Your house should have airtight insulation to prevent heat loss, which also will prevent moisture buildup in the walls. Read here about how to get the vapor barrier completely sealed.
A vapor barrier prevents the warm indoor air to seep into the insulation and create condensation. And it makes the house so isolated that the heat loss is reduced to a minimum. With a completely sealed barrier, you’ll meet the requirements for air tightness and the prevention of condensation.
The barrier is a powerful, durable plastic film, which should be carefully sealed at all joints. It should be used one and only one, barrier, and it should sit on the warm side of insulation. Whether you are building a new house or remodeling it is advisable to place the vapor barrier about 1/3 inside the warm side of the wall, to prevent unexpectedly to create holes in the plastic.
Misplaced vapor barrier can lead to extensive fungal decay and damage to the house’s load-bearing structures, and your insurance company may soon begin to argue when you ask them to cover the damage.
In the “old days”, there were leaky doors and windows that helped to provide a “natural ventilation”. As one began to fill in the insulation and sealed windows and doors, there came requirements that all the light parts of the building should be sealed, because the warm room air would seep into the insulation, in a construction with no vapor barrier.
In winter, the moist indoor air will condense in the insulation when it cools to the dew point. Similar to the dew on the mirror in a cold bathroom when you’ve taken a shower. The result is that moisture accumulates in the wall, and it leads to partially reduced insulation ability, and partly a much higher risk of mold and rot.
Proper placement of barrier.
To keep moist indoor air away from the insulation, put vapor barrier on the inside where the insulation is warm – it will get colder and colder, the farther out in the wall you get.
The vapor barrier should be airtight
Equally problematic as, or perhaps worse than, a misplaced vapor barrier is a leaky barrier. The moisture can quickly get through the plastic sheeting through bad joints and holes. A leak at about 1 mm, and 1 meter long, can easily generate 1 liter of water a day inside the wall.
Therefore, it is important that all joints are 100 percent clogged.