Background
Damp was identified at the base of the wall during the homebuyers survey of this recently purchased Victorian house in London. A damp proofer was asked to quote for work. Rising damp treatment was recommended costing £4,000 despite there having already been two previous attempts at damp proofing against rising damp.
Data loggers
The traditional method for measuring vapour involves the calculation of vapour pressure. However, this method is complex, terms of understanding and calculation, and requires specialised equipment. A more practical alternative is to utilise dew points. These directly correspond to vapour pressure. Essentially, the higher the amount of vapour in the air, the higher the dew point, increasing the likelihood of condensation and mould growth, regardless of the heating or insulation of a property.
There are various readily accessible meters, including data loggers that work with the Sensorblue app, for measuring dew points. Alternatively, you can use our tool at mouldpoint.co.uk to calculate it from temperature and relative humidity.
Understanding the internal dew point is beneficial for three main reasons:
1) When the internal dew point matches the external dew point, it indicates that vapour generated from everyday activities such as cooking, bathing, drying clothes, and breathing has been effectively managed with ventilation and dehumidification.
2) If the internal dew point is lower than the external temperature, there is no significant risk of condensation, and the likelihood of mould formation is greatly reduced. However, it’s important to note that mould can still develop if conditions remain 3°C above the dew point over a prolonged period.
3) The external temperature is both easy to measure and there are plenty of area forecasts, such as BBC Weather and or Met Weather.
In conclusion, from the data loggers
Reduce internal dew points to the outside temperature, by following recommendations, in essence;
- cook with tops on pots and pans,
- dry clothes outside or with a dehumidifier,
- overuse the bathroom extractor fan, keeping the door closed, with the fan running for at last 30 minutes after a shower,
- open windows and monitor internal dew points, using a dehumidifier to drop internal dew points below the outside temperature.
Evidence & Illustrations
Insulation is outside the scope of damp surveys but in my experience:
External insulation is typically better than internal insulation, but it is not always feasible, can be expensive and doesn’t always look so good (the same can be said of internal insulation, especially around elements such as window and reveals). The main risks are:
- Cold bridges, that is uninsulated zones, such as behind gutters and pipes. Keeping internal dew point at or close to the external temperature will mitigate this risk.
- Cracks and gaps in the insulation causing penetrating damp. Walls need six-monthly checks for cracks and gaps, and to be filled with an impermeable filler whenever these appear.
For internal insulation
- If there is timber below or close to insulation, such as a floor joist or ceiling joist, then impermeable insulation on the inside wall increases the risk of condensation forming causing rot, including Dry rot.
- The condensation largely comes from rain or solar driven moisture. The risk is reduced by covered walls, such as behind gap-free, impermeable render on the outside.
- Typically the best bang for your buck comes from PIR (Polyisocyanurate), such as Celotex and Kingspan, often attached to plasterboard (often used by damp proofers to both hide damp and reduce risk of condensation, which has merit but does not address the root cause of damp).
- Where there is timber below or within about 10 – 20cm of the top of the insulation, it is better to use a vapour active insulation, such as wood-fibre, Rockwool, or for thin areas such as eaves and reveals, Spacetherm (expensive but fantastic) or Wallrock thermal lining paper.
- For roofs you can use PIR above the ceiling so long as there is ventilation above.
- Like many things in life, you can be pragmatic, mitigating the risk;
- On the cold side of a building without prevailing wind, you could choose to use PIR.
- You could monitor humidity within the void near timber at risk. Rot respires, so will increase the local relative humidity keeping it high for a long period.
In your circumstances, and only after the wall exposed to rain has dried out, which will take about 9 months.
- There is a risk of rot, so the official answer is use vapour permeable insulation, but you can be pragmatic at your own risk:
- In the case of your chimney breasts, you could choose to use PIR with a gap of 50mm at the base of the wall, covered by skirting and a humidity probe, similar to the one I left.
- You could also use PIR insulation on other walls, so long as you monitor the sub-floorrelative humidity and anywhere near timber such as with a humidity probe.
1) the original damp proof course,
2) set of holes drilled horizontal from the outside,
3) plastic membrane in the corner.
As an aside, the render at the base of this wall has been exposed. While useful for seeing the various attempts at damp proofing, exposing it is subjecting the wall to increased rainwater. There is much debate about whether to render or not, and or leave a zone of un-rendered wall. Most experts focused on damp surveys will say, the walls least likely to suffer from damp are bare brick down to about 1M above ground, then rendered to the ground with an impermeable mortar and fillet of mortar between the ground and any hard standing surface. Buildings, especially modern properties are often covered with an impermeable coating on the outside. This is fine but cracks and gaps need to be regularly checked for and filled whenever they appear to stop rainwater passing through and being unable to evaporate out, unlike a permeable surface like a bare brick wall.
The conservatory and kitchen both were suffering from condensation, while heat loss increases the risk of condensation and mould, the root cause is insufficient ventilation and dehumidification.
To keep the property dry | |
|---|---|
| Repair render cracks | |
| Check and repair any missing external render to prevent water ingress, with a render similar to the original, typically a cement based mortar. Address external render cracks by raking out, that is slightly enlarging and removing loose material and filling with an acrylic based caulk for small gaps and cement based mortar for larger gaps over 3mm. The render should continue to the ground, with a fillet of cement based mortar between the wall and ground. | £500 |
| Use a dehumidifier | |
| Maintain relative humidity, especially at night when it’s cold outside, with a dehumidifier such as PureMate 20L/Day Portable & Compact Dehumidifier with 6.5 Litre (£179.99 Refrigerant), Duux Bora Smart Dehumidifier (DXDH02UK £300 Refrigerant) or Meaco DD8L Zambezi (£260 Desiccant – best for colder environments) – Which! Best buys. There are plenty of alternatives available. I choose a dehumidifier based on daily and tank capacity, low noise, whether it has a pipe out, and ease of programmable functions over other considerations such as weight and aesthetics. The technology is old and easy to manufacture. So if these brands are not available, you are sure to find another alternative. Evaluate based on daily capacity (20L minimum) and whether it has a continuous pipe out (use this to drip into a sink when away in winter). | £180 |
| Use fan to expedite drying | |
| Direct a fan onto the damp patch to speed up the drying process. | £50 |
| Monitor vapour with data loggers | |
| Monitor relative humidity and temperature against the wall or ceiling to observe the effects of improvements, such as ORIA Wireless Thermometer Hygrometer (20m (2 Packs)) – already in place. In addition monitor improvements on the damp wall a pencil line and damp meter such as dr.wood moisture meter, logging improvements each month. Monitor relative humidity and temperature against the wall or ceiling to observe the effects of improvements, such as ORIA Wireless Thermometer Hygrometer (20m (2 Packs)) – already in place. In addition monitor improvements on the damp wall with a pencil line and damp meter such as dr.wood moisture meter, logging improvements each month. | |
| Remove mould | |
| Consider sanding down mould where dry, and removing it with either bleach or anti-mould foam. I personally use HG Mould Remover Foam Spray, but imagine there is no major difference between brands. | £20 |
| Reduce vapour from cooking | |
| Cook with tops on pots and pans to reduce the production of vapour from cooking by 80%. | |
| Target vapour from bathroom | |
| TIP: Keep the bathroom door closed with the fan running or window open until the humidity from the shower has subsided – typically at least 30 mins after the last shower. | |
| Target vapour from clothes drying | |
| Use a tumble dryer (like a heat pump), dry in a vented room with the door closed (possibly a bathroom with the extractor running or window open), or use a powerful dehumidifier. Don’t dry clothes in an unventilated room. | |
| Monitor improvements | |
| I would buy a Dr. Meter Wood Moisture Meter 2 in 1 Pin & Pinless Multifunctional Water Detector £23.99 to log and monitor reduction to dampness in walls. | £24 |
| Remedial actions – estimate of costs | £774 |
Actions if all else fails | |
|---|---|
| Duct out kitchen extractor fan | |
| The main issue to getting ducting out the extractor fan is hiding the ducting. Ideally it should be using rigid ducting (not flexible as the ridges create eddies slowing air speed), as short as possible with as few bends. It can go up the chimney but would then need to be run for longer to account for the possibility of vapour travelling back down. | £200 |
| Consider targeted insulation | |
| Optimal actions – estimate of costs | £200 |
I conclude that there was no evidence of rising damp, which comes from groundwater in contact with walls. The risk of groundwater is very low. There were many signs of excess humidity causing condensation and mould.
Mould only grows where there is excess vapour, furthermore it is inhibited by salts found in rising damp.
. By taking a root cause approach, rather than a focus on blocking the symptoms of damp the savings are likely to be around £4,000. This contrasts with typical damp proofing contractor recommendations such as chemical treatment for rising damp and untargeted ventilation, such as PIV systems.
Flood risk: Risk of surface water (i.e. penetrating damp from rainwater, not rising damp from groundwater) See Flood Risk (note most flood risk is not from groundwater).
Sub-soil rocks: Near moderately productive aquifer with intergranular flow See British Geological Survey.
Therefore risk of Rising Damp: low risk, see explanation by Dr Robyn Pender of Historic England.
I confirm that I inspected the property on 240223SH0800. I conclude that there was no evidence of rising damp, which comes from groundwater in contact with walls. the risk of groundwater is very low. there were many signs of excess humidity causing condensation and mould. All buildings can be exposed to unvented vapour and external dampness to some degree. You will mitigate the risk of damp if you follow all our recommendations. This report is intended to be read in full including supplementary links in this report. Observations and opinions must not be taken in isolation. Given the right weather conditions almost any house can be affected by rain. Like any building, you need to be aware of the risks of damp arising in the future and should plan a programme of monitoring, prevention and maintenance accordingly. Damp Surveys Ltd reports are designed to provide you with an informed, independent expert opinion as to the cause of dampness in the property, together with any recommendations for further investigation or remedial work. We do not warrant any findings in this report unless we enter into a separate warranty agreement with you.
The survey was conducted during daylight hours. Damp will be more noticeable at night and when the weather is colder and more humid. Gutters are more likely to fail when full of leaves and during periods of prolonged rain and adverse wind. We make best endeavours but cannot guarantee being able to identify all forms of damp, rot and insect infestation affecting the property.
We carried out a careful and thorough inspection of as much of the property as was accessible. However, when a full inspection is not possible, we make a professional judgement regarding the likelihood of a defect being present. In certain circumstances, this may lead to a recommendation for further action to open up an area for additional investigation. We were unable to inspect the entire roof, all the guttering, and some of the drains. Similarly, inspection of woodwork or other parts of the structure that were covered, unexposed, poorly lit or inaccessible such as in the loft, cellar, or sub-floor void was not possible, and therefore we are unable to assert that such parts of the property are free from defect. There were no obvious signs of damp resulting from these limitations
Negligence claims must be made within 90 days of the damp survey site visit. Clients must make best endeavours to monitor humidity and temperature on a damp wall and the closest source of humidity, by purchasing and placing two data loggers for example the Smart Hygrometer measuring humidity on the wall and nearest source of vapour, before and after following our recommendations. These data loggers have sufficient memory capacity to store 90 days of data.
This report is for the sole use of the client (named on the invoice, but omitted here for anonymity) for whom the survey was undertaken and can only be relied upon for 90 days from the survey date. Unless expressly stated otherwise in this report, nothing in this report confers or is intended to confer any rights on any third party pursuant to the Contracts (Rights of Third Parties) Act 1999.
Simon Hichens
Site visit 240223SH0800
Simon Hichens BSc Chemistry RICS Qualified Expert Witness (Chartered Surveyors)
PCA Certificated Surveyor of Dampness & Timber in Buildings (Property Care Association)
Damp Surveys Ltd, 119 Longton Ave London SE26 6RF Reg. 10977488 (England & Wales)
Founder of damp.aï & Member of the Society for the Protection of Ancient Buildings (SPAB)
Our damp surveys are designed to provide a holistic diagnosis that identifies and deals with causes rather than a focus on symptoms. Symptoms will disappear over time, once damp is stopped at source. We look for risks of dampness, but unless moisture is present, we can’t guarantee to identify all risks of future moisture. Our reports:
- identify root cause(s) of major unwanted dampness within the property,
- identify major symptoms of unwanted dampness,
- identify major structural damage caused by dampness,
- recommend actions and estimated costs to stop damp at source and mitigate its effects,
- recognise that treating damp is often a staged approach, treating obvious causes first.
Understanding damp
For survey purposes, damp is defined as unwanted water. Water brings life. We need it. Unfortunately, fungus and insects also thrive in water. While a damp wall can cause decorative spoiling, it is not moisture, but the life it brings that causes the greatest concern in the home environment. FACT: vapour causes 85% of residential damp problems.
Independence and Methodology
Our only income is through damp survey fees. We are independent of contractors and never profit from remedial work. We do not receive or pay any fees or other inducements. Our motivation is peace of mind and practical, durable solutions. We use an array of equipment to identify the root cause of damp within walls. Our damp surveys follow guidance from RICS, Historic England and PCA joint position statement and support RICS’s highest standard, level 3 “Home Survey Standard” (HSS).
During the survey we assess likelihood of rising damp*, mainswater leak, penetrating damp, condensation, timber rot and woodworm. We profile the damp patches and consider the likely sources of dampness and test ventilation against building regulations requirements. We consider changes to the property and factors just before the first sighting of damp.
- Rising damp concern: A damp proofing contractor claimed your property had rising damp.
- Weather: dry overcast
- Property: Victorian semi-detached house
- Background: The property was bought recently
- Changes: open plan reception, open plan kitchen, chimney removal, chimney opened up, multiple attempts at damp proofing
- Occupancy: owner occupied
- Mould Colour: Mould was ground in the opened chimney breast
- Primary damp concern: damp at base of walls, especially near sub-floor vents
- Other damp concerns: damp in bedroom
- Compounding: heat loss from poor insulation
- Airbricks: There is sufficient sub-floor ventilation.
- Sub-floor humidity: There were no concerns about humidity in the sub-floor void.
- Tenure: house
- Attachment: a semi-detached
- Build: Victorian
- Walls: solid wall made of two brick widths (225mm)
- Exterior comment: The outside render has been removed at floor level, creating a significant risk of penetrating damp.
- Penetrating damp assessment: There is a significant risk of penetrating damp.
- Drains: The drains are of no special concern.
- Leak signs: No suspicion of a mains water leak
- Leak assessment: There are no obvious signs of a significant mains or wastewater leak.
- Floor: The suspended timber floor did not feel damaged.
- Loft: There appeared to be no significant damp or timber decay in the loft.
- Timber assessment: There are no suspicions of significant timber decay.
- Last dry: I understand that some damp issues were picked up when you bought, but these have become worse over time.
- Ventilation assessment: There is no kitchen ventilation. The bathroom ventilation could be better.
W.H.O. indoor air quality guidelines
“Management of moisture requires proper control of temperature and ventilation to avoid excess humidity, condensation on surfaces and excess moisture in materials. Ventilation should be distributed effectively throughout spaces, and stagnant air zones should be avoided.”
FACT: Houses contain 30 – 40 litres of water as vapour typically. Each day, each occupant adds on average about 1.5 litres as follows:
- Each shower results in 1/2L condensing (each additional shower in a series won’t add as much as the first, so long as the door is kept closed)
- Vapour from drying clothes releases about ½ litre per day if dried indoors
- Respiration releases about ¾ litre per day (of which ¼ l at night)
- Cooking produces about ¼ litre per day per person
- Unvented vapour can cause condensation, mould and allergenic dust mites. The simple rule for keeping your property dry and mould free is;
- Vent out as much vapour as produced, each and every day!
- Central heating and double-glazing reduce background ventilation. To overcome this you should use mechanical extractors. Where properties are rented, consider using monitoring devices (such as a Govee WIFI hygrometer) and if necessary, install continuous flow extractors, but beware there can be a thermal cost caused by continuous heat lost.
FACTS about Condensation and Mould
- FACT: warm air holds more water vapour than cold air. The warmer it is, the more air’s capacity to hold water vapour. Conversely the colder it is, the less capacity, until air cannot hold any more vapour. We call that the dew point or 100%RH. As soon air reaches capacity, condensation will form on the coldest surfaces.
- FACT: Mould and dust mites
- FACT: Mould only grows when air is humid for long periods. In the occupied home environment, it is 6 hours over 85% RH. For properties that are unoccupied for long periods, it is 75% RH for many days of consistent and high humidity. Avoid allowing the surface relative humidity of cold walls to exceed 85% RH by ventilating, dehumidifying, insulating, and ensuring airflow, particularly in cold corners, base of walls and ceilings, eaves, and cold window and door reveals.
- FACT: Dust mites are microscopic insects that can cause allergies. They grow in similar conditions to mould. So ventilate more to avoid allergies.
- FACT: Health concerns: There are no health and safety issues from damp noted. It is in the occupier’s interests to keep a property properly ventilated or dehumidified and temperatures above the dew and mould points throughout the property, thereby reducing the health risk associated with dust mites, bacteria, protozoans, as well as decorative spoiling caused by mould.
TIPS To Stay Condensation and Mould Free
- TIP: Reduce the production of vapour at source:
- TIP: Keep bathroom extractor fans running, or window open for at least 30 minutes.
- TIP: Keep bathroom doors closed at all times, and bathroom windows open safely.
- TIP: When filling the bath, run the cold water first then add the hot.
- TIP: Dry clothes outside, in a dryer, in a closed room with a dehumidifier or window open, never on radiators or heated rail without a door closed and a means of removing the vapour.
- TIP: Cook with tops on pots and pans, avoiding excessive boiling.
- TIP: Keep the kitchen door closed and extractor fan on where possible.
- TIP:Mechanical vents are best, but if you only have passive vents, such as a whole in the wall or chimney, then vents open with space for air to move, insulate where possible.
- TIP: Keep trickle vents open or alternatively, open windows on safety locks.
- TIP: Stop mould forming:
* Maintain external wall temperatures above 12°C, with low background heating.
* Declutter leaving space for warm air to circulate around cold surfaces.
* Where possible, avoid placing wardrobes and furniture against external walls.
* Avoid overfilling wardrobes and cupboards as it restricts air circulation.
* Use dehumidifiers or small wardrobe dehumidifying bags and replace regularly.
* Wipe mould and condensation off walls and clothes as soon as it appears. - TIP: Monitor relative humidity
* We recommend monitoring temperature and relative humidity with a data logger to see what is happening.
