Vapour Barriers

Vapour barriers are materials which effectively retard or stop the flow of water vapour these are normally produced in sheets or thin layers. An acceptable vapor barrier is one that has a moisture-vapor penance of not more than on perm. A perm is one grain of vapor transmission per square foot, per hour, per inch of mercury vapor pressure difference between the two surfaces. 

Insulation of Walls

Vapour barriers should be installed on the warm side of the insulation. They should be continuous surfaces of asphalt or waxcoated paper, aluminium or other metal foil sheets, or polyethylene film. They can be attached to the insulation as part of the manufactured product or installed separately in or on the warm side of the wall, floor, or ceiling. For greatest effectiveness, they must be continuous and allow 110 openings through which vapor may pass. Asphalt saturated felt paper is not a vapor barrier - it is however, a good moisture barrier and should be used on the outside of a building for that purpose. 

Insulation of Floors

Materials Used as Vapour Barriers 

One effective vapor barrier material is polyethylene film. It is a chemically an inert plastic, unaffected by acids, alkalis, and caustics; it is produced in rolls 3 to 20 ft wide with common thicknesses of 2.3.4 and 6 mil (1 mil= 0.001 in.). This film is useful not only as a vapor barrier for walls, ceilings, and floors, where the thinner grades are most often used, but also as a barrier to prevent the passage of moisture from the earth upward through a concrete slab laid on the ground. 

Insulation of a ceiling

Aluminium foil is used as a vapour barrier in several forms. One is the foil as a single sheet. Another is a thin layer of foil laminated onto a heavy backing of asphalt impregnated haft paper. Still another consists of two layers of foil laminated with asphalt cement.

Kraft paper coated with asphalt or wax also acts as a vapor barrier. sometimes two layers of paper are cemented with a continuous layer of asphalt. Whatever the material used, the application should be continuous-that is the principle. 

Moisture from earth can enter a building through a concrete slab on grade by travelling upward through the concrete slab, when it reaches the warm inner surface it evaporates and becomes water vapor. To prevent this, a moisture barrier should be laid between the earth and the concrete. Polythylene film is an excellent material for this purpose because it is impervious to the action of alkalis, decay, etc. 

Care must be taken in insulating existing buildings where no vapor barrier can be installed with the insulation. Where there is a likelihood of too much moisture vapor being generated in the building, a good vapor barrier can be provided by painting inside surfaces with two coats of lead, oil, rubber emulsion, or aluminium paint 

Insulation of a Cold Storage  

Vapour condensation problem is most critical in a cold storage where the temperature is brought down to 30°C, - 35°C. In a cold storage food stuffs, sea food etc. have to be stored for months together. Hence ideal conditions have to be created. So the building has to be insulated to keep the temperature low. The best way of insulating a building is to insulate it from the outside, completely shroud the building with heat absorbing materials so that no heat is transmitted into the building. 

Insulation of a Door

Take a glass tumbler and pour cold water Into it. Water drops condense on the glass because moisture moves towards the area. Here, glass itself being be vapour barrier, and thus the moisture when barred by this impervious barrier condenses on the glass itself. The same principle is applied in the insulation of buildings where moisture moving towards the cooler areas is slowed down by the walls but not My stopped when not fully insulted So when insulation is pock, moisture passing through the masonry walls collects on be surface of  insulation materials which decays over a period of time  due to constant contact with water. So a vapour barrier system is to  developed with a view to preserve the insulation material for a longer time, 

A vapour barrier is created by various materials, such as : 

1. Aluminium foil 
2. Polyethylene 

There are different two types of cold storages : 

1. large refrigerated ware houses, and 
2. cold storages manned by an individual.