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How to Deal with Damp

Damp can damage much more than a buildings appearance. It may lead to the deterioration of plaster and masonry, promote timber decay and create unhealthy conditions for occupants. Unfortunately, inappropriate treatments for damp commonly cause greater harm to old buildings than centuries of degradation. Over-reliance on electrical moisture meters frequently leads to unnecessary expense and damage through the retrospective installation in walls of horizontal damp proof barriers (damp proof courses or DPCs). Equally the harm done by modern solutions that aim to seal old walls rather than improve their ability to breathe is underestimated. An appreciation of how the basic construction of old buildings differs from that of new ones will help you avoid such misguided remedies.

Breathability

Old buildings must be allowed to breathe. Whereas modern buildings rely on keeping water out with a system of barriers, buildings that pre-date the mid-19th century are usually constructed of absorbent materials that allow any moisture that enters to evaporate back out.

Because most old buildings were constructed with solid walls without DPCs and originally had no roofing felt, rain or below ground moisture could both enter. This did not, however, mean dampness was inevitable. Before central heating was commonplace, heat from open fires drew in large quantities of air through loosely fitting windows and doors. This high rate of ventilation would have quickly evaporated moisture from permeable internal surfaces while the wind dried out any damp roof timbers or permeable external wall surfaces. An equilibrium was therefore established whereby the moisture being absorbed was equal to that evaporating. When upgrading an old building, you must maintain this equilibrium for the building to work as intended and remain dry.

Causes of Dampness

The main risks arise from:

Air moisture condensation: Energy-saving measures that reduce ventilation in old buildings such as double-glazing increase relative humidity. Humidity is also raised by modern lifestyles that generate large quantities of water vapour, from bathing, cooking and washing. Condensation will occur on any surface below the dew point (i.e. temperature at which saturated air releases surplus moisture vapour). Interstitial condensation within the pores of materials reduces thermal insulation and further increases the risk of condensation.

Penetrating damp: Roofs, chimneys, parapets and other exposed parts of a building are most susceptible to rain penetration, especially where access for maintenance is difficult. Junctions in roofs are potential trouble spots, with water exploiting defective lead flashings, mortar fillets, ridges or hips. Concentrated and prolonged wetting of walls and external joinery arises from poorly maintained rainwater fittings, and leaks from parapet and valley gutters can cause significant damage to structural roof timbers. Hairline cracks in pointing and render invariably admit moisture where cement mortar has been used for repair, rather than lime.

Internal spillage: This results from overflowing baths or showers, burst pipes, the gradual breakdown of pipe joints, leaks from washing machines or dishwashers, and accidental damage.

Below ground damp: This may be rising damp, which is neither as widespread as commonly thought nor a total myth, as sometimes now claimed. Floors become damp where the evaporation of moisture from below is inhibited by vinyl sheet, rubber-backed carpets or other impervious coverings. New concrete floors or impervious coverings also drive excess moisture into the bases of nearby walls (including chimney stacks), where it rises by capillary action. DPCs were not compulsory in walls prior to 1875 but this is only likely to become a problem where breathability is compromised. In addition to rising damp, below ground moisture can result in problems where ground levels around your building rise unduly.

Tips for Diagnosing Damp

Roofs and Rainwater Fittings

Inspect your roof during wet and windy weather to decide if a damp ceiling patch is due to roof leakage and/or condensation. Debris on the ground (broken slates, tiles and so on) or daylight seen inside lofts indicate possible roof problems.

Defective rainwater fittings may be most obvious during heavy rain, but stains on walls and plant growth provide further clues. Don't forget to check gulleys at ground level.

Condensation is diagnosed from diffuse areas of damp, beads of water droplets on hard shiny surfaces and mould growth on internal finishes. It is intermittent, like penetrating damp, but unrelated to wet weather.

Penetrating damp typically shows up as well-defined patches after heavy rain on south- and west-facing walls. Anticipate moisture ingress through hairline cracks in unsuitable hard, modern cement pointing or rendering.

Below ground moisture causing rising damp can extend up to 900mm above floor level, sometimes with a classic tidemark on finishes. Salts appear as white deposits but mould growth is rare.

Plumbing

Unusually high water bills (if metered) or a constantly refilling tank may suggest leakage.

DPCs and Alternative Systems Compared

Retrospective DPCs:

Physical:

  • Inserted by cutting in or during rebuilding.
  • Can cure rising damp but this drastic method is usually inappropriate.
  • Drawbacks: possible major structural problems; potential damage to historic finishes internally; unsuitable for randomly coursed walls; access difficulties; deterioration sometimes of masonry below DPC where moisture is concentrated.

Chemical:

  • Walls impregnated with chemical solution through holes at bottom to create waterproof barrier.
  • Widely used today but not always appropriate for old buildings.
  • Drawbacks: drilling holes inadvisable in flint, granite, etc; hard to form proper barrier in rubble walls with voids; holes unsightly; deterioration sometimes of masonry below DPC where moisture concentrated.
  • Cost: typically £195/m (including replastering).

Ceramic tubes:

  • Holes drilled to receive porous siphons approximately 50mm in diameter that absorb damp and evaporate it from each tube.
  • Sound in theory but problems may occur in practice.
  • Drawbacks: salt accumulation in tubes may increase moisture; air-flow sometimes inadequate; tubes commonly set in hard cement mortar; unsightly.
  • Cost: typically 125/m.

Electro-osmosis:

  • Electrical potential aimed at reducing capillary rise using electrodes bedded in wall.
  • Cheap but dearth of evidence that electro-osmosis is effective and system rarely used today.
  • Drawbacks: adjustment of current needed to match variations in damp along a wall usually impractical.

Other

  • An Austrian product presently under trial in the UK claims to inhibit the passage of water up a wall by inducing a local magnetic field. Achieved non-invasively with unit plugged into mains, typically in loft.
  • Likely cost: £3,000/unit (one unit covers an average-sized house).

Investigating Damp

Scientific analysis can be an essential aid for accurately diagnosing a damp problem but the importance of your sight, feel and smell should not be undervalued. Tests include the use of electrical resistance and capacitance meters, on-site moisture testers, hygrometers and salt analysis. Bear in mind though, that care must be taken when interpreting results. A frequent mistake is to diagnose rising damp purely on the basis of high electrical moisture meter readings. Elevated readings may indicate the presence of salts from past dampness or, if there are no salts, an on-going condensation or possible penetrating damp problem. Continued monitoring is often needed to establish the true cause of a damp problem.

Surveyors have a legal duty to follow a trail of suspicion. Regrettably, many still simply note the occurrence of high meter readings and pass on all responsibility for further investigation to remedial treatment contractors. These contractors have a vested commercial interest, encouraging over-specification. Should a mortgage company insist on work you believe is misguided, challenge this and consider obtaining a written report from an independent surveyor or architect.

Remedial Measures

Effective remedial measures depend on accurate diagnosis, but applying staged remedies can also be part of understanding the cause of a damp problem. Before deciding on more extensive work, the first remedy may involve nothing more than basic maintenance such as clearing a blocked rainwater gulley. Remedies will either cure dampness by addressing the cause (for example, improving drainage) or will manage it by treating the symptoms (changing washing or cooking habits, for instance).

Be sceptical of written guarantees, which are often loaded with get-out clauses and may have no insurance backing. The right approach from your contractor coupled with good workmanship is your best guarantee.

Controlling Air Moisture Condensation

Condensation can be treated by reducing air humidity or keeping surfaces above dew point temperature. Humidity is reduced by cutting the amount of moisture available or increasing ventilation by opening windows, etc. Tumble dryers should be vented to the outside if not of the condenser type, and clothes drying indoors is best avoided.

Temperatures are maintained above dew point with suitable heating. The permanent use of dehumidifiers is a poor substitute for efficient heating and adequate ventilation.
Condensation in chimney flues can be eliminated with proper linings. Redundant flues that have been sealed should be fitted with ventilation grilles or re-opened. Lofts should be well insulated and ventilated but make sure insulation does not restrict ventilation at the eaves.

Controlling Penetrating Damp

Reinstate dislodged and missing slates and tiles before damage occurs to roof timbers or plaster ceilings. SPAB, the Society for the Protection of Ancient Buildings, recommend that renovators avoid spray-on roof foams for the underside of roofs, or external bitumen coatings although other experts disagree. SPABs view is that they prevent proper inspection, hinder the re-use of slates or tiles and, by reducing ventilation, increase the risk of decay. Brush moss off roofs since it can block gutters and retain moisture, which may damage certain roof coverings in frosty weather. Also, clear gutters and rainwater pipes regularly, particularly if your building is surrounded by trees or perched on by pigeons. Parapet and valley gutters need to be cleared of snow to prevent melt-water rising above them and causing damp internally.

Re-point deeply eroded mortar joints in walls. Whilst cement is fine for modern buildings, it is important to use a lime:sand mix (preferably without cement) for most buildings pre-dating about 1900. Localised re-pointing is generally all that is required. Daub, lime mortar or oakum (ships caulking) are useful for closing gaps that may develop around the edges of panel infillings in timber-framed buildings. Where rain penetrates an exposed south- or west-facing wall, limewash, lime render and slate or tile hanging are traditional solutions although these cannot be employed without changing the external appearance of the wall. Sometimes installation of a ventilated dry lining system internally is appropriate. The use of colourless water-repellent treatments or plastic-based paints on old masonry is strongly inadvisable.

Controlling Below Ground Damp

The best solution to rising damp may well be to take measures that help your building breathe. Replacing hard cement render or pointing using a more suitable lime-based mortar often improves a damp wall and enables rising damp to dry out. Conversely, the application of waterproof renders and bituminous coatings tends to create or exacerbate damp problems. Where a floor has a modern damp-proof membrane (horizontal barrier or DPM) that is displacing moisture to the bottoms of walls, it may be sensible to replace this completely with a breathable construction or to at least provide a breathing zone for evaporation around the perimeter of the room. When underfloor heating is being installed, there are many situations where DPM-insertion can be avoided by employing materials such as lime concrete and expanded clay insulation.

Reducing or removing the source of moisture may also help alleviate rising damp. French drains can be an effective and relatively inexpensive answer but it is preferable not to site them directly against walls and rodding points must be provided. Otherwise, blockages can effectively convert them into a sump and increase dampness. Consider also the structural and archaeological implications.

Although retrofit DPCs can sometimes be appropriate, with an old building always consider first whether rising damp is actually too minor to matter and, if it is significant, whether more sympathetic ways exist of dealing with it. Where any timber is at risk of decay, for example, you might be able to simply isolate it. Similarly, the eradication of any contributing moisture from other sources such as rainsplash off closely abutting patios could obviate the need for more extensive remedial treatment.

Damp can be particularly troublesome in cellars but increased ventilation (including opening up redundant flues), re-pointing and lowering the water table locally can be effective. Failing this, it may be worth considering a dry lining system. Tanking (applying waterproof linings to walls and floors) is not recommended in old buildings.

Internal Finishes

To minimise the risk of future problems, lime plaster should usually be used for any replastering rather than the anti-sulphate or renovating plasters favoured by many treatment companies. Decoration with paints such as limewash and soft distemper, where possible, will maximise breathability. Old items of joinery removed during work should be carefully repaired and reinstated, not automatically replaced.

19 Comments

This is an excellent article! Thanks I found it very helpful. Can you please give more clarity on this statement: "Tanking (applying waterproof linings to walls and floors) is not recommended in old buildings." Can you please explain the reasons to this? Our neighbour has tanked his side of the party wall and is now putting pressure on us doing the same as we have rising damp. We live in a Victorian property built in early 1900. Many thanks!
Thanks for this article,great info..We live in an detached farmhouse c1850.thIck stone walls and never had a problem with damp. When we bought it 11years ago we installed an electro osmosis dpc and all was fine. Until 2 years ago we installed 2 wood burning stoves replacing the previously existing open fireplaces. Now we have a problem with damp rising up the wall . We installed vents recently (2months ago) but we are not sure if the dpc may have been broken during the building work, do you know is there a way to find out if the current has been broken? Or is it a case of closing up 2 open fireplaces and not enough ventilation?...WE also hang washing on a clothes rack above the aga..stopped that now, got a tumble drier today for the shed. Would appreciate any knowledge you could share. Many thanks Siobhan
Moisture is not good for any part of the house that is made of wood. Thanks for your informative discussion about damp.
I imagine the main reason for this would be that a modern tanking treatment might be at odds with the requirement for the walls of the older house to breathe. If any readers have any other suggestions, feel free to post them.
It is unlikely that your damp problem has been caused by a damaged dpc. It is most likely not rising damp but condensation. The reduction in ventilation from closing up two open flues and replacing them with narrower flues to wood-burning stoves is very considerable. Although you have added vents in the rooms for combustion air, this may not be adequate to compensate for the reduction in uncontrolled ventilation up the flues, resulting in greater levels of moisture laden air remaining in the house, and corresponding condensation forming in the coldest spots. This damp is most likely to show on any single glazed windows, the base of walls and the most exposed external walls - which can be mistaken for rising damp. This can usually be resolved by adding more ventilation - unfortunately with a corresponding loss of energy. You can add controlled ventilation (with heat recovery) into an old building, but it not easy or cheap! Michael Holmes is the Editor-in-Chief of Homebuilding & Renovating, Real Homes and Period Living magazines, and presenter of several property TV shows. He has self-built three times and renovated over 25 houses, and is the author of Renovating for Profit.

I have just had cavity wall insulation removed due to damp. It's dryng out from the gable wall had some damp specialists out conflicting information one said you dont need to do anything the plaster will dry out and seems dry meter reading low. One other said meter readings high in some areas and all the plaster would need to be removed damp membraine grometed onto the wall and re- plasterd. One other one said plasters ok but you could be getting driving rain and to get the outside gable wall treated with a silicon waterprofer that also allows the wall to breath. While this seems plausable and would not hard wht i want to know if there is no rising damp which i know is my case do i need to get the plaster removed and re plasterd. The cavity wall has been drying for 7 months my dehimidifier has not shown a reduction in the bucket being filling up. How long should it take to dry out. I have been told atleast 9 months.

Hi Kal,

I've reposted your question in the Q&A area as I believe you are more likely to receive a response to it there. The link is http://www.homebuilding.co.uk/community/qa/problems-with-plaster-and-damp

Kind regards, Sam (Online Editor)

hi
I believe i have air moisture problem ,i have recently moved into a 1920 solid wall property.
The worst rooms are down stairs and the room directly above it ,and smaller issues in neighboring rooms.
The whole house has wood chip wall paper with several layers of paint on .i have removed the wallpaper from two rooms so far ,these rooms had small areas of mold on and since the removal of wallpaper seem better ,so far ,i used a breathable paint ,matt.
The worst room i am in the process of doing ,as i am removing the wall paper the skim of plaster is coming off too! . Will this need a special kind of plaster ,when i repair it ?.I was going to put in an air vent ,but am unsure where to position this high or low ?
Should i have the two walls in question built out if so what with ? (the rooms are big enough to have this done)
As this is going to be my sons room i need to make sure when his furniture goes in ,no mold is going to grow behind it (this is whats also happening )
It has had double glazing fitted and i will be replacing this but unfortunately this will not be in the immediate future.
Would be grateful for any advice ,thank you ....great page glad i found it !!

This makes interesting reading, I have a victorian terraced property and we have tried to apply a waterproof paint to the walls to deal twith the damp, the dryer side and internal walls are fine but the the paint has begun to flake off at the front external wall and we have algal growth on the bricks. I am reluctant to install an invasive tanking system and have been considering puting in an extractor fan, or some sort of heat exchanger.

I am surprised you haven't mentioned positive input ventilation, I thought I had a damp problem but it turned out to be condensation, it's apparently very common in bungalows. I got an envirovent installed and it totally cured the problem, they

Sorry, pressed the wrong button,,,,

Also installed a humidity tracking filter less fan in the bathroom as the old one was a noisy old thing that hadn't been changed in 15ish years. What a difference these systems have made. Before it looked like it had been raining indoors there was so much condensation, now it's only on the coldest days that I get a small amount that disappears very quickly! Im no longer throwing out clothes that have gone mouldy in the cupboard and mould no longer grows behind furniture. The only downside to it is my hall feels a bit cooler where the fresh air comes into the house, but overall the difference to our house has been massive!

I had a number of problems with damp in my cottage and i could not work out what was causing it, i had a few companies in to diagnose the problem and they all insisted is was rising damp. Well as some of the issue was mould upstairs i knew this could not be right. I had a dutch firm call Holland Damp Proofing and they carried out a survey that pointed out numerous problems, some they could deal with and other of which they advised me what to do. It took about 18 months to resolve with their product and advice but it has finally gone. It was nice to have a company around the actually seemed to care and not just take my money.

Hi, Thanks for your information, I have found it really useful. We live in a brick built prpoerty circa 1850 and have inherited a mish-mash if issues due to the "up-grading" over the years. It has been rendered, centrally heated, cement floored at rear and double glazing. It aslo had a chemcal damp proof course. We do have damp problems dotted around the house, though I am now less concerned with them having read your article. However one concern remains, we have a large room which at the rear has a sloping roof which has been built with no insulation gap. Do we drop the ceiling and provide that layer or do we leave that as "natural" ventilation and put in a multi-fuel stove to improve our heating issues? I am now very conscious of the inadvertent damage which has been done by eleminating necessary & health air flow.

Great article, also read the condensation one which was helpful too. Have a 1930 semi and having problems with ground floor lounge. Two external walls, one north facing. When we moved in a few years ago we ere aware of slight damp issue but the previous owner had some dpc work done a few years prior and we decided to look at what the cause may be. There was a ramp breaching the dpc on the north facing wall and we removed this. Since then we've also cleared cavity, repointed & added airbricks. The property already has cavity wall insulation. problem continues and we get salts forming on the north facing wall and it often feels damp to the touch. We have no tide mark but sometimes get small amount black mould, only in corner. I ensure room ventilated and always keep door open. Heating has also been upgraded. Problem only occurs when weather gets colder and so I feel its condensation? Perhaps as this is a cold room? We did get a "specialist" some time ago who recommended new dpc but we're sceptical, especially after reading this and other articles. Our gut instinct is still condensation and that perhaps we need to insulate some more, we have sanded floor boards so are probably losing heat there, however we'd appreciate any advice or information we may have missed.

Sorry, thought I should add we're considering whether the plaster may be too soft on the north facing wall and/or installing some underfloor insulation. The underfloor area has been checked and cleared (12 months ago) as there was previously a damp earth smell whichhas now gone. Thanks for any advice.

I have cavity wall insulation that has failed due to incorrect installation (grid format m1.5-2mtr apart as well as penetrating water on inside of middle bedroom PVC window and dampness(either side of this window in main and middle bedroom cupboard and a kitchen that resembles the artic. It's all on the gablend wall.

My issue is, I am in a block of 4 where a full size 3 bedroom houses are built above 2 bedroom maisonette/flats. The water penetrating issue is and has been an ever increasing issue which originates on the property above and has laid empty since 2010. I need to make my property wind and watertight (1970's build) as the render is shot. But the water (tho appears as a trickle in and out of my window damp issue needs urgent attention) Isssulation needs extraction/drying and properly redone.

My hurdle is, can this be done rectified without the property above as communication/inclussion of above empty property and owners is stopping me from making my property warmer. Is it more practical to just reinsulate inner walls and forgo cavity or exterior insulation more practical or is there a system where I can issolate my self from above property as far a waterpenetration goes by way of a inner cavity drainage system (crude device I know) I just want my property to be warm and secure from elements.

In 2010 I had a specialist company in to check cavity on gable end via camera and confirmed it has failed with large voids of insulation. But because non communication or consent from property owners above I am held in catch 22 situation (I am a disabled veteran and at present my house is not working for me from cold and dampness point of view) No governmental or Utilities companies will entertain or discuss options as they insist they have to do both my property and the property above. I am at the stage where I would pay to do both but no consent from owners.

I do not know who or what I can do I have no experience in this field except wishing to get this all done so I can get on with taking care of my condition and my home warm. I am in Central Scotland.

Excellent article! - I own a c.1900's Victorian terrace that suffered damp badly on the external walls (solid) - so bad infact that the plaster completely deteriorated and mould grew everywhere. Turns out the biggest cause was hanging clothes out to dry, having the central heating on permanently and never ventilating the rooms. We also had a rainwater pipe that was not connected to the roof gutter correctly and so water was running down the face of the wall causing most of the damp.

Thanks for your comment Martin.

We've actually just published another article looking at the various causes of damp in more detail. It sounds like it may be a bit late for you as your issue, but it certainly explains the key cause well (i.e. damp is just moisture that can't escape).

http://www.homebuilding.co.uk/advice/existing-homes/renovating/causes-of...

There is the link just incase you wanted further reading on damp!

Regards,
Lindsey

Excellent articles, many thanks.
My challenge is penetrating damp. We have a SW facing wall, the outer surface of which consists of rectangular knapped flint with lime mortar, built around 1820. During last winter's persistent driving rain we had considerable damp on the inner surfaces, mostly on the first floor where the outer wall is exposed to the full force of the weather. The gutters and roof are in good condition. The mortar is in fair condition, some minor gaps, some of it a bit soft.
So, do I a) repoint with new lime mortar?
b) try one of these breathable waterproofing products?
c) move house?
All comments and advice much appreciated.