Making Sense of Housewraps

■ BY FERNANDO PAGES RUIZ

hen I started building houses nearly 30 years ago, we lapped lightweight #15 asphalt — or rosin-impregnated building paper directly over the stud framing before installing the siding. Nowadays, concerns with energy-efficient construction and moisture infiltration have focused a great deal of attention and no small amount of high-tech chemistry on this thin layer of paper. Although some builders still advocate the felt-paper barriers of yesteryear, most have switched to plastic-based housewraps, products designed to stop air infiltration and wind-driven rain while allowing water vapor to evaporate—a great concept.

However, like everything high-tech, new solutions come with new problems. The range of choices and the precise installation requirements of modern housewraps challenge builders with terms like spun — bonded, polyolefin-based moisture, and air-infiltration fabric. Even if the technical terminology is hard to remember, learning how to install these products correctly is im­portant. Yet a quick look around a construc­tion site reveals that most builders, with thirty years or with three behind the ham­
mer, are having a hard time handling this new technology.

There’s plenty of confusion surrounding weather-resistive barriers. Many home­owners and builders don’t know which prod­uct to choose, some builders never learned how to install it correctly, and many have no idea what housewrap does in the first place.

What Does a Housewrap Do?

Placed beneath the siding, housewrap is a second layer of defense for your home. When installed properly, it performs three basic functions (see the drawing on the facing page).

First and foremost, housewrap acts as a backup barrier that keeps water off the struc­tural sheathing and framing. Properly in­stalled siding is the first line of defense, but sometimes wind-driven rain and snow still find a way through. Housewrap also func­tions as an air barrier that stops hot — and cold-air movement through the wall cavity. As long as joints are sealed properly, house — wrap is designed to cut utility costs and

increase comfort by reducing air infiltration and potential drafts.

The real magic of housewrap lies in its third function: allowing the free passage of water vapor so that wall cavities and framing lumber can dry to the outside of the build­ing, reducing the threat of mold and rot. Without this feature, installing housewrap would be like putting a thick raincoat over your house: great for keeping out the rain, but terrible at releasing water vapor from within. Instead, housewraps are designed to act like a Gore-Tex® jacket, allowing water vapor to pass through the building envelope in case moisture problems arise.