If your home has suspended timber floors, as most Victorian, Edwardian and interwar homes do, there is a void beneath the floorboards. Air bricks in the outside walls let air flow through that void and carry moisture away. When they are blocked, the space under the floor can become damp, still and ideal for decay.
Why sub-floor ventilation matters
Moisture rises from the ground beneath a suspended floor all the time. Cross-ventilation through air bricks on opposite sides of the house keeps that moisture moving out. Without it, humidity in the void rises, timber joists and floorboards absorb moisture, and conditions become right for wet rot, dry rot and woodworm.
How air bricks get blocked
- Paths, patios and driveways raised over the years, covering low air bricks
- Flower beds and soil heaped against walls
- Extensions built across the original air bricks without new ventilation
- Air bricks blocked deliberately to cut draughts
- Rendering or repointing that has filled the grilles
- Leaves and debris collecting in the openings
Signs of a damp sub-floor
- A musty smell, especially in ground floor rooms
- Floors that feel soft, springy or uneven
- Skirting boards that are decayed or have come away from the wall
- Mould on the underside of carpets or on floorboards
- Woodworm holes in floorboards or skirtings
Draughts versus damp
It is tempting to block air bricks in winter to stop cold draughts. Please don’t. Insulating between the joists and draught-proofing gaps in the floorboards is a far better way to keep warm without cutting off the ventilation the timber needs.
What we do
During a survey we check the number and condition of air bricks, look at ground levels outside and, where needed, lift boards to inspect joists and the sub-floor. Fixes can include clearing or adding air bricks, installing periscope vents where ground levels are high, and treating or replacing any decayed timber. See our dry rot and timber treatment service.
