Identifying your damp type first
Treatment depends entirely on the type of damp. Using the wrong treatment wastes money and leaves the problem unsolved. A 2014 study by the Centre for Window and Cladding Technology found that 70% of homeowner-diagnosed damp cases were misidentified.
Rising damp
Horizontal tide mark at 1-1.5m above floor level. High moisture meter readings at floor level, declining with height. White salt deposits or brown hygroscopic staining. Caused by missing or failed damp proof course.
Penetrating damp
Isolated damp patches near windows, gutters, chimneys, or at high level. Worse during or after rain. Moisture meter readings high around the patch, normal elsewhere. Caused by external water ingress (failed pointing, blocked gutters, cracked render).
Condensation
Widespread surface moisture on walls, windows, and ceilings. Black mould in corners and behind furniture. Worse in winter. Moisture meter shows relatively uniform readings. Caused by warm, humid indoor air hitting cold surfaces.
If unsure, get a specialist damp survey from a qualified surveyor (RICS or Property Care Association member) before committing to treatment.
Treatment for rising damp
Rising damp treatment has two stages: stop the capillary rise with a new DPC, then deal with hygroscopic salts in the existing plaster.
Stage 1: Install a damp proof course
Chemical injection (most common):
Drill holes at 100-120mm intervals along the mortar bed at DPC height (150mm above external ground). Inject silicone or silane-based cream under pressure. The chemical diffuses through the masonry over 4-6 weeks, forming a water-repellent barrier equivalent to a physical DPC.
Cost: £40-70 per linear metre. Typical terraced house (10m of external wall): £400-700 including 20-30 year guarantee.
Effectiveness depends on wall saturation, mortar quality, and product choice. Modern silane creams outperform older solvent-based liquids. BRE testing shows 85-95% reduction in water absorption when installed correctly.
Physical DPC insertion (heritage or failure cases):
Cut a horizontal slot through the wall at DPC height using a specialist saw. Insert a plastic or slate membrane, then repoint the slot. This creates a guaranteed physical barrier but is 30-50% more expensive and requires temporary wall propping.
Cost: £80-150 per linear metre. Used in listed buildings where chemical treatment is not approved, or where chemical injection has failed in dense or very thick walls.
Stage 2: Replaster with renovating plaster
Even after a new DPC stops capillary rise, hygroscopic salts remain in the old plaster. These salts absorb moisture from the air, so the wall continues to appear damp and decorations fail.
Solution: hack off all affected plaster to 300mm above the highest tide mark. Apply salt-retardant renovating plaster (Limelite, Dryzone Damp-Resisting Plaster, Sika MonoTop). This porous, lightweight plaster allows salts to migrate to the surface and evaporate harmlessly rather than trapping them behind a dense finish.
Cost: £80-120 per m². A single room with 20m² of affected wall area costs £1,600-2,400 for hacking off, replastering, and skimming.
Do not use standard gypsum plaster or cement render. These dense finishes trap salts behind them, causing the plaster to blow (lift away from the wall) within 12-18 months. BRE Good Repair Guide 6 (2005) identifies incorrect replastering as the primary cause of apparent treatment failure.
Before DPC installation: check for bridging
Bridging (external render below DPC level or raised ground) must be fixed before DPC installation, or the new DPC will be bypassed.
Check:
- Ground level is at least 150mm below the DPC.
- External render stops at or above the DPC line.
- Internal plaster does not bridge the DPC (less common but can occur with cement render).
BRE Good Repair Guide 6 found that 60% of rising damp cases in buildings with an existing DPC were caused by bridging rather than DPC failure. Fixing bridging is often cheaper and simpler than DPC replacement.
Treatment for penetrating damp
Penetrating damp is water entering through the wall face due to external defects. Treatment focuses on stopping the water at source, then dealing with internal damage.
Fix the external cause
Repointing: Failed mortar joints allow rainwater to penetrate. Rake out loose mortar to 15-20mm depth and repoint with lime mortar (for pre-1919 buildings) or 1:3 or 1:4 sand-cement mortar (for modern buildings). Cost: £40-80 per m².
Gutter and downpipe repairs: Blocked or leaking gutters allow water to overflow onto walls. Clean gutters twice yearly (spring and autumn). Replace cracked sections. Ensure downpipes discharge at least 1m from the wall base. Cost: £200-500 for gutter cleaning and minor repairs.
Render repairs: Cracked or hollow render allows water behind the wall face. Patch small areas (under 1m²) with matching render. For larger failures, strip and re-render the entire wall to ensure a consistent finish. Cost: £60-100 per m² for full re-render.
Roof repairs: Missing tiles, failed lead flashings, or blocked valleys cause leaks. Replace missing tiles, repoint ridge tiles, renew lead flashings. Cost: £500-2,000 depending on extent.
Internal replastering (if salts present)
If penetrating damp has been present for months or years, hygroscopic salts may have accumulated in the plaster. Even after fixing the external cause, these salts continue to absorb moisture from the air, causing damp staining.
Solution: hack off contaminated plaster, allow the wall to dry for 4-8 weeks, then replaster with renovating plaster or standard gypsum (if the wall is fully dry and salts are minimal).
Cost: £80-120 per m² for renovating plaster, £50-80 per m² for standard replastering.
Treatment for condensation
Condensation is the most misdiagnosed damp type. It is caused by warm, humid indoor air hitting cold surfaces, not by water coming through walls or floors. Treatment focuses on reducing indoor humidity and warming cold surfaces.
Improve ventilation
Ventilation removes moisture-laden air before it condenses on cold surfaces.
- Trickle vents: Install window trickle vents to provide continuous background ventilation. Cost: £10-30 per vent, fitted.
- Extractor fans: Install or upgrade bathroom and kitchen extractor fans. Humidistat-controlled fans activate automatically when humidity rises. Cost: £80-200 per fan, installed.
- Positive input ventilation (PIV): A loft-mounted unit pushes fresh, filtered air into the home, creating positive pressure that expels humid air. Effective for whole-house condensation. Cost: £400-800 installed.
Increase heating
Warmer surfaces reduce condensation risk. Maintain minimum indoor temperature of 18-21°C in occupied rooms, even when out during the day. Use timer controls to maintain background heating rather than letting the house cool completely.
For cold external walls, consider:
- Internal insulation: Insulated plasterboard (50-100mm thickness) warms the internal wall surface, reducing condensation. Cost: £60-100 per m².
- Secondary glazing: Reduces cold bridging through single-glazed windows. Cost: £150-300 per window.
Reduce moisture sources
- Vent tumble dryers to outside or use condenser models.
- Use lids on pans when cooking. Use extractor fans or open windows.
- Dry laundry outside or in a well-ventilated room.
- Avoid portable gas heaters, which produce 1 litre of water vapour per litre of gas burned.
Surface treatments
Anti-condensation paint: Contains insulating particles (vermiculite or perlite) that warm the surface slightly, reducing condensation risk. Apply after improving ventilation and heating. Cost: £20-40 per litre, covering 6-8m² per litre.
Anti-mould wash: Fungicidal wash kills existing mould before redecoration. Does not prevent condensation but removes health risks. Cost: £10-20 per litre.
Treatment for basement and cellar damp
Below-ground rooms require different treatments due to hydrostatic pressure from surrounding ground.
Type A: Barrier waterproofing
External tanking membrane applied to outside of basement wall or floor, preventing water entry. Only practical during construction or major excavation. Cost: £100-200 per m².
Type B: Cementitious waterproofing
Cementitious slurry coating applied to internal wall and floor surfaces. Forms a water-resistant barrier but relies on structural integrity. Vulnerable to cracking if the building settles. Cost: £60-100 per m².
Type C: Cavity drain membrane systems
Dimpled plastic membrane fixed to internal walls and floor, creating an air gap. Water penetrates the structure but is channeled behind the membrane to a sump pump. Most reliable method for retrofit basement waterproofing. Cost: £80-150 per m².
How to choose the right treatment
Get a correct diagnosis
Use a moisture meter to measure wall moisture at different heights. Rising damp shows high readings at floor level, declining with height. Penetrating damp shows localised high readings near the source. Condensation shows relatively uniform surface readings.
If moisture meter readings are ambiguous, get a specialist survey from an independent surveyor (RICS qualified or Property Care Association member). Avoid free surveys from treatment contractors, who may have a financial incentive to diagnose the most expensive problem.
Fix the cause before treating symptoms
Replastering or redecorating without addressing the moisture source is a waste of money. The damp will return within months.
Always:
- Install or repair DPC before replastering rising damp zones.
- Fix external defects (gutters, pointing, render) before replastering penetrating damp zones.
- Improve ventilation and heating before redecorating condensation-prone rooms.
Use the right plaster for the damp type
Renovating plaster: For rising damp and long-term penetrating damp where hygroscopic salts are present. Allows salts to evaporate harmlessly. Cost: £80-120 per m².
Standard gypsum plaster: For condensation and short-term penetrating damp where salts are absent. Cheaper and easier to work with. Cost: £50-80 per m².
Lime plaster: For pre-1919 buildings with solid walls and breathable construction. Allows moisture to evaporate naturally. Cost: £80-120 per m².
When to get professional help
DIY treatment is possible for condensation (ventilation improvements, anti-condensation paint) and minor penetrating damp (repointing, gutter cleaning).
Get professional help for:
- Rising damp treatment (chemical DPC injection requires specialist equipment and most manufacturers require professional installation to activate guarantees).
- Full room replastering (renovating plaster has specific application requirements).
- Basement waterproofing (design and installation must comply with BS 8102 for Building Control approval).
- Structural repairs (subsidence, wall tie failure, lintel cracks).
Sources
- BRE Good Repair Guide 6 (2005): Treating rising damp in houses
- BRE Digest 245 (2007): Rising damp in walls - diagnosis and treatment
- Building Regulations Approved Document C (2022): Resistance to moisture
- BS 8102:2009: Code of practice for protection of below ground structures against water from the ground
- Centre for Window and Cladding Technology, University of Bath: Damp diagnosis study (2014)