Weather-resistive barriers help prevent cold air leaks
Ever slip on a lightweight windbreaker on a blustery, rainy day and feel warmer? A windbreaker doesn’t add much in the way of insulation and it only keeps out some of the rain, but really cuts down on how much wind works its way through all the little gaps in your clothes, so you end up feeling a lot more comfortable.
Such is the concept behind house wraps, of which Tyvek is probably the most widely recognized brand. Technically called a weather-resistive barrier, or WRB, house wraps are designed to cut down substantially on the amount of cold air and drafts that can leak into your home through gaps in the exterior finish. The material wraps around the entire exterior of the home, under the siding, and can be applied directly to plywood or oriented strand board (OSB) sheathing, over foam insulation boards, or directly to the studs.
An important distinction between today’s house wraps and older wrap materials, such as felt paper, is that modern house wraps are not completely waterproof. While they will stop the bulk of “solid” water from passing through them–for example, a driving rainstorm while the house is still in the framing stage–they are designed to allow moisture vapor to pass through them. With today’s tighter house construction, where moisture can become trapped in wall cavities with the potential for mold and structural damage, this water vapor transmission ability is an important feature.
JUST ROLL IT ON
House wraps are relatively easy to install, although it is a two-person job. They come in large rolls–up to 10 feet wide and 150 feet long–and are specifically designed to cover the walls with a minimal number of seams. What seams there are in the installation are covered with tape for further seal against air infiltration.
Installation begins at one corner of the structure. Wrap approximately 6 to 12 inches around the corner, adjusting the starting point so that the vertical fastening guide marks are aligned over a stud. Adjust the roll up and down so that the bottom edge of the wrap material covers the space where the bottom sill plate of the wall meets the top edge of the foundation. Fasten the edge of the roll in place at this point.
Unroll and fasten the wrap material around the house, like wrapping a big gift box. The material is unrolled right over window and door openings and around corners without cutting. The wrap should overlap both of the top plates. On tall walls or second stories, the wrap on the upper wall should overlap the lower wall at least 6 inches.
House wrap materials are best fastened by using nails with large heads or with plastic washer heads, or with 1-inch wide staples. Staples with narrow crowns, such as those from a conventional staple gun, are liable to tear through the material in windy conditions. For metal studs, use screws with washers.
At each window and door opening, make a cut through the wrap material in the shape of an inverted letter “Y,” then cut again along the underside of the header, creating three flaps. Fold the flaps into the building, wrapping them around the sides and bottom of the rough wall opening, then staple the wrap material to the inside of the framing. Finish the installation by installing side and bottom flashing material over the top of the wrap, and installing a top flashing under the wrap above the window.
For maximum protection against air infiltration, all seams in the wrap material should be taped. This includes horizontal overlap joints between the upper and lower floors, and vertical joints where the seams occur.
House wraps can also be installed over framing while the wall is still lying down. Start at one corner as outlined above, and allow at least 6 inches overlap at the corners and top, and a sufficient amount at the bottom to overlap the lower plate and floor framing and extend to over the top of the foundation when the wall is stood. Stand each wall and secure them in place, then overlap the house wrap material and seal with tape.
House wraps such as Tyvek, Typar, Celotex, and other brands are readily available at lumberyards and home centers in a variety of sizes, along with fasteners, tape, and header flashing material. Complete installation instructions are typically provided with each roll.
Remodeling and repair questions? E-mail Paul at [email protected].
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Copyright 2006 Inman News