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Moisture movement through building components

Short answer

Moisture moves by capillary rise, gravity, wind pressure, hydrostatic pressure, vapour diffusion, condensation, and hygroscopic absorption. To stop it, provide DPC at plinth, surface waterproofing, cavity walls, integral waterproofing, tanking, site drainage, roof treatment, ventilation with vapour barriers, and use dense materials.

Explain how moisture moves through building components (types and forces of moisture movement) and the remedial measures to stop moisture entering a building.

Moisture movement is the passage of water, as liquid or vapour, through the pores, cracks and joints of building materials and components. It causes dampness, which weakens materials, harms health and spoils finishes.

Types and forces of movement

TypeForce that drives itExample
Capillary riseSurface tension in fine poresWater rising from the ground in walls
Gravity / percolationWeight of waterRain leaking through a roof or cracks
Wind-driven (pressure)Wind pressure on rainRain pushed through wall joints
Hydrostatic pressureWater headWater table pressing on a basement
Vapour diffusionDifference in vapour pressureHumid kitchen vapour moving into a cold wall
CondensationVapour cooling below dew pointWater on cold surfaces
Absorption by hygroscopic materialAffinity of the material for waterWood and bricks taking moisture from the air

Capillary rise is greater in fine-pored materials such as brick, and it is large in clay and silt soils.

Remedial measures

  1. Damp proof course (DPC) at plinth level (25 to 40 mm cement concrete 1:2:4 with a waterproofing compound), and DPC at parapets and sills.
  2. Surface treatment: waterproof cement plaster, water-repellent paint, pointing of joints.
  3. Cavity walls to break the path of moisture.
  4. Integral waterproofing: add compounds (Pudlo, Dr. Fixit) to concrete and mortar.
  5. Membranes / tanking: bitumen, polythene sheet or bituminous felt on basements.
  6. Good site drainage: slope the ground away, provide an apron around the building and subsoil drains.
  7. Roof treatment: slopes, flashings, brick-bat coba, proper gutters and downpipes.
  8. Ventilation and vapour barrier: prevent condensation by ventilation and by vapour barriers on the warm side.
  9. Good materials: dense, well-burnt bricks and non-absorbent stones.

✓ Answer checked by Claude Opus 5.5 (high effort)

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