Interior or exterior. To get the most insulating value for your money, compare the total costs of insulating an area to a specific R-value. For many insulating jobs, such as those in your attic and basement, doing it yourself can save you money. As you've seen with a glass of ice water, condensation occurs when warm, moist air touches a cold surface. When this happens in your home, it can cause water or frost damage, mold and mildew.
Vapor barriers slow the movement of water vapor through building materials.
A good vapor barrier will allow very little moisture to pass through it. If you're installing insulation, you should always include a vapor barrier. Investing time and money in a vapor barrier is insurance against future moisture damage. If you are building a new home, a vapor barrier should be placed under the concrete slab. In Insulation such as polyurethane and polyicynene insulation does not need a vapor barrier when installed.