The building envelope comprises the following physical structures - foundation, roof, doors and windows. These physical structures protect the building from external physical dangers, but they are permeable enough to transfer water vapor from the air into the insides of the building or vice versa. Water vapor can harm the insulation of the building, apart from causing physical damage to the walls, windows, and other physical structures. Thus arises the need of a vapor barrier. A vapor barrier refers to any physical structure made up of plastic or metal foil, which prevents the diffusion of moisture from external surroundings to the internals of a building. it can be used to protect the floor, ceilings and walls of a building from moisture intrusion. It is sometimes referred as a vapor retarder, as it retards or stops the flow of moisture into the interiors. But these barriers are often excluded from the construction plan or are just installed incorrectly. Purpose of a Vapor Barrier As was mentioned earlier, a vapor barrier helps prevent the diffusion of moisture and water into the interiors of a building. The moisture can cause lots of harm, as it can affect the insulation system of the building. Due to changes in the atmospheric pressure, especially in the winter season, water vapor travels from the interiors of a building to the outside surface, which is comparatively much cooler. As the temperature of the vapor reaches dew point, the water vapor condenses. Condensation harms all the building envelope components, no matter what they are made of. It causes the wooden structures to rot and also rusts the steel. Installing a vapor barrier prevents this condensation problem and thus also prevents the insulation system from damage. Note that water vapor can permeate the building from anywhere, be it the basement (through the soil), walls (surrounding air), or even the roof. Installing One

1. Aluminum foil
2. Polythene plastic sheet
3. Glass sheets
4. Metal sheets
5. Vapor retarder paints
6. Paper backed aluminum
7. Asphalt-coated kraft paper
8. Extruded polystyrene
9. Metalized film

Vapor barriers are preferred by home owners who want to prevent their house structures from water damage. They are literally a requirement in chilly weather conditions, and should be installed on the insides of the building envelope. It should preferably be installed on the warmer side of the insulation. This is because, water vapor condensation (which is to be prevented) occurs when the water vapor moves from the warm side of the wall to the cold side. Barriers should preferably be installed on the inside of the rooms, and installing them on both the sides of the insulation should be avoided, as it taps harmful amounts of moisture. The installation is very easy and can be done by anyone.

• Step 1: The surface on which the barrier is to be installed should be dry and free from any kinds of debris. Fill all the gaps, like on the window sides or on the doors with caulk. This would make the surface consistent and plain.
• Step 2: Choose a proper material for the vapor barrier. The most commonly used material for vapor bags is polythene. They are available in different sizes (from 8 feet to 20 feet) and also thickness. Just make sure that the material you choose is of good quality. Now spread this material on the surface neatly. When installing it on the wall, make sure that the material is enough and some part of it falls below the base level. This will prevent the water from seeping in again into the walls.
• Step 3: To fix the barrier, you just need to nail or staple it. Nailing or stapling the barrier is just enough to keep it fixed.
• Step 4: Seal the creases using a waterproof tape. There are specially designed tapes available for this very purpose.
• Step 5: To prevent damage to the newly installed barrier, apply finish to the wall within 1 month. You are done.

Hope the article will help you install a vapor barrier. This will help you prevent the corrosion of the walls due to moisture, and safeguard the walls and other house structures for a long time.