Damp in London Properties: Types, Causes, and Remediation

**1. Condensation (the most common type by far)**

Condensation occurs when warm, moist air meets a cold surface and the water vapour condenses into liquid water. It is responsible for the vast majority of damp and mould problems in London homes, but is frequently misdiagnosed as rising or penetrating damp.

• *How to identify condensation damp:*
• •Appears on cold surfaces — external walls (particularly corners), around window frames, on north-facing surfaces
• •Often accompanied by mould growth (black mould from Cladosporium or Aspergillus species)
• •Worst in winter when internal-external temperature differential is greatest
• •Appears most in rooms with high moisture generation — bathrooms, kitchens, poorly ventilated bedrooms
• •The affected surfaces are cold to the touch

*What causes it:* Condensation is caused by the combination of moisture-generating activities (cooking, showering, breathing, drying clothes), inadequate ventilation (extractors not running, windows kept closed), and cold surfaces (cold bridging at wall junctions, uninsulated walls, single glazing). In London's Victorian terraces, poorly insulated solid walls and inadequate ventilation are the main contributors.

• *Genuine remediation:*
• •Improve ventilation: trickle ventilators in windows, running extractor fans consistently in bathrooms and kitchens, whole-house MVHR systems on more comprehensive renovations
• •Reduce moisture sources: tumble dryer vented externally, not drying clothes on radiators, covering pans when cooking
• •Improve surface temperatures: insulation of external walls reduces the cold surface temperature and significantly reduces condensation risk
• •Positive Input Ventilation (PIV): a whole-house ventilation unit that supplies fresh, filtered air to the house and creates positive pressure that pushes moist air out through natural ventilation points

**2. Rising Damp**

Rising damp occurs where ground moisture rises through the base of a wall via capillary action — the porous brickwork and mortar act like a wick, drawing moisture upward from the ground.

• *How to identify rising damp:*
• •Appears on ground-floor walls, rising to typically 1–1.2m from floor level (rarely higher — capillary forces cannot raise water much beyond this)
• •Tide mark appearance — salts deposit at the damp front creating a white crystalline staining
• •Often associated with spalling (flaking) plasterwork, particularly at the base of the wall
• •More pronounced on external-facing walls
• •Damp meter readings are consistently higher at low level and reduce toward the top

*What causes it:* Rising damp occurs where the damp-proof course (DPC) is absent (pre-DPC buildings, common in London's oldest stock), or has failed, been bridged (by raised external ground level, paths laid against the wall, or internally by plaster running below the DPC line).

• *Genuine remediation:*
• •Check for and address DPC bridging first — this is the most common 'rising damp' cause and the cheapest to fix
• •Chemical DPC injection: a chemical water-repellent is injected into the wall at DPC level, creating a water-repellent zone. This is the standard treatment for genuine rising damp.
• •Replastering with renovation plaster (not standard backing plaster) that is salt-resistant
• •Note: many 'rising damp' diagnoses are incorrect — have an independent surveyor diagnose the problem before spending money on treatment

**3. Penetrating Damp**

Penetrating damp occurs where water enters the building through the external fabric — through failed pointing, cracked render, defective roofing, failed flashings, or poor window/door detailing.

• *How to identify penetrating damp:*
• •Localised damp patches that appear in wet weather and dry out in dry weather
• •Can appear at any height, not just ground level
• •Often associated with an identifiable external defect at the same location
• •Damp patches follow the pattern of water entry — below a failed flashing, adjacent to a poorly sealed window, below a parapet gutter

• *Genuine remediation:*
• •Find and fix the source of water entry — this always comes first. Treating the internal surface without fixing the external defect will not solve the problem.
• •Common sources: re-pointing of brickwork and masonry joints; re-rendering of cracked or blown render; relaying or re-sealing flat roof membranes; replacing or re-seating flashings; re-sealing window and door frames
• •Once the source is fixed, the internal decoration can be addressed after adequate drying time

The damp-proofing industry has a well-documented history of misdiagnosis — where condensation or penetrating damp is diagnosed as rising damp and an expensive (and unnecessary) chemical DPC treatment is recommended.

**Why misdiagnosis happens:** Damp-proofing companies often profit from the treatment they recommend. A chemical DPC injection and re-plastering package costs £2,000–£6,000+ for a single wall, compared with £200–£500 to fix failed pointing or improve ventilation. The financial incentive to recommend the expensive treatment is significant.

• **How to get an independent diagnosis:**
• Before commissioning any damp treatment:
• •Ask a RICS-registered building surveyor to assess the damp — not a damp-proofing company surveyor. A surveyor who is paid per report (not per treatment) has no incentive to over-diagnose.
• •Ask the surveyor to use a Protimeter or similar accurate moisture meter and to take readings at different heights and on different walls — not just the affected area
• •Ask the surveyor to explain their diagnosis and to distinguish between the three types of damp

**The BRE's view:** The Building Research Establishment (BRE) has published research suggesting that genuine rising damp (as opposed to condensation or penetrating damp misdiagnosed as rising damp) is much rarer than the damp-proofing industry suggests. The BRE's position is that condensation accounts for the vast majority of damp and mould problems in UK homes.