When you’re outside in a rainstorm, you will usually have some type of protection to keep from getting wet. It could be an umbrella, a raincoat or something similar. For your home, there is a similar device that keeps moisture out called a vapor barrier. Vapor barriers come in different forms as well. For example, there are paints that work as a vapor barrier. This is very cost effective if you’re painting the walls anyway. You just need to make sure that the area is covered well by the paint, since a missed patch won’t only be unsightly, it is also an inefficient vapor barrier.
Using a polyethylene plastic sheet is a common vapor barrier, but you should make sure that you are careful when installing it, since it is easy to rip it on nails or wooden splinters thus rendering it useless.