What’s wrong here?

We’re often asked for advice on installation from customers and the trade so it’s always useful to see a photograph of the work proposed.

Taking a look at the picture above, we saw a number of problems, here’s our thoughts and later a solution …….

  • The air vent appears to be blocked in a number of holes, especially the bottom row which is below the ground level and should have been cleaned regularly.
  • The air brick has been painted over, time and time again reducing the size of the vent holes.
  • The ground level has been built up and the stones are blocking the lower holes.  This is one of the biggest causes of damp and reduced ventilation under floor boards of period homes and can lead to increased levels of condensation inside the home, formation of wet rot and dry rot and reduced  air in properties for combustion of open fires and combustion appliances.
  • The air brick shown in the photo is a clay/terracotta air brick and even when new wouldn’t have allowed very much air in for ventilation,  see previous blog (clay vs cast iron comparison). Consideration must be given to upgrading clay and terracotta air bricks if the mm2 surface area of the holes is especially low and the vent is used to keep sub floors from rotting , reducing condensation or providing air for combustion appliances.


The priority here is to lower the ground level on the outside of the property. If the outside floor level has breached a damp proof course it will create damp along inside walls and floors which would also encourage the formation of dry and wet rot.

If the outside floor level cannot be lowered then we would suggest removing the clay 9×6 air brick and replacing it with a 9×3 brick along the lower course and mortaring in a 9×3 cast iron air brick with a minimum free area of 4500mm2 on top. Care should be taken to ensure the ventilation duct fashioned using mortar from the air brick and into the property is clear. If the ventilation path is required to direct air below floor boards inside the property then plastic ducting may be used within the wall either by drilling a core hole and inserting plastic tube or installing a periscope vent combined with the air brick.

It is essential that surface water is directed away from the property so the air brick doesn’t create entrance holes allowing water to run into basements or below interior floors. If the outside floor levels can be lowered it may still be worth considering replacing the clay air brick for a more efficient vent as once cleaned, the air brick in the photograph would only  provide a free area for ventilation of 8x8x6x4 = 1536mm2.

All our 9s3 cast iron air bricks have a free area of at least 4500mm2