Building Science Moisture and its movement

 

Moisture and its movement through building component

1. Moisture or damp is water particles that appear on the surface of the materials.

2. If we talk this in the building, it is one of the most important elements affecting the living condition inside and deteriorating the strength and durability of the building components .

3. Damp appears in any surface of the building ; on the ceiling, on the wall, on the floor and on the other surfaces inside or outside.

Sources of moisture

1. Rain water

2. Ground water

3. Water from condensation

Phenomenon of condensation

                The air that exists everywhere in the atmosphere is a mixture of dry gasses and water vapor Therefore, moisture is present normally in the atmosphere in the form of water vapor. This water vapor exists in the form of finely divided particles of superheated steam at the given temperature (dry-bulb temperature). The amount of water vapor that a given quantity of air can hold increases with the temperature. If the air temperature is increased, it would take more water particles and if the air temperature is lowered the water particles diffuse each other and at a definite temperature, the air can no longer hold molecules in vapor state.

        When air at any particular temperature contains as much as water vapor as it can hold, the air is said to be saturated and the condition of air is said to be moisture saturation state. The particular temperature at which the air is at the state of moisture saturation is known as its dew point. If the temperature of air is increased from dew point, the excess vapor can no longer be held by the air will be deposited on the surface as condensation.

Building Science Moisture and Vapor management

Surface condensation and interstitial condensation

        When the temperature of any surface within a building is at temperature below the dew point of adjacent air, some of the water particles in the air will condense on that surface, this is called surface condensation. Depending on the nature of the surface , the moisture may either be absorbed by the material (perhaps remaining unnoticeable) or it may appear as water on the surface.

Surface condensation will not occur if,

1. The temperature of the surface is kept above the dew point of the adjacent air by adequate heating or by sufficient insulation behind the surface.

2. The humidity of the air is limited so that its dew point is below the temperature of the surface. With the occasional or intermittent surface condensation, an absorbent surface is advantageous, as it can retain a limit quality of moisture until condensations change and re-evaporation can take place.

        Condensation in buildings is not necessary confined to exposed surfaces, but may, under certain conditions occur within a material or on a surface within the thickness of a wall, roof or floor construction. This is called interstitial condensation.

Vapor diffusion

        The flow of water vapor through a porous building material or composite slab is analogous to the flow of heat through the structure. Convection current transfers heat and moisture at the fluid-solid boundary. Conduction heat transfer is similar to vapor diffusion through a porous material and its resistance to moisture flow varies with density as does thermal resistance but in the opposite sense.

The sources of water vapor in an occupied buildings are as follows:

• People (transpiration releasing 0.7kg per 24 h).

• Cooking

• Washing, bathing, drying clothes

• Humidifiers and open water surfaces

• Animals (domestic animals exhale than people produce)

• Combustion of paraffin

 The effects of the moisture

1. Efflorescence (disintegration of bricks, stones, tiles etc. to power like material)

2. Softening and crumbling of plaster.

3. Bleaching and flanking of paints with formation of patches.

4. Warping, buckling and rotting of timber.

5. Corrosion of metals (particularly ferrous metals)

6. Damage of electrical fittings

7. Growth of fungus and termites

8. Unhygienic condition to occupant in the building

9.  Damaging sound and thermal insulation.

Movement of Moisture

        Almost all construction materials used in the buildings absorb water to some extend. There are definite phenomena for the absorption and movement of this water in the building components. Following 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

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

        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 subsurface water condition.

       Subsurface drainage is the method of diverting underground drain may be directed towards the low land or a drywell of adequate capacity. Subsurface drainage is also be directed to a sump pump and the 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 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 the rain in time and place. A general idea could be drawn from the amount of annual rainfall. Besides, the wind substantial effect on the intensity, strength and the direction of rain. Rain penetration into and through building surfaces is governed by capillary action, momentum, surface tension, gravity force and air pressure. Capillary forces draw rain water into the pores and tiny cracks, while the remaining forces direct rain water into larger openings.

Following 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

         In general, moisture vapor moves from warm side to cold side of the building components. As the temperature depends on the climate there must be clear idea of climate. Water vapor moves in two ways; by vapor diffusion and by air transport. The mechanism differs for both the case. It 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 from an area of higher vapor pressure to an area of lower vapor pressure, as well as from 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 the area of lower air pressure.

Vapor diffusion barrier

        There are the materials that control the entry of the water vapor into the building components by the mechanism of vapor diffusion. The vapor diffusion barrier may be required to control the diffusion entry of water vapor into the building components from the interior, from the exterior or from both the interior and exterior of the building. Vapor diffusion barriers are permeable to water vapor. These material allow water vapor to pass through them. Materials which are generally classed as permeable to water vapor and act as vapor diffusion barrier are: unpainted gypsum board and plaster, fiberglass insulation, cellulose insulation, dimensional lumber and board lumber, unpainted stucco, some latex-based paints, masonry, bricks, light weight asphalt-impregnated building papers, asphalt-impregnated fiber board sheathings, house wraps etc.

        Some materials are semi-permeable to water vapors. Following materials fall under this category; plywood, expanded polystyrene (EPS), extruded polystyrene (XPS), fiber based insocyanurate, heavy asphalt impregnated building papers, most latex based paint etc. depending on the design, specific condition of the building component and climate. All of these materials may not be considered to act as vapor diffusion barrier. Most often, these materials are treated as vapor permeable. There are many materials which are generally considered as impermeable to water vapor.

        Followings are some of these materials, rubber membranes, polyethylene film, glass, aluminum foils, oil paints, bitumen impregnated Kraft paper, almost all types of wall coverings and their adhesives, foil faced insulating non-insulating materials etc.

Precaution in use of vapor barriers

        The main purpose of the installation of the vapor barrier is to establish overall resistance and make the surface able to keep the dew point gradient below the thermally produced temperatures. The designer must be very serious on the proper installation of the vapor barriers.  Followings are some of the point that have to be considered while performing installation of the vapor barriers.

1. Use the materials with low thermal resistance but a high vapor resistance, such as aluminum foil, plastic sheets, roofing felt, gloss paint etc.

2. Providing vapor barriers on or near warm sides

3. Providing vapor barriers after the insulation

4. Vapor release on cooler sides

5. No barriers on both sides.