House Plus). Ask the contractor how the lining handles moisture, and what happens at window reveals and floor joists.
FAQ
Questions people ask
Can hollow block walls be cavity filled?
No. There's no continuous cavity to pump. SEAI's technical standards say cavity hollow block walls are not suitable for cavity wall insulation. The options are internal or external insulation.
How do I insulate a hollow block wall?
From the inside with dry lining, or from the outside with external insulation. SEAI pays €3,500 for internal and €6,000 for external on a semi-D (seai.ie, 2 November 2026).
Is internal or external better for hollow block?
External keeps room sizes and stops rain getting into the block, but costs more and may need planning. Internal is cheaper and leaves the outside alone, but takes some floor space and needs care with moisture.
How common is hollow block in Ireland?
4.2% of BER certificates for homes built before 2011 record it as the main wall. It's most common in Meath (11.0%) and Kildare (10.9%).
My hollow block house was pumped with bead. What now?
Get an independent borescope survey in writing, then talk to the installer. See our removal guide for when taking it out is the answer.
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 (5 public sources, checked 29 October 2026)
Sources (2)
Prices and facts on this page were checked against these published pages. We are not connected to these businesses.
- Passive House Plus: passivehouseplus.co.uk
- Electric Ireland: electricireland.ie