Movement and Control of Moisture

Movement of moisture

Almost all construction materials used in the building absorb water to some extent. There are definite phenomena for the absorption and movement of this water in the building components. Followings are different forces governing the movement of moisture through the building components.

  1. Capillary action
  2. Wind loads (momentum of water particles)
  3. Surface tension of the building component.
  4. Gravity forces
  5. Air pressure
  6. Diffusion (occurs due to difference in vapor pressure)

Moisture control

 

  • Ground water control

 

The ground water and rain water percolating in to the ground causes great problem to the buildings. This is very sensitive work and due to the attention has to be paid to control the entry of moisture. Besides, condensation is also quite frequent in ground floor and basements.

 

Followings are the methods of moisture control in the substructure of the building.

 

  1. Damp proofing
  2. Water proofing
  3. Subsurface drainage

 

Either of the methods is used under particular ground condition.

Under damp proofing, it is meant by the application of simple damp proofing paints or membrane to control the capillary infiltration. Under water proofing, it is meant by deliberate application of impervious layer under all adverse weather condition. Subsurface drainage is the method of diverting underground water away from the foundation and basement. Depending upon the condition of the ground, the perforated underground drain may be directed towards the low land or a drywell of adequate capacity. Subsurface drainage can also be directed to sump pump and water collected may be pumped out. After this, there must be proper water proofing treatment in the foundation and walls of the basement.


  • Rain control

 

Rain is the most important factor to control in order to construct a durable building. Rain may enter in the building in many ways. Rain is more dependent on the climate and varies from place to place. It is difficult to forecast the intensity of rain in time and place. A general idea could be drawn from the amount of annual rainfall. Besides, the wind has substantial effect on the intensity, strength and direction of rain.

 

Rain protection into and through building surfaces is governed by capillary action, momentum, surface tension, gravity forces and air pressure. Capillary forces draw water into rain water into pores and tiny cracks, while the remaining forces direct rain water into larger openings.

 

Followings are the means of rain penetration control in practice:

 

  1. Capillary breaks
  2. Obstruction of horizontal openings
  3. Drip edge or kerfs on horizontal openings
  4. Use of flashings
  5. Creation of pressure equalization

  • Vapor control

Water vapor moves in two ways, by vapor diffusion and by air transport. The mechanism differs for both the cases. If may happen that the means of effective control of the vapor diffusion may not be effective for air transport.

 

Vapor diffusion is the movement of moisture in the vapor state through a material as a result of vapor pressure difference (concentration gradient) or a temperature difference (thermal gradient). It is not the movement of moisture as a result of air movement. Vapor diffusion moves moisture form an area of higher vapor pressure to an area of lower vapor pressure as well as form the warm side of the building component to the cold side. Therefore, the moisture will migrate by diffusion from where there is more to where there is less. The movement of the moisture from warm side to the cold side of the building component is called ‘thermally driven diffusion’. The moisture condenses on cold surfaces acting as dehumidifiers pulling more moisture towards them.

 

The air transport is the process of movement of moisture present in the air from the area of higher air pressure to a area of lower air pressure.