Making Sense of Housewraps
These plastic-based barriers help to keep wind and water at bay, but only if you choose the right product and install it correctly.
Synopsis: If you don’t understand what housewrap does and how it’s designed to work, you are likely doing more damage to your house than you are good. This article demystifies the functions of a housewrap, discusses the pros and cons of several brands, and answers both common and in-depth questions about these products. Sidebars in the PDF below discuss the growing popularity of rainscreen housewrap membranes and modern pneumatic cap nailers, and building scientist Paul Fisette presents an argument for the use of felt paper.
When I started building houses nearly 30 years ago, we lapped lightweight 15-lb. 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 you can’t remember the technical terminology, you have to learn how to install these products correctly. Yet a quick look around a construction site reveals that most builders, with 30 years or with three behind the hammer, 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, others 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.
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, housewrap is designed to cut utility costs and increase comfort by reducing air infiltration and potential drafts.
But 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.
Side-by-side comparison is often pointless
Nowadays, any approved weather-resistive barrier, from #15 felt to high-tech housewrap, touts the dual benefit of being a weather resistive drainage plane that also allows the passage of water vapor. But not every product balances these two features equally. To add to this confusion, housewraps are now available in dozens of varieties, so how do you choose? Unfortunately, there’s no easy answer.
For more photos and information on how to pick the right housewrap, click the View PDF button below.
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