You know where to insulate, but there’s still one more thing you need to know about—vapor barriers. Well-insulated attics, crawlspaces, storage areas, and other closed cavities need to be well ventilated to prevent excessive moisture build-up.
It doesn’t’ t rain inside your house, but it does dew. Walk out on your lawn on a cool summer morning and you’ll find the ground wet—not with rain, but with dew. Pour yourself a glass of ice water in a nice, warm house and you’ll find the outside of the glass wet—from dew. Install insulation without a vapor barrier and dew will form, which will rot wood, corrode electrical wiring, and erode plaster and wall board. If it doesn’t’ t rain in your house, why do you have dew?
Warm air holds more moisture than cold air does. Your breathing adds moisture to the air. Cooking increases the humidity inside the house. And so does a humidifier. When that warm, moisture-laden air hits the cold surfaces in unheated portions of the attic and walls, dew forms. But if you can place a moisture- blocking barrier between the cold surface and the heated air, dew won’ t form. Unfortunately, insulation won’ t do that job by itself—in fact, most insulation materials would collect the moisture, and wet insulation doesn’t’ t work. So, you need a vapor barrier for basement and between the warm portions of the house and the insulation.
Many bales and blankets of insulation have a vapor barrier already attached. All you have to remember is to install the insulation with the vapor barrier closest to the heated area (Don’ t panic. You won’ t have to hold this book in one hand while you install the insulation with the other. The instructions on how you place the vapor barrier are printed right on the vapor barrier.) For other forms of insulation, it is necessary to lay down or nail up a vapor barrier (usually aluminum foil, saturated kraft paper, or plastic sheeting) before installing the insulation.
Often, when insulating a cockloft or exterior wall, it is impractical or impossible to install a vapor barrier in basement. In those cases, you can create a moisture-blocking effect by papering the wall with vinyl wallpaper or by painting the wall or ceiling with two coats of good-quality oil-based or metallic paint.
