Domakihub How to Fit Cavity Wall Vents and Stop Damp for Good
Proper cavity ventilation is the single cheapest fix that protects a wall for decades.
Reviewed by Domakihub home & decor pros · 2026-06-23 · 5 min read
Cavity wall vents — sometimes called air bricks or cavity sleeves — keep the air space between your inner and outer leaf of masonry moving, which is the primary defence against rising damp, timber decay and mould. The video above shows the full installation process; the steps below give you everything you need to prepare, select the right product and either do it yourself or brief a builder confidently. For Greek homes, holiday rentals and hotels where walls are often concrete-frame with a single-skin infill, the principle is identical: you need a clear, insect-proof path for air to travel through the thickness of the wall.
Why Cavity Ventilation Matters
A sealed or blocked cavity traps moisture-laden air. In coastal or island climates — Crete, the Aegean islands, Ionian coast — salt air and winter rain push humidity into masonry, and without airflow that moisture condenses on the cooler inner leaf. The result is efflorescence (white salt bloom), spalling render and, over years, structural corrosion of reinforcing steel in concrete-frame buildings.
Older Greek buildings often used terracotta air-brick inserts (aeristires) at ground-floor level; many have since been plastered over during renovation. Fitting new cavity vents is usually a straightforward DIY job, but if your survey reveals bridged damp-proof courses or saturated insulation you will need a licensed damp-proofing specialist before any vent work makes sense.
Choosing the Right Vent for the Wall
The three common types are: a simple terracotta or plastic air brick (single-leaf walls or perforated-block infill panels), a telescopic duct sleeve (for a genuine two-skin cavity of 50–100 mm), and an in-line fan vent for rooms where passive ventilation is insufficient. Measure the wall thickness carefully — drill a small pilot hole and use a depth-marked rod — because telescopic sleeves must bridge the exact cavity width to prevent mortar bridging.
In Greece, single-skin concrete-block external walls are common in newer construction. These still benefit from weep vents at the base of the external render to drain condensation, even when there is no true cavity. For heritage stone buildings or listed structures, check with the local archaeological service (Eforia Archaiotiton) before cutting any openings: permits may be required.
How to Fit a Cavity Wall Vent: 5 Steps
- 1
Mark and drill the opening
Choose a position near the base of the wall, between mortar joints where possible, and at least 150 mm above the finished floor or ground level. Use a rotary hammer drill with a core-drill bit sized to your vent — typically 100 mm or 125 mm diameter. Drill slowly on the outer face, withdraw the bit regularly to clear dust, and stop as soon as the drill breaks through to the cavity. Keep the drill level: a sloping hole traps water rather than draining it.
- 2
Clear the cavity and check for blockages
Shine a torch into the cavity and use a piece of stiff wire or a small camera snake to check that the air path is not blocked by fallen mortar, old insulation batts or debris. Clear any obstruction. If you find that the cavity has been fully filled with blown insulation (a common retrofit from the 1990s), do not proceed with a standard sleeve vent — the insulation will simply spill out. In that case, consult a damp specialist about venting options that work with filled cavities.
- 3
Prepare and insert the duct sleeve
Cut the telescopic sleeve to the measured wall thickness, leaving a 5 mm tolerance. Apply a thin bead of exterior-grade silicone sealant around the outside face of the outer duct section. Push the sleeve into the hole from outside, extend it to touch the inner leaf, then twist to lock any tabs. The outer grille face should sit flush with or just proud of the external render surface, never recessed, so water cannot pool against it.
- 4
Secure and seal the inner grille
On the inside, fit the inner grille or fly-screen cover — this prevents insects and rodents from entering. Most kits use a friction-fit or a pair of screws into plugged holes. Apply a thin bead of interior-grade flexible sealant between the grille flange and the wall surface to prevent draughts around the perimeter. Do not use rigid cement here; differential movement will crack it within a season.
- 5
Test airflow and finish
Hold a lighted incense stick or a thin strip of tissue near the inner grille. You should see a steady draw of air through the vent, especially on a day with any wind. If there is no movement, check that the outer grille is not still covered by its transit film and that no mortar has slumped into the sleeve during installation. Once confirmed, point any gaps around the outer face with a matching mortar mix and allow to cure before painting or rendering over the flange.
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FAQ
- How many vents does a wall need?
- A general rule is one 100 mm round vent (or equivalent open area) per 1.5 m of wall length for subfloor or cavity ventilation. For a typical Greek ground-floor room of 4 m × 5 m you would place vents on at least two opposite walls so air can cross-flow. Always space them at the same height so the pressure differential drives movement.
- Can I fit cavity wall vents myself, or do I need a licensed contractor?
- Drilling through a non-structural infill panel and inserting a sleeve vent is DIY-friendly and needs no permit in most Greek municipalities. However, if the work involves a structural wall (pillar-and-beam frame walls are structural infill), you must not cut openings larger than a vent core without a structural engineer's sign-off. Damp-proofing work that involves injecting chemical DPC or applying tanking membranes to a basement is a specialist trade.
- My vent is fitted but I still have damp — what now?
- A vent only cures damp caused by poor cavity ventilation. If the source is a failed external render, a cracked roof parapet, rising damp through a missing or bridged DPC, or condensation from high indoor humidity (common in rental kitchens and bathrooms), ventilation alone will not fix it. Have a qualified surveyor identify the moisture source before spending further on remediation.