---
title: "Moisture movement and condensation effects"
subject: "Building Technology"
chapter: 1
key: "semester_6/building_technology:moisture-condensation"
source: https://www.bctengineer.com/q/semester_6/building_technology/moisture-condensation
site: BCT Engineering
---

# Moisture movement and condensation effects

Building Technology · Chapter 1: Functional Requirements of Buildings

- Asked in: 2067 Asar (old course), 4+2+2 marks
- Video: [Principle of Avoiding Condensation Explainer](https://www.youtube.com/watch?v=k_QXw1BCZ2A) — Marius Radoane
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/building_technology/moisture-condensation
- Whole chapter: https://www.bctengineer.com/md/semester_6/building_technology/chapter-1

## Question

How does moisture movement occur in buildings? Explain the different types of condensations and their effects on building materials.

## Short answer

Moisture moves as liquid (capillary, gravity, wind, hydrostatic) and as vapour (diffusion and air transport). Condensation types are surface, interstitial (concealed), and cavity condensation. Effects include damp patches, mould, timber rot, steel corrosion, frost damage, wet insulation, and damage to finishes.

## Answer

#### Moisture movement
Moisture moves through building materials in two forms.
1. **Liquid movement**: by capillary action (rising damp), by gravity (percolation through roofs and cracks), by wind pressure (driving rain) and by hydrostatic pressure (basements).
2. **Vapour movement**: by diffusion from higher to lower vapour pressure, usually from the warm, humid side to the cold side, and by air movement carrying vapour through leaks.

If vapour meets a surface below the dew point temperature, it condenses to water.

#### Types of condensation
1. **Surface condensation**: Occurs on the visible inner surface of a wall, window or ceiling that is colder than the dew point of the room air. It is common in kitchens and bathrooms in winter.
2. **Interstitial (concealed) condensation**: Vapour passes into the wall or roof and condenses inside the material, where the temperature falls below the dew point. It is unseen and more harmful.
3. **Condensation in cavities** (roof voids, cavity walls).

#### Effects on building materials
- Damp patches and stains, peeling paint and plaster.
- Mould and fungal growth, bad smells and health problems.
- Rotting of timber and corrosion of reinforcement and steel.
- Frost damage and spalling of masonry.
- Wet insulation loses much of its insulating value.
- Dripping water damages furniture and finishes.

#### Control
Ventilate humid rooms; insulate to keep surfaces warm; use a vapour barrier on the warm side; use dehumidifiers and extract fans.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Which is a way liquid moisture moves in a building?
   A. Diffusion from low to high vapour pressure
   B. Condensation inside cavities only
   C. Evaporation from dry walls
   D. Capillary action causing rising damp
   Answer: D. Liquid moves by capillary action, gravity, wind pressure and hydrostatic pressure. (Key point: Liquid movement)

2. In which direction does vapour usually diffuse?
   A. From the cold side to the warm side
   B. From the ground to the roof only
   C. From the warm humid side to the cold side
   D. It does not move through materials
   Answer: C. Vapour moves from higher to lower vapour pressure, usually warm humid to cold. (Key point: Vapour diffusion)

3. Why is interstitial condensation more harmful than surface condensation?
   A. It occurs only on windows
   B. It produces no water
   C. It happens only in kitchens
   D. It occurs unseen inside the material
   Answer: D. It forms inside the wall where temperature falls below dew point, so damage goes unnoticed. (Key point: Interstitial condensation)

4. Where should a vapour barrier be placed to control condensation?
   A. On the cold side
   B. On the warm side
   C. On the outer face of the roof only
   D. Below the foundation
   Answer: B. The vapour barrier on the warm side stops vapour entering the wall. (Key point: Control)

## Related in Building Technology

- [Moisture movement through building components](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Amoisture-movement) — Building Technology, chapter 1, asked 7×
- [Effects of moisture and methods to stop it](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Amoisture-effects-control) — Building Technology, chapter 1, asked 3×
- [Heat, ventilation, light, sound, orientation, moisture](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Abuilding-considerations) — Building Technology, chapter 1

## Related in other subjects

- [Design network for two buildings with fiber](https://www.bctengineer.com/question.md?key=semester_8%2Finternet_and_intranet%3Anetwork-design-guidelines-two-buildings) — Internet and Intranet, chapter 5