Penetrating damp in buildings

Damp in buildings may be apparent from:

• Damp patches...
• Mould growth, which is a cause of respiratory allergies.
• Mildew, salts, staining and ‘tide marks’.
• Damage to surface finishes.
• Corrosion and decay of the building fabric.
• Slip hazards.
• Frost damage.
• Poor performance of insulation.
• Damage to equipment, or electrical failure.

The most common causes of persistent damp in buildings are:

• Condensation (surface condensation or interstitial condensation).
• Rising damp.
• Penetrating damp.

Penetrating damp is moisture that penetrates laterally through the fabric of a building from the outside, typically as a result of; leaking roofs or pipework, blocked or damaged guttering, cracks in walls, and so on.

Penetrating damp is most common high on south-west facing walls that are exposed to driving rain. It is particularly common in older buildings which do not have a cavity in the external wall construction. Properly constructed cavity walls should drain penetrating water back to the outside through ‘weep holes’. However, penetrating damp can occur in cavity wall constructions where insulation has not been properly installed, where cavities include obstructions, where cavities have been incorrectly filled or where wall ties have been incorrectly installed.

Damp can often be seen visually, but diagnosing the cause can be difficult and can involve specialist advise and equipment, such as; electrical resistance meters (although the presence of salts will affect resistance as well as damp – see Rising damp for more information), carbide meters, thermal imaging cameras, and so on.

It is only when the source has been identified that the problem can be tackled. This may be as simple as repairing damaged items, such as; damage to flashing, missing or broken slates or tiles, damaged pipework, cracks in walls, old or damaged sealants, damage to chimneys, parapets and other parts of the building that are difficult to see, deteriorating pointing and so on. However, where problems are concealed, such as incorrectly constructed cavities, they can be very difficult and disruptive to rectify.

There are a number of external treatments available to prevent penetrating damp, such as cladding external walls and the use of colourless water-repellent treatments. However, this may just conceal the problem, and increasing the impermeability of external walls could increase the likelihood of surface condensation inside the building or interstitial condensation within the structure itself.

Breathable treatments are available, but again, it is important that the source of the problem is tackled.

Treatment generally also involves remedial work to any corroded or decayed elements of the building fabric, such as hacking off and replacing existing plaster, replacing timbers, re-decoration, and so on.

NB Guidance on the code of practice for property flood resilience, published by CIRIA in 2021, defines water ingress as: ‘The seepage or entry of water into an area it should not be in.’

See also: Leaks in buildings.

[edit] Related articles on Designing Buildings Wiki

• Assessing moisture in porous building materials.
• Breather membrane.
• Building damp-free cavity walls.
• Carbide meter.
• Cold bridge.
• Condensation.
• Damp.
• Damp-proof course.
• Damp proofing.
• Defects in brickwork
• Defects in stonework.
• Dew point.
• Diagnosing the causes of dampness (GR 5 revised).
• Does damp proofing work?
• Efflorescence.
• Flashing.
• Humidity.
• Insulation.
• Interstitial condensation.
• Leaks in buildings.
• Mould growth.
• Rising damp.
• Rising damp in walls - diagnosis and treatment (DG 245).
• Spalling.
• Tanking.
• Treating brickwork with sealant or water repellent.
• Understanding dampness.
• Weathertight.