A vapor barrier is an essential part of a building, but it’s often installed incorrectly or omitted entirely. Daily living activities -- washing your dishes and laundry, showering, even breathing -- as well as evaporation of surface water from sources such as humidifiers, fish tanks, toilets and damp basements creates moisture in your home. In winter, this moisture tries to get from the warm interior of your home to the cold exterior in the form of water vapor. Installing a vapor barrier on the (warm) interior wall of your cement brick house will prevent condensation and reduce air leakage through your walls and insulation. If you install the vapor barrier on the cold side of the insulation, i.e., the inside of the brick, you will trap moisture inside the walls and make the moisture problems worse.

You can make a vapor barrier out of any unbroken surface that's impermeable to water vapor. Polyethylene plastic is a good material to use, in a thickness between 0.002 and 0.008 inches (0.05 to 0.2 millimeters). Impregnated kraft paper, even with a layer of foil, is not a good choice because there's no way to close the gap between one sheet and the next, so the moisture will move right between the sheets.

Because no vapor barrier can be perfect, and some water may still get in, you must create a venting path for the water to get through the insulation. After this, you can install your vapor barrier by stapling it over your insulation or by using mastic. Overlap the tabs on the insulation's paper backing and staple them to the edges of the wall studs. Make sure to overlap several feet of vapor barrier at all joints so there are no gaps through which the vapor can travel. Also staple the vapor barrier to the window frames to prevent air from leaking around the windows. Finally, leave plenty of excess material in all the corners so the plastic won't be torn when installing the wallboard. Once your vapor barrier is installed, you're ready to put up your interior walls.