Understanding damp and condensation in older houses

Why older Edinburgh and Lothians homes get damp

Walk through any Victorian tenement in Marchmont or a stone-built farmhouse outside Dalkeith and you will find walls that have stood for well over a century. That is a testament to the materials used – sandstone, lime mortar and slate – but it also means these buildings breathe in a way modern ones do not. Older properties were designed to let moisture move through the fabric and evaporate away. Seal them up with cement render, plastic paint or modern insulation and you trap that moisture inside, where it eventually shows up as staining, a musty smell or peeling plaster.

Edinburgh's east coast climate adds to the challenge. Driving rain from the south-west soaks exposed elevations, haar keeps external surfaces wet for long spells, and many tenements have weathered chimney stacks and gutters that quietly drip down through the wall for years before anyone notices. Understanding which of these is actually happening in your house is the single most important step you can take. Get the diagnosis wrong and you can spend thousands on a treatment that never had a chance of working.

Condensation: the most common cause by far

If you have black mould around window reveals, in the corners of bedrooms, behind wardrobes or along the top of external walls, condensation is almost certainly the culprit. It happens when warm, moist air inside the house meets a cold surface. Cooking, showering, drying washing indoors and even breathing all add water vapour to the air, and older houses often have cold bridging at wall junctions and single-glazed sash windows that act like a magnet for it.

The classic mistake is to assume black mould means rising damp. It rarely does. Look for the clues:

• Mould appears in cold spots, corners and on north-facing or sheltered walls.
• Windows and glazing stream with water on cold mornings.
• The problem worsens in winter, when windows are kept shut.
• There is no tidemark or crumbling skirting at floor level.

The fix is rarely a chemical injection. It is more about managing moisture at source: extractor fans in kitchens and bathrooms that actually vent outside, trickle vents or controlled window opening, not blocking chimneys that were designed to ventilate, and avoiding drying washing on radiators where you can.

Rising damp: real, but far less common than claimed

Genuine rising damp does exist, particularly where the original slate damp-proof course has failed or been bridged by a raised patio, path or concrete slab. The telltale signs are a horizontal band of damage up to about a metre above floor level, salting and crumbling plaster, and rotting skirting boards. It almost always starts at the base of the wall and moves upward.

What muddies the water is that many properties are diagnosed with rising damp when the real issue is something else entirely. Before accepting that diagnosis, it is worth checking whether:

• External ground levels have been built up above the original damp course.
• Render or cement pointing has bridged the damp-proof course.
• A leaking downpipe or blocked rhone is soaking the base of the wall.
• Internal floors have been covered with impermeable materials that trap moisture.

Where rising damp is confirmed, the correct answer is usually to reinstate or install a proper damp-proof course and, crucially, to replaster in a breathable, salt-resistant render. Skipping that last part is why so many injected walls still show problems a few years later.

Penetrating damp, roofs and rainwater goods

Penetrating damp comes in sideways – through the wall, the roof or the chimney. It shows up as damp patches that grow after heavy rain, often on upper floors or on a particular elevation exposed to prevailing weather. On older Edinburgh stock, the usual suspects are:

• Failing roof coverings – slipped or cracked slates, perished flashings around chimneys and dormers, and worn mortar skews at gable ends.
• Blocked or broken rhones and downpipes, which pour water down the face of the wall instead of into the drainage system.
• Cracked or porous harling on rendered properties, letting water drive straight into the stone behind.
• Defective chimney stacks, where open pots, missing cowls and eroded flaunching allow rain to run down inside the flue.

These are often the cheapest problems to fix and the most satisfying, because once the water source is stopped, the wall can dry out and the decoration will usually hold.

Diagnosing before you spend

A proper diagnosis looks at the whole building rather than one patch of wall. Start outside and work systematically: check gutters and downpipes in the rain if you can, look at ground levels around the base of the walls, and examine the roof and chimney from a safe vantage point. Inside, note exactly where the problem appears, how high it reaches, and whether it changes with the weather.

A few shillings spent on a cheap moisture meter and a thermometer can tell you a lot. Cold surface plus warm moist air equals condensation. Damp that tracks rainfall equals penetration. A horizontal band rising from the floor equals rising damp. If two of these overlap – which they often do in older tenements – deal with the water source and ventilation first, then reassess.

Getting the work right

Older buildings need breathable repairs. Cement renders, gypsum plasters and plastic masonry paints trap moisture and push the problem deeper into the stone, which is why so many well-intentioned renovations create new damp issues within a few years. Lime mortars, lime washes and breathable insulation let the fabric dry out naturally, and they are the right choice for anything built before the mid-twentieth century.

Where you are dealing with a tenement, remember that the roof, rhones and chimney stacks are usually common repairs shared with neighbours, so it pays to raise the issue through the factor early rather than patching your own flat and watching the water return. Tackle ventilation and heating together, keep gutters clear twice a year, and treat any damp patch as a symptom rather than a problem in itself. Find the source, fix it properly, and give the building time to dry. Older houses reward that patience generously.