Hollow Block Is Not a Cavity

The hole is the size of a two-euro coin. Behind it: darkness, a little dust, the suggestion of an empty room. The man with the drill says cavity. Aoibheann, who lives in the house, says grand. The quote is for beads. The van is for a hopper. The wall is not a cavity. The darkness is the inside of a block.

A cavity is a sandwich: two leaves of masonry, a gap, jam if you like. Hollow block is a chocolate bar. The holes are the chocolate. You do not pump jam into a KitKat and call it lunch. Dublin has been calling KitKats sandwiches since about 1968, and the heating bill has been the punchline.

The cells are the block

Let us invent a person. We will call her Aoibheann. 1972 mid-terrace, Coolock. Grey hollow block, a dash that has gone the colour of weak tea, a wall that looks, from the garden, like every other wall on the road. Someone drilled a hole, saw air, and wrote “cavity fill” on a clipboard. The clipboard is not a survey.

A hollow-block wall is usually one leaf. Two hundred and fifteen millimetres of concrete, give or take, with vertical cores running up through each block the way holes run through a breeze block, because that is what it is. The cores are not a corridor. They stop at the mortar. Every bed joint is a lid. The “empty room” behind the two-euro hole is a pocket the size of a block, stacked on another pocket, stacked on another, none of them talking to each other the way a cavity talks to itself from eaves to damp-proof course.

A cavity wall is a different animal. Outer leaf, inner leaf, a gap. The gap is the job. You can, if it is clean and dry and empty, fill it. The live piece on the air brick that is not a letterbox is about that animal: a wall with a nose and a plughole. This essay is the other animal. If the Coolock terrace is one leaf of hollow block, treating it as a sandwich is how you buy a hopper for a wall that cannot take one.

A chocolate bar is not a sandwich

The mix-up is visual. Drill a cavity wall and you see a gap. Drill a hollow block and you also see a gap. Both gaps are dark. Dark is not a specification. The cavity gap is a continuous slot between two walls. The hollow-block gap is the cell of the unit you just punctured. One of those is a costume you fill. The other is the wall.

SEAI’s own homeowners’ guide to wall insulation is blunt about this, which is a relief, because Dublin folklore is not. Three wall types, not two: cavity, solid, and hollow block. The first question on their list is not “how cheap is the bead”. It is “what type of wall do I have?” Hollow block is the third thing, and the third thing does not get a hopper.

Heat does not care about your quote. Heat walks the path you left open. In a hollow-block wall the path is the webs: the concrete between the cells, the mortar lids, the dense ladder from the weather to the sitting room. Fill some of the pockets and the webs are still a radiator. Building Science’s digest on thermal control is the boring rule: continuity, or you are decorating. Beads in random cores are a lucky dip, not a coat.

The hopper is the wrong costume

Wrong survey, wrong product. That is the whole plot, and it is not a metaphor.

A hopper job assumes a cavity: a gap you can inject, a weather of beads that fills a slot and stays there. Hollow-block cores are closed boxes. Pump into them and you fill the box you hit. Some cores take material. Some refuse. The webs stay naked. The inner face stays a map of cold patches that do not match the invoice.

Moisture is the other punchline. Water gets into masonry. It is supposed to be able to leave. Fill isolated cells with stuff that likes holding still air — and, inconveniently, still water — and you have given the rain a cupboard with no drain. Building Science on vapour barriers is the adult sentence: keep water out, and let it out if it gets in. Trapping it in closed cores is how a “filled” hollow-block wall grows a musty band at socket height. Mould is physics with spores, not a lifestyle.

Aoibheann’s sitting-room wall is colder on the chimney breast than it is beside the window, which is the wrong way round if you believed the clipboard. The clipboard believed in a cavity. The wall believed in physics.

What the survey is actually looking at

You cannot diagnose hollow block from a Facebook photograph of a grey dash. Finished is a paint word. A surveyor looks at the wall as a wall.

Thickness first. A typical Dublin hollow-block leaf is about two hundred and fifteen millimetres. A cavity wall is two leaves plus a gap, and it is fatter: often around three hundred millimetres at the opening. If the whole wall is one block thick, it is not hiding a sandwich.

Then the hole, properly. A borescope, not a two-euro coin and a torch of optimism. Are you looking at a continuous slot that runs past several blocks, with an inner and outer leaf you can name? Or are you looking at the core of the unit, mortar above, mortar below, webs left and right? Knock it. Hollow block sounds like a block. A cavity sounds like two walls having a conversation through a gap.

The Housing Agency exists because Irish housing is a stock, not a vibe: decades of types, built in batches, Coolock included. 1960s and 1970s Dublin put a lot of people into hollow-block terraces and semis and called it a house. It is a house. It is not a cavity house. Guessing from the dash is how a hopper quote survives a grant form.

If the person quoting you will not tell you whether the wall is one leaf or two, they are quoting a costume. Costumes do not get grants. Walls do.

The grown-up coat

The grown-up path is not “just fill it”. The grown-up path is a coat on the outside, or a lining on the inside where the wrap is the wrong job.

External wall insulation Dublin is the weather coat: boards fixed to the outer face, mesh, render. It does not live in the cells. It puts a continuous layer over the whole elevation, webs and all, and heat has to walk through the boards instead of the concrete ladder. That is continuity. That is the thing the hopper was pretending to be.

Internal lining is the other conversation: boards on the room side, a bit of floor area gone, kitchens and radiators in the way. It is a real costume. It is not a hopper either. SEAI will not talk about pumping a hollow-block core and calling it a cavity fill.

SEAI does not pay for a feature wall. The whole-surface rule is the rule. Wrapping the front and leaving the gable as original grey is a cream photograph with a cold side. Passive House Plus has spent years on Irish fabric: the boring drawings where the coat actually goes around the house. A wrap that stops because the hopper quote was cheaper is not a strategy. It is a mood.

The money, without folklore

These figures are fixed by house type, not a percentage of a quote, and they are not a prize for filling the wrong wall.

Cavity wall insulation is the other costume, and it applies when there is a cavity. Standard: apartment €700, mid-terrace €850, semi-detached or end-of-terrace €1,300, detached €1,800. Higher figures if you are on a qualifying welfare payment: apartment €900, mid-terrace €1,100, semi-detached or end-of-terrace €1,700, detached €2,300. Still a cavity grant. Still not a hollow-block grant. Aoibheann’s terrace does not get those numbers for a hopper aimed at cores.

The wrap is €3,000 for an apartment, €3,500 for a mid-terrace, €6,000 for a semi-detached or end-of-terrace, and up to €8,000 for a detached house. Internal lining, if that is the other conversation, is €1,500 / €2,000 / €3,500 / €4,500 across those same four types. None of those numbers is a you-pay band for a wrap. We do not invent one.

The house has to have been built and occupied before 2011. Approval comes before work. If work starts before SEAI says yes, the grant is gone. If SEAI does not approve, no work starts, and you pay nothing. That is the grant guarantee as Retrofit Dublin runs it. Retrofit Dublin is SEAI-registered, and Ireland’s official LoftZone installer. The phone is 01-233-7477. The 15-year guarantee is on the insulation, not on the folklore that a dark hole is a cavity.

The Irish Times on the heating bill you can cut without folklore still starts with the fabric you already own. RTÉ’s report on the January retrofit package put the wall money in the same announcement as the rest of the 2026 push. Neither of them will write you a hymn about beads in a Coolock core. That is our job.

Fabric first, then the machine

A whole-home energy strategy is an order of operations. You stop the leaks. Then you change how you make the heat. The cheapest first move in most Dublin houses is still the hat — attic grants €1,100–€2,500 — then the walls. Cavity if the camera says there is a gap, clean, dry, empty. A wrap if the wall is hollow block: boards on the outside, continuity over the webs, not a lucky dip in the cores. Then the machines.

Retrofit Dublin does home energy upgrades in that order on purpose: fabric, then systems. A heat pump behind a hollow-block wall you “filled” with optimism is a very efficient device heating a chocolate bar. Do not buy an expensive machine to feed a survey you refused to finish.

If the quote still says cavity and the wall is still one leaf, Claim Your Grants.