Oikodomisis

Rising (Capillary) Damp: How Water from the Foundations Destroys the Render

It's a familiar sight to most owners of older ground floor houses or basements: The paint right above the skirting board starts to swell, the render cracks, a white powder makes its appearance, and finally the material crumbles and falls to the floor.

You scrape the wall, fill it, paint it again, and the next year the problem returns in exactly the same spot. This endless and costly cycle is due to rising (or capillary) damp. Let's see how this phenomenon works and why simple painting will never save you.

What Is Rising Damp? (The "Sponge" Phenomenon)

The building materials of your house (concrete, bricks and traditional render) are porous. They are full of microscopic holes and small tubes (capillaries).

If your house's foundations are in contact with damp soil and proper waterproofing (damp-proof course) was not installed during construction, the wall's bricks start to behave like a sponge you rested in a puddle of water. Groundwater "climbs" upward, defying gravity.

  • Usually, rising damp stops climbing at a height of 1 to 1.5 meters from the floor (that is, where gravity defeats the capillary rise).
Rising damp mechanism - water climbs from foundations

The Real Culprit: Salts (Efflorescence)

Many believe that the water itself is what brings down the render. Wrong. The real destroyer is the salts (nitrates, sulphates, chlorides) that are dissolved in the groundwater.

  1. The water, full of dissolved salts, rises through the brick and reaches the render.
  2. There, the water finds the room's air and evaporates.
  3. The salts, however, cannot evaporate. They stay behind, trapped inside the small pores of the render.
  4. As the water dries, the salts crystallize. During their crystallization, they increase their volume vertically. This swelling creates terrifying pressures inside the render, resulting in it "blowing up" from the inside out. This white powder (like fluff) you see on the wall, is the salt crystals.
Salt crystallisation inside render pores

3 Classic Mistakes That Make the Problem Worse

If you are facing rising damp, avoid these "quick fixes" that actually worsen the situation:

Three mistakes: repainting, waterproof paint, tile cladding

Mistake #1: Simple Filling & Painting

As we explained, the new filler will fill with salts again in a few months. It's money thrown away.

Mistake #2: Plastic/Waterproof Paints

If you paint the lower part of the wall with a heavy elastomeric or 100% waterproof paint to "seal" the moisture, the water will simply look for another way out. Since it cannot evaporate low down, it will "climb" higher on the wall, transferring the problem half a meter upwards.

Mistake #3: Stone or Tile Cladding

Cladding the damp wall with tiles simply hides the problem from your eyes. The moisture will continue to weather the bricks internally and will cause a strong smell of mold.

The Definitive Solution: Dehumidification and Special Renders

To stop rising damp, the intervention must be done on two levels: at the water source and at the wall's surface.

Siloxane chemical injections + renovation render

Step 1: Barrier at the Source (Chemical Injections)

The most modern and reliable method to "cut" the water is chemical injections. Specialized crews open a series of holes low on the wall (in the first row of bricks) and introduce special hydrophobic resins (siloxanes). These resins spread inside the brick's pores and create a horizontal, impenetrable barrier. Water can no longer rise.

Step 2: Dehumidifying Renders (Renovation Renders)

Even if you stop the new water from rising, the old render is already contaminated with salts and must be demolished (torn down) at least half a meter above the visible sign of moisture. The new mortar to be applied is forbidden to be simple cement render or gypsum. A certified Dehumidifying Render (Category R - Renovation plaster) must be used.

The Secret of Dehumidifying Renders: These materials have been manufactured with a macroporous structure. Their pores are huge (like honeycombs). They allow the wall to "breathe" freely, while giving plenty of space to the salts to crystallize inside them, without breaking the surface. Thus, the wall dries naturally and the paint remains intact for decades.

Related Articles

Salt Efflorescence on Plaster: White Stains & Solutions Moisture Behind Plaster: Leaks vs Condensation Dehumidifying Renders (Macroporous): Rising Damp Solution Interior Mould on Plaster: Thermal Bridges & Ventilation

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