What is vapor barrier speaks to why use a vapor barrier and explain vapor barrier.
How does it affect building structure insulation? Water vapor is a gas and exerts vapour pressure on the surfaces that contain it. The more water vapour an air mass contains, the greater the vapour pressure.
During winter heating conditions, the air inside a building is at a higher temperature and contains more water vapour than outside. This is referred to as relative humidity. Higher temperature air is able to hold more water than cold temperature air therefore the humidity (moisture content) is relative to the temperature of the air. This is why cold outside air may have a fairly high relative humidity but hold very few grains of moisture.
This is especially true in areas of the building with many occupants or where cooking, wet industrial processes, bathing or washing take place. The result is a net difference in vapour pressure acting from inside to outside, causing water vapour to diffuse outward directly through the various layers of the building structure. If the rate of diffusion is high enough and the drop of temperature great enough, water vapour will reach its dew point and condense within the structure.
If moisture accumulates, insulation may lose its effectiveness and materials may be damaged by rusting, mildew, decay, freeze thaw or other destructive processes. If water runs out of the assembly, building finishes and furniture may be damaged. Water vapour can also be carried through an assembly by air leakage through gaps in materials.
During summer cooling conditions, in hot humid weather, the diffusion of water vapour through building structures is reversed. Water vapour is driven from the warm moist outside air toward the cooler drier inside ambient zone. In most of North America the summer differential is not as great as the winter differential. And the cooling season is shorter than the heating season. Where heating seasons predominate, control of water vapour in buildings focus primarily on the flow of vapour from interior to exterior though conditions of reverse inward flow also warrant consideration.