What are damp problems in walls?

Damp problems in walls are wet patches, salt efflorescence, flaking paint, mould or a musty smell caused by more moisture entering a masonry wall than can dry out. The cause varies greatly per situation. If you find the source first, you avoid spending money on the wrong solution.

The main causes

• Rising damp: ground moisture rises by capillary action through the foundation into the masonry, especially if a damp-proof course such as a trasraam is missing or damaged. Hygroscopic salts in the wall then also draw moisture from the air.
• Penetrating damp: rainwater gets through the outer wall, for example through poor pointing, cracks or a solid wall without a cavity. In a cavity wall, mortar bridges or dirty wall ties can carry water to the inner leaf.
• Condensation: warm, humid indoor air condenses on cold spots such as thermal bridges, corners and walls behind cupboards.
• Leaks: broken gutters, downpipes, waste pipes or water pipes.
• Construction moisture: moisture from new masonry, concrete or plaster that has not yet dried out.
• Moisture from the crawl space: a wet, poorly ventilated crawl space releases moisture into the floor and walls.

In practice: recognising the cause

• Does the wet patch appear after rain, on the weather side of the house? Think of penetrating damp or a leak.
• A tide mark with white salt crusts at the bottom of the wall that stays all year round? That points to rising damp.
• Mould in corners and behind furniture, especially in winter? That is usually condensation, so a ventilation or insulation problem.
• On masonry, a moisture meter only gives an indication: salts raise the reading. So compare readings at different heights and locations.

Common mistakes

• Sealing a wall with waterproof paint or an impermeable plaster layer, so the moisture can no longer get out.
• Applying a water-repellent treatment to a facade while the pointing is still poor.
• Insulating on the inside without first solving the source of the moisture.

Always start with the cheap checks: gutters, downpipes, pointing and ventilation. If in doubt, have an independent damp survey carried out before you choose a drastic treatment.

Related terms

• Construction moisture
• Condensation
• Thermal bridge
• Trasraam
• Crawl space
• Moisture meter
• Mortar bridge

Want to learn more about construction terms? Visit our knowledge base at fredsdiyplans.com.