A vapor-barrier (aka vapor-retarder) prevents moisture from passing into the insulation. In northern climates, like Michigan, the heat vapor mostly tries to escape from the warm house to the cold outdoors. A vapor barrier prevents the moisture-laden vapor from coming into contact with the cold. With walls as thin as 3.5 inches, it is critical to prevent the transfer of moisture through the wall system.
The Importance of Vapor Barriers in Certain Climates
As with anything, the greater pressure prevails. Hot air has greater pressure than cold air. So, the hot air seeks to replace the cold air.
Hot air contains moisture. In the winter months, when the hot vapor seeks to escape your house, it takes the moisture with it. It is very important to keep the moisture from escaping. Therefore, vapor barriers are placed toward the inside of the house in cold climates.
If the moisture is allowed into the wall system, it will damage the insulation and wall framing members.
Source: Energy Codes
Information on Vapor Barriers
Vapor Barriers explained at “Do it Yourself”
A simple diagram from the US Department of Energy showing the importance of a vapor barrier, especially in a northern climate like Michigan.
Air barrier vis-a-vis Vapor barrier Are you planning to use polyethlene plastic as a vapor barrier? Read this first! From the Building Science Corporation. This two-page article provides a nice introduction to vapor barriers and addresses the use of polyethylene as a vapor barrier in a northern climate.
Use of Vapor Retarders Technical Bulletin The Insulation Contractors of America Technical Bulletin 6 page document provides more information on vapor barriers and their use in crawl spaces, basements and other areas.
Vapor Barrier From the Building Science Corporation this 6 page document introduces the need for vapor barriers and their proper application according to climates. Page 2 addresses the use of polyethylene as a vapor barrier.
Vapor Barrier Wall Design From the Building Science Corporation This 19 page document provides builders with technical aspects related to wall assemblies and vapor permeability. This examines different regions in the United States and various permeability factors of materials used on exterior walls.