The Air Brick Is Not a Letterbox

The air brick on the gable has a CV. It is not a letterbox. It is not a draught. It is the wall’s nose and the wall’s plughole, and someone in 1998 stuffed it with newspaper because the hall was cold and the brick looked like a grill for post.

A cavity wall is two leaves of masonry with a gap. The outer leaf is supposed to get wet. That is its job. Water runs down the back of that leaf and is supposed to leave through openings at the bottom: weepholes, air bricks, the little terracotta honeycomb that faces the garden like a polite colander. Paint those holes shut and the gap becomes a cupboard. Cupboards hold things. This cupboard holds rain.

A hole in the brick is not a defect

Let us invent a person. We will call him Colm. 1980s semi, Walkinstown. Yellow brick. A row of terracotta air bricks at damp-proof-course height, each one a grid of diamond holes, each one doing two quiet jobs that nobody thanks them for.

Job one: let the cavity drain. Driving rain soaks the outer leaf. Water finds the back face. Gravity does the rest. A tray or a damp-proof course. Then an opening. If the opening is open, the water leaves, the way a gutter leaves if you have not stuffed it with leaves. If the opening is a memory, the water sits. The inner leaf, which was promised a dry life, starts taking calls from the garden.

Job two: let the wall, or the void under the floor, breathe. Some air bricks serve the cavity. Some serve the underfloor. They all look like the same grill. They are not the same grill. Stuffing the wrong one is how you get a wet wall and a floor that smells like a tent.

Colm’s painter did not know this. Painters are hired to make things look finished. An air brick does not look finished. It looks like a hole someone forgot to brick up. So it got a coat. Then another coat. Then the diamonds filled in, and the wall kept the rain as a souvenir.

The wall has a plughole and you put a cork in it

Irish houses fail moisture in committee, not in one cinematic leak. Technical Guidance Document C is the official rulebook for site preparation and resistance to moisture: the damp-proof course, the cavity, the openings that let water out. It is not a style guide for how the brick should look from the road.

Technical Guidance Document F is the ventilation book. Air bricks that serve an underfloor void are not optional jewellery. Block them and the timber starts a hobby. Block the ones that serve the cavity and the hobby moves into the wall.

The live piece on what happens when you pump beads into a wet cavity is the next chapter. This chapter is the plot twist that made the cavity wet in the first place: the vents got stuffed, or painted, or mortared, or foam-gunned, because someone thought a hole in a wall was a letterbox, or a draught, or a defect that needed tidying.

How a honeycomb becomes a cork

There are several costumes, and Dublin has worn all of them.

Paint. Masonry paint loves a grid. Two coats, three coats, a Saturday spent freshening the brick, and the diamonds are a texture instead of a path. You can still see the air brick. You cannot see through it. Light should move. Air should move. If neither moves, you have a terracotta plaque with a CV it can no longer perform.

Mortar. A pointing job that treated every opening as a gap to close. Weepholes at the base of a cavity are often just open perps — a missing blob of mortar on purpose. A tidy pointer fills them. Tidy is not a thermal strategy. Tidy is how you build a pond in a wall.

Stuffing. Newspaper. Expanding foam. A sponge. A child’s sock. The folklore is that air bricks are draughts, and draughts are the enemy, and the enemy can be defeated with whatever is under the sink. The hall feels less bitey for a week. The cavity starts holding a drink. You have draught-proofed the plughole and called it comfort.

The fill itself. A hopper that does not protect the openings will blow beads or fibre into the air bricks from the inside. The wall looks insulated from the street. The plughole is now a bead trap. Water arrives. Water stays. You have insulated a swamp and called it a coat.

A blocked vent is a wet wall with a waiting list

Water in a cavity does not send an email. It sends a cold patch over a socket. Skirting with a tide mark. A musty band at damp-proof-course height that you blame on the house being old. The house is not old in a mystical way. The house has a blocked nose.

A wet cavity is a bad place to put insulation. Insulation is good at holding still air. It is also, inconveniently, good at holding still water. Fill a cavity that cannot drain and you have not insulated the wall. You have given the rain a hotel. The inner leaf gets cold and damp. Mould is physics with spores, not a lifestyle.

If the camera later says the gap is already a fish tank, filling it is the wrong job. The wrap — external wall insulation Dublin as a weather coat, not a paint job — lives outside both leaves. It does not sit in the pond. You start by asking whether the plughole still works. Openings first. Camera second. Coat if the cavity has already lost the argument.

What the camera is looking for

You can often see this from the footpath, which is a rare gift in wall work. Walk the elevations. Look at damp-proof-course height. Look under windows, where trays are supposed to weep. Look at the gable. Count the terracotta honeycombs. Put a finger to them. Air should move. Light should move. If the diamonds are a smooth painted rectangle, the wall has been draught-proofed into a cupboard.

A surveyor still puts a borescope through a small hole, because the street cannot tell you whether the cavity is empty, filled, or wet. Dry empty gap, open weepholes: a fill can still be a conversation. Openings painted or stuffed, water on the back of the outer leaf: stop. Clear the vents. Let the wall dry. Then argue about insulation.

If the person quoting you will not look at the air bricks, they are quoting a hopper. Hoppers do not get grants. Openings do. A responsible crew protects every honeycomb before a bead goes near the wall. That is not fussiness. That is the difference between a drain and a cork.

The money, without folklore

These are the current individual-grant figures, taken from SEAI’s own wall page. They are fixed by house type, not a percentage of a quote, and they are not a prize for pumping a wall that cannot drain.

Cavity wall insulation, standard: apartment €700, mid-terrace €850, semi-detached or end-of-terrace €1,300, detached €1,800. That money is for a cavity that deserves a fill: clean, dry, empty, and able to weep. It is not a bounty for filling a cupboard. Higher cavity figures exist 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 sponge grant.

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 €8,000 for a detached house. Internal lining is €1,500 / €2,000 / €3,500 / €4,500 across those same four types. The official wall numbers live on the SEAI wall insulation grant page. The plain-English table, edited 28 May 2026, lives on the Citizens Information page for Better Energy Homes.

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. The phone is 01-233-7477. The 15-year guarantee is on the insulation, not on the folklore that a hole in a brick is a letterbox.

Technical Guidance Document L is the energy rulebook: U-values, the score for how easily heat walks through a wall. A dry filled cavity can be a decent score. A wet filled cavity is a score wearing a certificate and a smell. The Irish Times set out the 2026 second-wall change when the package landed. RTÉ put the insulation-grant increases in the same January announcement. Neither of them is going to write you a hymn about a painted air brick. That is our job.

Open the nose, then argue about the coat

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 insulation. Then the walls — all of them — and the walls include the holes that let the cavity drain. External cavity if the camera says the gap is clean, dry, empty, and the air bricks still do their job. A wrap if the cavity is the wrong job. Then the machines.

Retrofit Dublin does home energy upgrades in that order on purpose: fabric, then systems. A heat pump behind a wall that is holding rain is a very efficient device heating a sponge. Do not buy an expensive machine to feed a plughole you refused to uncork.

You can keep the quote that treated the air bricks as decoration. It is a historical document of optimism. The honeycomb is still a plughole. Paint is still a cork. Do the hat. Open the vents. Look with a camera. Apply before anyone drills. If SEAI does not approve, no work starts, pay nothing. Phone 01-233-7477.

If the air brick is still a letterbox and the cavity is still a cupboard, call 01-233-7477 and Claim Your Grants.