Where Should A Vapor Barrier Be Installed?

Vapor barriers are installed on the inside of a frame wall between studs and sections of drywall.  The vapor barrier will be covering insulation placed between the studs.  Most vapor barriers are constructed with polyethylene plastic sheeting.  The vapor barrier is intended to keep water vapor from the interior of the home from moving through the drywall to the warm side of the insulation.  If the insulation gets wet, it will lose its thermal resistance.  It may also lead to mold.  Both are not good!

While polyethylene plastic sheeting is one of the most popular materials used for vapor barriers, there are other vapor barrier materials available including sheet metal, glass, asphalt coated paper, plywood, brick and bitumen coated kraft paper.  Climate will determine which is best in a particular application but polyethylene plastic sheeting is a good solution in many cases.  
Whatever kind of vapor barrier is used, it is advisable to place it on the  warmer side of a wall (towards the interior of the structure).  In addition, it is very important to seal the vapor barrier as any leakage will allow moisture to form and cause damage behind the drywall.   Be sure to tape all seams with tuck tape…it’s a bit more expensive but gets the job done properly and will last.  
In addition to walls of a home, there are other places where vapor barriers can be installed.  Examples would include attics, crawlspaces and wine cellars.  In the case of new construction, you will want to discuss placing a vapor barrier under the foundation with your contractor.  
One last point of clarification!  Many people are confused by the difference between the terms “vapor barrier” and “moisture barrier.”  A moisture barrier (like Tyvek) is installed on the exterior of a frame wall, on top of the sheathing and under exterior brick or siding.   Moisture barriers prevent water from entering the wall cavity.   A vapor barrier is installed on the inside of a frame wall between the studs and the drywall.  As mentioned above, vapor barriers prevent water vapor from the inside of the home moving thru drywall and condensing on the warm side of the insulation between studs.  
Good luck with your project!
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