It is important that our homes have a certain level of humidity for our general comfort. However, too much caused by cooking, showering, and clothes drying for example can lead to problems, particularly in cold and poorly ventilated houses.
Damp problems, or more accurately condensation problems, can cause bronchial diseases and asthma, dust mites and mould. However few, if any, are untreatable or unsolvable.
Firstly, to dispel a fable – although rising damp doesn’t half full mythological status since it is possible, it is also very unusual and very unlikely. All modern buildings are required to have a damp-proof course (dpc) to combat rising damp – very few if any Georgian or Victorian buildings had dpcs, and very few Georgian or Victorian buildings have rising damp!
When it does occur, it occurs because of the capillary action of masonry, drawing moisture up like a sponge. In order for it to do this the masonry must be sitting in water, as in a pool of water. If your house is sitting in a pool of water, then you have problems – if it isn’t then you don’t have rising damp.
All other damp problems are condensation problems. Generally speaking they happen when warm, humid air comes into contact with a cool surface. At this moment, the warm air cools and becomes saturated – it can no longer hold onto the moisture within it. This is known as its dewpoint. The water falls out of the air and condenses onto the cool surface – windows most noticeably.
But not only windows – the water will condense onto the coldest surfaces in the building, quite often corners of cooler rooms, behind furniture where there’s no airflow, on unused chimney breasts, on cold bridges linking the internal fabric to the external, and on the inside of solid walls subjected to prevailing winds externally for example.
Interstitial condensation is arguably more serious and very difficult to appreciate since it occurs within the building construction, and can pass unknown for a long time leading to serious structural damage such as dry rot, rusting, or plaster failure.
To an extent this has become a modern problem – we’re more likely to use and heat our homes intermittently these days, and generally do so without open fires. Back in the day, coal fires would burn continuously keeping the building warm and well-ventilated. Cold-bridging would be less of an issue because the building was warmed through and kept warm all the time, and we simply didn’t generate as much steam.
Ventilation is essentially the key to solving most damp problems, and natural cross ventilation is the best way to ensure all areas in our homes remain condensation-free. Before this even, ventilating effectively at the source of the moisture is very important, which is why bathrooms and kitchens should be well-ventilated and the hot and humid air prevented from travelling to cooler parts of the house if at all possible.
This may come at an energy price, as it is difficult to provide the amount of ventilation required to kitchen and bathrooms without mechanical means. However, it is well worth it and there are now extractor fans available with humidity sensors and heat exchangers.
Another method to combat condensation problems is to maintain adequate heating throughout the house – constant low background heating generally costs the same as short bursts of high heating, but will reduce the likelihood of condensation forming, particularly if occasional rooms are kept slightly warm.
Constant heating attempts to eliminate cold surfaces, and another way of doing that is to make sure that your home is well insulated and all windows are double glazed – condensation cannot form on a wall or window if it is warm! Beware though, the moisture will go elsewhere, and solving one problem of damp could create another in another room.
Adding insulation may increase the potential for interstitial condensation, which is why one method of modern construction is to utilise vapour barriers within wall and roof construction, that ensure moisture cannot enter the building fabric.
Another, more natural method, is to employ hygroscopicity and build breathing walls. These allow moisture to pass through them, ensuring that at no point can interstitial condensation occur. What is more, they are able to de-humidify rooms by soaking up moisture and heat, and re-humidify rooms by letting it out again.
Indeed, applying water impermeable materials to older permeable materials, like re-plastering a wall with cement rather than lime, is likely to create more of a damp problem than it solves.
If you have damp problems then rest assured they are all curable, and mostly so using inexpensive methods, sometimes even just a slight change of behaviour!