Closing Up the Box

On the exterior of the walls, a layer of a water-repellant material will probably be applied, especially in cooler climates. Think of your addition as a gift package, and just like a tie or child's toy, it needs to be appropriately wrapped. In the case of a building, however, the material won't be colorful paper. It'll be one of several products described as air-infiltration retarders.

In the past, papers impregnated with asphalt (tar paper) or rosin were applied to the exteriors of houses, after the sheathing went on but before the siding did. More recently, proprietary products like Typar and Tyvek have superceded such papers. The new house wraps are high-tech sheets of olefin fibers that are stapled in place. This clever extra skin allows moisture vapor to escape the wall cavity yet also stops wind from moving freely into the wall.

The house wrap should cover all wall surfaces. Flaps should fold in at win­dows and doors. An important step that is sometimes omitted is the application of a specially made seam tape over joints between sheets of the wrap and around win­dows and doors. Without it, the wrap will not form a complete air barrier.