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Last edited 25 Jan 2021
Treating brickwork with sealant or water repellent
Traditionally, the external brick walls of buildings were generally constructed without a cavity between the inner and outer leaves, however, they were thick enough to prevent moisture penetrating from the outside to the inside under most normal circumstances.
Since the early part of the 20th century, most external brick walls have included a cavity between the inner and outer leaves, meaning that any moisture that penetrates through the outer leaf runs down the outer face of the cavity and is drained to the outside.
In both these cases, if moisture does penetrate from the outside to the inside, it is generally because there is a defect in the wall construction. It could be that the wall has cracked, or there is a fault in the pointing, or gaps within the mortar, or the coping at the top of the wall is defective, there is a problem with a drain or rainwater downpipe, or the cavity has been breached or poorly filled. There could also be problems with condensation or rising damp that might give the appearance of penetrating damp.
However, some very poorly constructed external brick walls, typically around the late part of the 19th century and the early part of the 20th century are not sufficiently thick to prevent penetrating damp (sometimes just a single brick thick) and they do not include a cavity. As a result, even very minor defects that can be very hard to identify (such as minor movement, or small cracks behind pointing) can result in significant damp problems inside the building.
There are two basic approaches for this:
- Sealing the brickwork with a non-breathable sealant.
- Treating the brickwork with a breathable water repellent.
 Non-breathable sealants
Non-breathable sealants can be applied to the external face of a wall where they form a film making it impermeable to moisture. These sealants are typically manufactured from materials such as; acrylics, stearates, urethanes, silicone resins and so on. However, these sealants will generally also make the wall impermeable to air movement, meaning that any moisture that does get into the fabric of the wall (such as water vapour from the inside, or interstitial condensation) will not be able to evaporate to the outside. This can result in a wall that is sealed, but nonetheless damp, and at risk from mould growth, frost damage, and so on.
Because of the problems they can cause, they are generally not recommended. However, it should be noted that some products described as sealants are actually breathable, or partially breathable, see water repellents below.
Water repellents can also be applied to the external face of a wall. They then penetrate through the fabric of the wall, making it repel moisture. However, the wall remains permeable to air movement, and so water vapour is still able to pass through the wall, reducing the likelihood of a build up of damp from the inside. Water repellent treatments include materials such as; siloxanes, silanes, silicates, methyl siliconates, and so on.
The internal face of walls can be tanked with waterproof plaster, preventing moisture that penetrates the outside of the wall from reaching the inside. However, this may leave the fabric of the wall damp and elements such as joists within the construction may be at risk of deterioration. It can also be difficult to achieve complete tanking, for example where there is a junction between the wall and a floor, or ceiling, and subsequent movement, or works undertaken to the building can breach the tanking.
Brick walls are intended to ‘breathe’. They are permeable to moisture and water vapour, and this allows them to dry out if moisture penetrates them from the inside or outside. Any action taken to prevent this can have unforeseen consequences and should not be undertaken without careful consideration. It is always best to try and identify defects and correct them, rather than leaving the defects and trying to cover them up. Applying a sealant, repellent or tanking will not correct the underlying problem.
The Brick Development Association states, 'We believe the use of sealants and water repellent treatments on brickwork is totally unnecessary and take the view that properly specified and built work performs perfectly satisfactorily in resisting water penetration by wind driven rain, without their need. Brickwork inevitably gets wet in rainy weather and dries out later. This is the way it works and has done so for thousands of years.'
Whatever solution is adopted, it is likely that action will also be necessary to treat problems inside the building which have been caused by the damp, such as mould, rot, defective plaster, and so on.
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