“Over and over again we see people not doing proper perimeter flashing prior to putting in a window or not doing proper head flashing,” McKinstry says.
On a window, the housewrap or moisture barrier should be laid first (see drawing below). Though some contractors argue for installing side flashing before laying in the window, McKinstry and others who work in climates with heavy wind-driven rain argue for installing the window first.
“It’s very important to flash the sides right,” McKinstry says. “Where we have seen penetration is on the bottom along the outside edges. That’s where windows fail, at the corners.”
It’s also essential when installing the window to avoid caulking underneath the bottom flange. Caulk there only traps moisture inside.
The head flashing is the final piece to be installed. This piece must be installed underneath the housewrap, so any water that gets behind the wrap will shed out and over the window flange instead of behind it.
“That’s one thing lots of guys don’t understand,” McKinstry says. “You have to tuck the head flashing underneath something.”
Flashing is also required at joints between two pieces of siding. This is an area where installers often err, particularly when installing fiber cement or vinyl. The most common error, Streano says, is the failure to account for contraction and expansion.
“Guys typically just butter the joint with caulk and don’t leave a gap,” Streano says. “But you have contraction and expansion, so the caulking will fail and that gap will open up.”
Rather than caulking the seam, Streano’s crews first apply a back flashing using 60-minute tar paper, then butt the two pieces of siding together.
“You’re going to get some wind-driven rain in there,” he says, “so now there is an additional piece of flashing that gets that rain out of the wall immediately.” As added moisture protection, some contractors will create a rain screen before installing siding (see drawing on previous page). A rain screen is a system in which the siding is separated from the moisture barrier by attaching furring strips at regular 16-inch intervals. This separation creates an air cavity that equalizes air pressure behind the siding — which deters wind-driven infiltration — and also encourages drying and drainage of any moisture that condenses behind the siding.
“We know that moisture is going to get in there and we want to manage it,” Streano says. “We want to facilitate, not prohibit, airflow. Mold likes a stagnant, moist climate, so you want to get air flowing through the space between the wall and the siding.”
(For more information about using rain screens, click on “WebXtra” under the “Magazine” tab at www.remodelingmagazine.com.)
BETTER DRAINAGE There’s little consensus among remodelers as to which moisture barrier works best. McDaniel prefers a combination of 30-pound felt paper and Tyvek FlexWrap. McKinstry favors impregnated black tar-paper, having followed the lawsuits that resulted from failures in first-generation synthetic wraps.
But Streano sees an advantage in new synthetic wraps such as Tyvek DrainWrap, GreenGuard’s Raindrop, and Valeron Vortec. Designed with channels to facilitate air flow and drainage, these products provide an effective alternative to the more costly rain-screen approach, Streano says.
“These products are all moisture barriers,” Streano says, “but they’re also able to drain moisture and facilitate the drying of the system. They don’t have the same capability to drain as the [rain screen], ” he adds, “but they do a very good job, relative to a regular moisture barrier, of draining excess water as well as facilitating air flow. And they’re only a fraction of the cost of installing a traditional rain screen.
McKinstry says he’s finding increasing demand for rain screens but, either way, “ventilation is so key in this area. If you have good ventilation, even if you get moisture, it dries out.” — David Zuckerman writes frequently on construction topics from his home in Brooklyn, N.Y.