The simplest way of finding out whether you have a problem with damp walls in your house is to take a look at them and see whether you can spot any of the following common symptoms:
- Peeling wallpaper
- Black mould
- White salt stains
- Blistering emulsion and/or internal plasterwork
You might also notice a nasty, damp smell. This could be what makes you notice the problem in the first place.
What’s the problem with damp?
The most obvious issue with damp is how it looks, but always remember that it often appears much worse than it really is. Apart from that, the most important thing to think about is any adjacent timber structures. Floor structures or timber frame inner leaves can quite easily suffer from fungal decay or infestations of wood-boring beetles which need moisture to survive.
It isn’t always as simple as it might seem to diagnose the actual cause of a damp issue in the home. Firstly, you’ll want to consider the extent to which a wall is exposed to the elements. Forceful wind and rain can enter a wall where the mortar joints are eroding, and cracked or loose render, and blown brick or stonework faces can soak up water like a sponge. Also think about how high the surrounding ground levels are, whether you have any leaky gutters, whether there are any splashes hitting the wall from passing traffic, and whether the windowsills are eroding.
A crack will usually develop gradually at junctions such as window frames and wall panels where water can easily creep inside. Climbing plants can make things more complicated by harbouring damp, whilst ivy is notorious for its ability to dig into soft mortar joints.
Cavity walls usually resist damp best due to the air between the two layers, although damp is still able to arise where something has bridged the gap, such as bits of mortar crossing over due to badly carried out construction. A missing cavity tray can make things worse. It’s also not unheard of for cavity wall insulation to become sodden with water, and thus start to work in the opposite way to that which it’s supposed to and begin sucking heat out of the building.
In the home, condensation is a common cause of damp as the water trickles down walls, where it is absorbed and often mistaken for ‘rising damp’. The real issue is the way in which humid air is able to creep into hidden areas of the wall structure, where it condenses and can potentially cause long-term damage.
How to Remedy the Problem
Firstly, you’ll want to sort out any issues with leaky water from guttering of pipework that has broken.
Then check that the windowsills project far enough from the wall, with distinct drip-grooves beneath so that rainwater is thrown clear and doesn’t end up trickling down the wall. Re-cut any eroded grooves, and replace decayed sills.
In places where they have eroded, mortar joints need to be raked out and repointed with the right mortar mix and style for the property. Be careful when repointing, as sometimes the supposed cure can lead to worse problems than the original. Old solid walls which have been repointed in a cement mortar or rendered in cement can suffer from trapped moisture. The moisture stuck in the wall can’t evaporate and so causes more damp. The way to deal with this is to use softer, traditional lime-based mortars or renders which are flexible and breathable. Your house actually needs to breathe to let out moisture, so blocking the walls up with cement is not a good idea.
Some more good ideas
- Avoid common ‘plastic’ paints on solid walled buildings, and opt for traditional limewash or clay paints instead. These let the walls breathe, and have a deep, natural appearance.
- Climbing shrubs may well look lovely, but for the sake of the building it is best that they’re removed altogether.
- Tile-hung walls should be well-maintained as any broken or slipped tiles can allow water to seep through to the wall. Damp can also come in from above, usually because of poor sills or joints. The simplest solution is to keep up with the maintenance, replacing slipped or missing tiles and poor windowsills as when necessary.
- Damp is particularly problematic at lower wall levels, where it can look like ‘rising damp’. However, the issue can often be sorted by lowering ground levels. The damp-proof course (DPC) ought to be at least 150 mm higher than the external ground level – usually two courses of bricks. Anything lower than this and the ground levels should ideally be reduced.
- Protect exterior walls from traffic spray by adding cladding, either with tiles or traditional weatherboarding. You could also use a render finish, making sure to cut off the render at its base just above the DPC level with a projecting lip (bellmouth drip) so that the render is not in contact with the ground and doesn’t bridge the DPC. An absolute last resort is to inject a damp-proof course, which is usually quite expensive, largely ineffective, and sometimes rather destructive.
- To sort out any condensation-related damp, humid air needs to be expelled with extractor fans from the rooms where most of the water vapour is produced. These will usually be bathrooms, cloakrooms, kitchens, and utility rooms. Tumble driers are especially good at producing lots of steam and need to be vented to the outside. Ventilating rooms is also important, by fitting trickle vents to windows and keeping open fireplaces open in old homes.
- Cheaply constructed Victorian homes will sometimes have rear additions built in 115 mm-thick bricks, which are usually rendered. These walls, being thin and cold, tend to be prone to condensation and dampness. It’s usually advisable to dry line these walls with insulated plasterboard to improve their thermal efficiency.
- Be aware that anything which forms a bridge between the two layers of a wall, such as a metal wall tie in a cavity wall, or solid masonry window reveals, can act as a thermal bridge and potentially attract condensation and damp. Better insulation should make indoor surfaces warmer and stop humid air from condensing.
Walls must always be allowed time to dry out once the problem has been eliminated. The drying time is typically one month per inch of wall thickness. Any damp plasterwork contaminated with salts will probably require hacking off before being replaced.