13 - Building Pathology - Dampness Flashcards
What is dampness?
The presence of unwanted moisture in the structure of a building, either the result of intrusion from the outside or condensation from within the structure.
How do you identify dampness? What does it look like?
- Visible wetting of walls, ceilings and floors
- Blistering paint
- Bulging plaster
- Sulfate attack on brickwork
- Mould on surfaces and fabrics
- Musty smell
What are the forces that propel water from the outside into the interior of a building?
- Capillary forces - water flowing through masonry or hairline cracks, usually by thermal catalysts.
- Kinetic forces - wind-driven rain will force water into the depth of a wall.
- Pressure differential - ventilation and air-conditioning systems may cause water to be sucked into the voids.
- Gravity - water can drip in through imperfections in flashings, gutters, roofs, and parapet walls.
- Surface tension - water will tend to follow a wet surface. It will flow around corners and edges.
What are the different ways moisture could enter the building?
- Rising damp
- Penetrating damp
- Condensation
- Weathering
- Trapped construction water (new builds)
What is rising damp?
Moisture rising up from the ground through capillary action normally up to 1m high with tide marks.
How is rising damp caused?
- Lack of DPC/DPM
- Inadequate lapping of DPC/DPM
- Bridging of an existing DPC/DPM (often by external rendered finishes or raised ground levels)
- DPC/DPM failure through natural deterioration or damage caused by building movement
- Splashing from rain or downpipes where DPC is less than the recommended 150mm above ground.
- Increase in ground water levels (e.g. man-induced changes to the water table, leaking drains, blocked land drainage systems, leaking water mains and springs) - likely if rising damp occurs fairly suddenly.
What height can rising damp reach and what factors can affect this?
- Rarely higher than 1.5m
- Depends on:
- Supply of water
- Pore structure of materials
- Rate of evaporation
- Heating within building
- Chemicals in ground and walls - efflorescence can block capillaries through which water evaporates, thus driving water further up the wall
What problems are associated with rising damp?
- Lead to outbreaks of dry or wet rot under the right conditions
- Reduce the strength of building materials, such as chipboard and plasterboard
- Reduce the effectiveness of insulation
- Damage decorations
How would you identify rising damp within a building?
- Visual inspection of possible causes (lack of / bridging of DPC/DPM etc.)
- Characteristic tide mark that does not extend beyond the lower part of the wall
- Damp contours can be pinpointed with a moisture meter
- Damp limited to usually 1m-1.5m above ground and readings above peak will drop quickly
- High percentage of moisture content in timber skirtings
- Salt analysis using a calcium carbide meter determines a high level of nitrates and chlorides, which are contained naturally in the subsoil
- Areas of dampness appear to get wetter in humid conditions due to hygroscopic salts (nitrates and chlorides) brought up from the ground, which attract water in from the atmosphere
UK ONLY - BRE Digest 245 (Rising Damp in Walls - Diagnosis and Treatment) contains detailed guidance on rising damp identification and remediation
What steps would you recommend to eliminate rising damp?
- Identify the source of rising damp and remove or provide a barrier, for example:
- Lower the ground level (where DPC is breached or ground level is not 150mm below DPC)
- Repair leaking drains/water mains
- Unblock land drainage systems
- Replace DPC or provide new where non-existent - Replace plaster/finishes where hygroscopic salts may still be present
- Renew to height 300mm above level of rising damp
- Re-plaster with a cement-based plaster (not gypsum plaster as most of these cannot prevent the passage of hygroscopic salts and quickly breakdown in wet conditions), preferably containing a waterproofer or salt inhibitor
What different methods of installing/replacing DPCs are available?
Physical replacement - You can install a new damp proof membrane to act as a damp proof course. This is a much bigger and more complicated process that involves taking out each brick along the failed mortar course and installing a new physical damp proof membrane. More expensive, disruptive and can only be laid on horizontal course (not suitable for rubble walls).
Chemical injection cream - choose between complete kits or individual cartridges of PermaSEAL PRO DPC Injection Creams. The cream is injected or hand-pumped into specially-positioned holes in the mortar course. Once inserted, the damp proofing cream reverts to a liquid. This allows it to penetrate the bricks and achieve complete absorption. As it cures, it creates a powerful water-repellent barrier and a new chemical DPC that stops water from rising up the wall.
Cheaper, lines the pores with a water-repellent solution however effectiveness depends on its successful penetration of the wall and lack of ‘viscous fingering’ (chemical spreads out and does not form a continuous barrier)
(e.g. PermaSEAL PRO DPC Injection Cream)
Electro Osmotic DPC - a titanium wire (anode) is secured around the wall at DPC level and connected to a small power supply, which causes moisture molecules to repel down the wall back into the ground (not supported by the BRE)
What guidance is available for rising damp problems?
- BRE Digest 245 (Rising Damp in Walls - Diagnosis and Treatment) - rising damp identification and remediation
- BS 6576:2005 - Code of practice for diagnosis of rising damp in walls of buildings and installation of chemical DPCs
What is penetrating damp?
Moisture/water from outside a building moves into the interior through the walls, roof, or ground.
Enters via a porous bridge transferring moisture through capillary action or there might be a hole in the building fabric.
What are the different mechanisms/forces of water ingress relating to penetrating dampness?
- Capillary forces - water flowing through masonry or hairline cracks, usually by thermal catalysts.
- Kinetic forces - wind-driven rain and splashing.
- Pressure differential - ventilation and air-conditioning systems may cause water to be sucked into the voids.
- Gravity - water can drip in through imperfections in flashings, gutters, roofs, and parapet walls.
- Surface tension - water will tend to follow a wet surface. It will flow around corners and edges.
What are the common causes and routes of penetrating damp?
- Slipped roof tiles
- Inadequate chimney/parapet flashing
- Copings without drips or not bedded on DPCs
- Leaking gutters (lack of correct support, damaged joints, lack of regular clearing)
- Overflowing hopper heads
- Leaking downpipes (broken joints, rusting cast iron downpipes to rear against wall)
- Continuously running cistern overflows (not discharging water clear of wall)
- Blocked gulleys (resulting in water splashing against wall)
- Cracked render or movement cracks in brickwork
- Defective pointing (recessed joints that could lead to frost action)
- Cavity ties (upside down so drip ineffective, mortar droppings resting on ties, uneven courses resulting in ties sloping toward inner leaf)
- Inadequately fixed cavity insulation boards causing bridging of the cavity from outer to inner leafy
- Poorly fitted windows and doors
- Sills without drips
- Poorly designed thresholds
- Missing vertical DPCs
- Driving rain on solid walls in particularly exposed situations (on cliff sides/west side of the Pennines)
- Vegetation growth to damp/shaded brickwork
- Repointing older buildings with cement mortar - stronger but less breathable than lime mortar, reducing the rate of evaporation. Also not as flexible so more likely to crack through drying shrinkage, thus allowing water ingress
- Applying strong external renders (for the same reasons as using strong mortars)
- Walls built in denser materials (e.g. granite or engineering bricks) - lack the ability to absorb water so most water runs down the face, which makes minor cracks more critical in allowing moisture ingress. Evaporation is also restricted
What are the problems/effects associated with penetrating damp?
- Damage plastering and wallpaper (decorations)
- It can cause toxic mold spores to grow.
- Lead to outbreaks of dry or wet rot under the right conditions
- Reduce the strength of building materials, such as chipboard and plasterboard
- Cause chemical reactions in building components (e.g. sulphate attack)
- Reduce the effectiveness of insulation
How would you identify penetrating damp within a building?
- Distinct damp patches with well-defined edges
- Often in localised areas
- Moisture readings show sharp change from wet to dry
- Patches of efflorescence (crystallisation of sulphates and carbonates present in building materials)
- Timber in area of damp has high moisture content
- External inspection may reveal obvious defects (e.g. cracked render/brickwork, damaged downpipes etc.)
- Deep wall probes indicate high readings in centre of wall
- Line of dampness on internal plasterwork corresponding with mortar joints where cement mortar/dense wall materials have been used
- Measure wall temperature, air temperature and RH to eliminate condensation
- Salt analysis shows zero level of nitrates and chlorides, eliminating rising damp
What steps would you recommend to eliminate penetrating damp?
- Identify the source of penetrating damp and remove or provide a barrier, for example:
- Replace defective rainwater goods
- Introduce DPCs beneath copings or vertical DPCs around openings
- Repair cracked render/brickwork
- Unblock gulleys/rainwater goods
- Replace poorly installed cavity ties - Application of a hydrophobic masonry paint (repels water but still allows the wall to breathe) may also be beneficial for solid walls that experience excessive rain penetration
What is Condensation?
- Condensation - change of water vapour naturally present in air into liquid water
- The amount of water vapour the air can hold depends on its temperature (the warmer the air, the more water vapour it can hold)
- If moist air comes into contact with a cold surface, the air will be cooled and its ability to hold water will reduce
- Once the air falls to a temperature where it can no longer hold the amount of water vapour present (i.e. it becomes saturated), liquid water will form on the cold surface
- Warm moist air comes into contact with cold surfaces which releases the moisture. Generally at high level.
What is meant by the terms ‘dew point’ and ‘relative humidity’?
- Dew point - the temperature at which the air becomes saturated and will condense
- Relative humidity - moisture content present within the air, referred to as a percentage of the amount of water vapour it can hold at that temperature. Warm air can hold more moisture than cold air, so as the air temperature falls, the maximum amount of water the air can hold also falls.
What is the acceptable RH in habitable spaces?
NZS 4303:1990 Ventilation for acceptable indoor air quality recommends relative humidity no greater than 60% in habitable spaces specifically to minimise the levels of allergenic or pathogenic organisms such as fungi and dust mites.
What causes condensation and high moisture levels?
- Moisture from leaks and damp ground
- Water released from household activities such as showering, washing, cooking and even breathing
- Moisture released from unflued gas heaters
- Inadequate heating and ventilation.
What problems are associated with condensation?
- Mould growth, particularly where RH remains above 70% for long periods (usually more than 12 hours)
- Health risks to the elderly, young children, asthmatics and those with weakened immune systems
- Can encourage timber decay where timbers are subject to prolonged moisture exposure
How would you identify condensation within a building?
- Wall has a ‘misty’ surface
- Stains or streaks of water runnin gdown a wall (particularly in bathrooms, kitchens and below windows)
- Damp patches with no definitive edges
- Dampness behind wall cupboards or inside wardrobes against external walls (areas where air circulation is restricted)
- Localised dampness at potential ‘cold bridges’
- Patches of mould growth
- Humidity (measured using a hygrometer), insulation and ventilation levels as well as heating and living patterns must also be taken into account
