Damp and Condensation Guide.......
...reprinted here are some details from his article by permission of Joe Dix , additional information and some photos by Robin Clark........................prompted by friends living in France , but hopefully useful to everyone...

Damp and condensation can be a problem in all homes , understanding a little more about them will help to deal with the problem .

There are really two main sources of the damp in our homes ...
Steam created from cooking , boiling kettles , baths and showers all create water vapour that will stay in the home unless you take steps to deal with it.......
Damp which can penetrate via the foundations of older homes , or seep in through the walls or roof due to poor construction or maintenance.......

Condensation..........
is more noticeable where there is a colder area in the home.........water vapour will stay in the air unless the temperature of some item ....often a water pipe or window to the outdoors , is much cooler than the rest of the house......the water vapour is cooled where the warm moist air comes into contact with the cold surface , the vapour condenses out as droplets on the cold surface , and can eventually drip down onto the floor or windowsill whatever.......

This may appear to stop when the house warms up during the day , but the water drops will usually just evaporate and become part of the vapour carried by the warm air around the home , and will reappear if the temperature drops . The only way to change this cycle is to open doors and/or windows or if an extractor fan is fitted in the home , to use it...

For this reason , all new homes built today in the U.K. must have an extractor fan installed in bathrooms .
If the problem needs to be resolved quickly , or if it is winter and opening doors is not reasonable , the use of a de-humidifier is recommended . These work on the same principle as a refrigerator , except where the ice box might be , a fan draws air across the cold surface where the condensation occurs . The water then drips into a container which can be emptied down a drain or connected permanently to one .
Most units have a variable control to set the degree of water vapour removal . It is incidentally the same as distilled water , and can be used in some steam irons .

Warm air can hold more water vapour , cool air less , and as the air cools eventually it has to give some up , so there is always a point where the water drops appear as condensation , referred to as the dew point .

Meters are available to enable one to measure the amount of moisture in the air from outlets such as MAPLIN......the indication is as 'relative' humidity in between zero and 100% ........zero is totally dry air and 100% is totally saturated ...but be aware that the indication is only true for one temperature , ie. if the temperature drops , the humidity indication will increase as it is getting closer to the saturated 'dew point' condition .
This doesn't mean that there is suddenly more water vapour , only that the 'relative percentage' is closer to the 100% because the temperature has changed.......
There are graphs available to explain this , useful if you are keeping records.......

The bad thing about condensation is that it can damage the finish on some furniture and fabrics , can make tiled floors feel very cold for example , and of course the dreaded mould can grow .
A variety of moulds and mildew love warm moist places , the limits for most are 70% relative humidity or higher , and 15 degrees C. or higher . So it does not have to be very warm , just moist , and you will find black mould appearing in corners , around window frames , under sinks and basins , anywhere that the condensation collects .
If you manage to keep your home dry , below 50% for example , there will be far less opportunity for mould to establish itself .

There are health implications in living in a mould infested home . Some persons are more sensitive to the spores , which float in the air , those suffering from athsma for example may experience more frequent or more intense attacks . The spores are too tiny to be trapped in normal air filtering equipment , and the best alternative is to use laboratory or medical units .........these will normally include an ultra-violet lamp which destroys any biological airborne items such as spores . This may alleviate the health symptoms , and will stop the mould from spreading ...

Damp.....
So in spite of paying attention to the above listed sources of moisture , are there still places that feel wet , discoloured wallpaper maybe ...???... even curling up and separating from the wall..????......that musty damp smell...?????.....then the source of the moisture may be damp penetrating from outside the home.....?????????..

Obviously the roof is the first suspect , if there are any damaged tiles or slates , then rain can get into the roof space . This may drip down onto the plasterboard of the bedroom ceilings and become absorbed . Plasterboard can hold a great deal of moisture but will get soft as it becomes saturated and usually exhibit mould growth too .......
The water can run sideways from the broken tile/slate and run down or soak into walls , again causing the plaster to become soft and loosen the adhesive used on wallpaper , thus the wallpaper becomes discoloured and starts to curl up away from the wall...

Another source of trouble which can cause the same problems , is guttering........is any of the guttering , or are any of the downpipes damaged or broken and leaking..........????..
This may be apparent just by visual inspection from outside .A useful indicator is a green patch on a wall .Even when it is not raining this tell-tale guide can show where water is splashing against the wall . Bricks are not totally waterproof , in fact they can allow water to soak through over time , some bricks more than others .

All homes built in the last 100 years should have cavity walls , this was an idea which stops damp soaking though from outside , as there is an air gap between the two walls .This does not always work perfectly , especially at places like windows and doors , where the gap has to be bridged , and the builder may have inadvertently left a path for the water to soak through .....

The second big suspect is the dreaded 'rising' damp.........................due to capillary action ,( the natural tension in water which can be seen in a tumbler where the water curls up where it contacts the glass surface at the edge) , water can soak up from the ground outside or under the home.....

There will always be damp in the soil , rainwater can be held there for days or weeks , and if there is not a dampproof course in the brickwork , the damp can creep upwards though the brickwork and plaster inside , up to about 30 inches above ground level . This is the reason that old homes have wooden panelling around the walls up to about three feet above the floor .....

To prevent this modern homes will have a layer of DPC built into the brickwork , this is sometimes visible from outside as a black stripe or layer in one of the mortar joints . Modern versions are plastic sheeting , available in narrrow strips for this purpose , previous products were bitumin based , heavy felt saturated with bitumen , and slate .

Some stone will not allow water to pass easily , and slate is one of them . Very similar to roof slates , the pieces were laid end to end in one of the mortar joints , if costing allowed it a second layer was sometimes added with the gaps staggered to make it totally effective .
All DPC layers have to be six inches or more above the ground level (15cm) to be effective , or splashing can bridge the DPC and wet the wall above it .

For a home with no DPC installed at the time the house was built , there are now chemical products which can be introduced into the wall by drilling holes and allowing the chemical to infuse into the bricks.........Some techniques are available to insert slate or similar strips into a slot cut into the mortar joint but are difficult to use , can affect the stability of the wall and are viewed with mixed feelings by insurance companies ......

In an old house where there was a suspended wooden floor , ie. an air gap under the floor between it and the ground , there should be air bricks in the walls below floor level to allow the moisture to escape .
In cases where the moisture/rainwater seeping into the ground has been trapped here , the floorboards can actually rot away as the damp encourages the growth of moulds , and the presence of insects which attack the wood by living apon it .
This may be more of a problem in internal rooms with no external walls depending on the water table ( the naturally presence of water in the ground due to rivers nearby) , and lack of foresight by the builders......
In cases where a decision has been made to remove the rotten wooden floor and fill the space with concrete , it is important to understand the part played by a waterproof membrane .
Simply filling the space with rubbish/hardcore and covering with concrete may not stop the damp . If there is still moisture in the ground it will soak up through the concrete and make a permanent cold damp room .
Maybe not a common mistake but it has happened............

Early concrete floors should have a layer of shingle,gravel or other small pebbles under the concrete , topped by sand to prevent the wet concrete from migrating into the gravel . This acts as a 'capillary break' to stop the damp creeping up into the new floor . Rising damp can only progress through soil , soft bricks , clay , fine sand etc....the larger spaces around pea shingle or gravel are too large for the moisture to advance ........

Later concrete floors should have a waterproof membrane such as heavy polythene sheet applied under the concrete to block the moisture . This is a technique which can improve even a garden shed , if a concrete base is planned it is a simple matter to add a sheet of polythene under the top two or three inches of concrete to block the damp and make a dryer storage space .

There are testers available to check how moist surfaces are , these are simply resistance meters with two probes and a battery which give an indication of how damp a material is , because the presence of water lowers its electrical resistance ..
Some of these use meters with a green sector and a red sector , some have leds of different colours to indicate resistance .....a normal multimeter can also be used if it has a resistance checking range , the main problem in use is getting a consistent contact with the relevant material . As the probes are usually spikes made of stainless steel , just pushing them into wood or soft plaster to the same depth in various places on the material , can give some relative information . Brick is a bit harder , as the probes will not penetrate without a lot of force , sometimes the mortar in between the bricks is soft enough to allow checking .....

Action
So now you should have enough information to decide where your damp is coming from ..............if it is trapped condensation it is moderately easy to get rid of .........
Fixing the roof or guttering is well documented , and lots of builders provide services in this area..
Rising damp is the hardest to tackle even if there is an obvious source . If any part of the home is below ground level for example or local building work has changed the ground level you have an obvious suspect........trees and bushes touching the wall can also introduce damp...... .........

As mentioned if a DPC is built into the walls , it must be at least six inches above the surface to be effective , this may involve digging the soil away from the walls .
A common fault is to lay paving such as a patio up to the wall , building up the actual ground level and forgetting the six inch rule .....or not being aware of it .
If the house is built into a hillside , or is a basement , then it may not be feasible to simply move the soil away from the wall , there are two ways to tackle this , either dig a trench along the affected wall and apply waterproofing on the outside , then fill the trench in ....or apply waterproofing techniques on the inside of the wall........
There are similarities here , a smooth surface is required in order to apply coats of bitumen or similar high tech modern equivalents and so this may require the rendering of the wall before any other treatment.......
The actual process can depend apon local regulations , and may involve layers of plastic sheeting adhered to the bitumen coating .
There is a product available which looks like a solid sheet of bubble wrap , the dimples create a path for the water to drain down to the bottom of the wall . In extreme cases a perforated pipe can be laid along the bottom of the wall , protected by Geotex fabric to prevent fine soil particles from blocking the pipe...........................the pipe itself forming a drain and routed off to a soakaway ......

When a similar technique is applied inside the building/room , the drain can be led outside or to a deeper hole in a corner, a 'sump' , where a pump takes the water away .
Other Facts
Bricks themselves vary a great deal in quality and moisture resistance . Some are sold as 'facing' bricks meaning they are harder and more resistant to water penetration . These often have a decorative finish on one or two side faces as they are expected to be visible in the finished wall...
.Internal walls were made of softer lower quality bricks for many years , as these were cheaper to obtain than the facing bricks , in modern homes building blocks are now used instead , made from combinations of ash and cement , often containing bubbles of air or nitrogen which improves the insulating qualities and incidentally also makes them lighter in weight ...

Any areas in a building which are expected to be subjected to moisture should have been built with special engineering bricks . These are often a dark purple in colour due to the mixture of clay and additives such as ground up cinders and clinker from industrial waste , or a fairly bright red colour and a very smooth finish , almost the same as red tiles . Again these are more expensive and so use in some homes was limited .

The worst case for damp is a single brick wall (not a double cavity wall) , made from softer bricks not intended as facing bricks , with no DPC . This combination was used in building homes years ago to save cost , and the outside wall was rendered with a cement render possibly painted too , to make it more waterproof . This works ok to stop rain soaking through the walls , but is subject to rising damp.....

If the outer surface is not protected in some way , the water soaking into these soft bricks can actually cause damage to the brick itself if subjected to frost ..........The water freezes and creates tiny cracks in the brick , these increase as more water is trapped until a section of the brick face actually flakes off and falls to the ground , in a process known as 'spalling '........This is often seen in garden walls where no protection was provided , and soft porous bricks used...
This is not only a problem for bricks.......stone houses/walls also suffer from the same problem . The more porous stones such as sandstone and limestone are especially vulnerable . As the natural stone finish is usually the main reason for using stone , painting or rendering is undesirable and so some care is needed in construction to reduce the effects of frost in this way.........

One fact not often understood is the mortar in the joints.............when building with softer stone , the mortar should be a lime mortar which is itself more porous and allows the moisture trapped in the wall to escape more easily.........the worst thing to do with these soft stones walls is to use a cement mortar which is already fairly highly waterproof , coupled with a plasticiser which makes it easier to use , but also even more waterproof , when initially building or later re-pointing during renovation ....................this truly traps water in the wall and forces it to try and escape via the stone surface , leading to the 'spalling ' problem......

The harder stones like granite , flint , slate , and some shales do not suffer from this at all as they are not porous rocks...... .
When renovating old stone properties an examination of the mortar used initially and in subsequent changes , coupled with any evidence of spalling can indicate the best approach to use in subsequent work ....
There are documented cases of whole buildings having to be re-pointed with lime mortar due to the wrong choice made a few years previously...............or the internal or external render being removed and replaced with a lime render to allow the wall the breath better .........
Once water has penetrated any wall , it must be allowed some way to escape even if other measures are taken to stop the ingress of further water ........ie. if you have fixed the leaking guttering , the wall can hold the damp for months afterwards , even when forced drying is used . A good rule of thumb for all forms of stone and brick wall is at least 12 months to dry out for every 12 inches of thickness of the wall ........
Case Histories
Property A
A modern four bedroom split level house in the U.K., 25 years old , with some wood construction .
There always seemed to be a lot of damp in the house , and after few years the wooden shiplap planking on the front of the gables and the side of the dormer windows showed signs of rotting , soft areas and paint flaking .
The owner reported damp in the loft too , with cardboard boxes becoming soft and discoloured , and in mid winter condensation dripping from inside the roof ......
There was no extractor fan fitted anywhere in the house and the general level of moisture in the air , displayed on a meter , was shown to be between 70% and 80% . there was black mould in some corners ........

Adding an extractor fan to the bathroom and another to the kitchen and instructing everyone on their use , and running a de-humidifier on the upstairs landing soon reduced the humidity to 50% ....a more normal figure.....
A check of the three loft doors showed that they were not at all airtight , and all the factors pointed to the warm moist air escaping from the home through the loft doors into the loft space , and subsequently leaving the loft via the gaps in the shiplap planking .
The planking was thus always kept damp and quickly succumbed to the rot .(The eaves ventilation was in-adequate due to the split level nature of the home) .
In winter the inside of the roof being the coldest place which the moist air contacted , allowed condensation to develop apon the inside surface , which would then run down the sloping roof and drip from any convenient nailhead or wrinkle in the roofing felt .

Initially the de-humidifier , only a small domestic model , was removing a litre of water each day from the home . This quickly reduced over six months to about a litre each fortnight .

In other countries such as Canada , where they experience wide extremes of temperature , there are strict rules surrounding the use of wood in construction . Any walls or ceilings to the outside or loft space which are wood frames and plasterboard covered , must have a waterproof membrane installed under the plasterboard , and this has to be sealed at corners and edges , and around power points and any other places where air might escape . This stops any warm moist air from contacting cold wood , which could cause similar rot problems as described above ...............
Property B
A 200 year old cottage of stone with no DPC .......exposed stone outside , rendered inside.......on a south facing slope...
There was general damp throughout the ground floor , in one room there was black mould in one corner , and the tongue and groove ceiling had some signs of white mould or mildew . Under strong light from a torch there was also mildew on the inside of the window and door frames , and on a wooden shelving unit in this same room.........

Quick and fairly easy checks with a spirit level , a long straight piece of wood , and a tape measure established that the north side of the property (the up hill side ) had an outside ground level some half metre/18 inches above the inside floor level .
There was also a row of bushes against the wall here too.......the outside wall surface was visibly tinged with green in this area..........
Initial test with a de-humidifier showed 82% humidity in this worst room , a check for resistance all over the lower walls inside the room revealed that there was damp present for about the first two feet up from the floor .......
The de-humidifier was left running constantly and was removing 2 litres per day for the first week in this same room , this gradually dropped to about one litre per week .
The outside ground level was reduced to 12 inches below the inside floor level , and a gully created to divert rainwater from this area , this involved removing the bushes.

In previous renovations the stone work had been pointed with varying degrees of skill , in some places there were the roots of vegetation growing into the wall.........
The initial construction was thought to be in the manner of a dry stone wall , but with mud added for stability , being some 16 inches wide at the base , tapering slightly to the tops , some three metres in places . The stone itself being a form of slate .....
The colour of this mud was quite red compared to the normal soil colour which was more yellowish . Local custom has it that the mud should be raked out of the joints where visible and the joints cleaned , and then pointed , when renovation work is done . It appeared that a cement mortar had been used previously , in some places many years before , with no evidence of a problem with spalling .

Over the whole wall there were areas where the pointing had been poorly done , or the mud had not been cleaned away before pointing was done , allowing poor adhesion , cracking , water ingress and frost damage ..
The area was cleaned correctly and the joints washed with hand garden spray..............(power washers are to be used with caution in this application..)....
It was obvious that the mud from higher up the wall was very dry and dusty , whereas that nearer to the ground was soft and malleable giving a good indication of which areas may have allowed water to soak into the wall or how high up the wall the rising damp had progressed .

After re-pointing all the suspect areas it became a simple task to keep the humidity below 70% by running the de-humidifier occasionally , the black mould disappeared after washing the area infected and has not returned .