Choosing Flexible Flashings - Continued

Installing Flexible Flashing

On most job sites, peel-and-stick flashings are installed without a lot of fuss. Typically, the flashing is cut to length, the release paper is removed, and the flashing is pressed in place by hand (Figure 7). But the easy way may not be the right way. Some manufacturers recommend that substrates should be primed before installing their peel-and-stick flashing, and that pressure should be applied with a roller, not the palm of the hand.

Figure 7. Builders use a variety of methods for flashing windows. This installer has chosen a belt-and-suspenders approach, installing strips of FortiFlash, a rubberized-asphalt flashing, on top of strips of E-Z Seal, a kraft-paper flashing laminated with polyethylene and fiberglass reinforcement.

Is a primer necessary? Self-sticking flashings often adhere better to a primed surface than an unprimed surface. Manufacturers that recommend priming generally focus on concrete and masonry as the most problematic surfaces, partly because those surfaces can be dusty or damp. Other manufacturers specify that OSB and gypsum sheathing need to be primed, and a few recommend priming metal and plywood. Most manufacturers agree that in cold weather, a self-sticking flashing will adhere better to a primed than an unprimed surface. There is a Catch-22, though: When it's too cold for peel-and-stick, it may also be too cold to apply primer.

In any case, few residential builders are likely to take the time to prime plywood or OSB sheathing before using peel-and-stick, which is one

reason some manufacturers omit the recommendation. If you do decide to prime, remember to use the primer recommended by the flashing manufacturer, since the wrong primer may cause compatibility problems.

Hand pressure or roller? Many, but not all, manufacturers recommend that their flexible flashing should be installed with a steel or hard-rubber J-roller — the same type of roller used for gluing plastic laminate countertops. Many manufacturers' reps admit that this recommendation is widely ignored, but doing so carries some risk: When it comes to priming and using a roller, the bottom line is that builders who deviate from a manufacturer's recommendations can't expect any support from the manufacturer if something goes wrong.

Use With Care

Despite the versatility of flexible flashings, they have their limitations and must be used with common sense. Some builders have reported adhesion problems with peel-and-stick. Others note that too much peel-and-stick can create a wrong-side vapor barrier.

How tacky? Peel-and-stick doesn't always stick. "I've been to sites where I've seen the peel-and-stick already half falling off the housewrap," says Patricia McDaniel, owner of Boardwalk Builders in Rehoboth Beach, Del. Poor bonding can be due to a variety of factors, including low temperatures and dirty substrates (Figure 8). Manufacturers agree that the adhesive bond of peel-and-stick flashings varies over time. Initially, for the first month or so, the bond should actually get stronger. But no one really knows when, if ever, the bond strength may begin to fail.

Figure 8. Peel-and-stick flashing adheres poorly to dirty substrates or when applied in cold weather.

Building scientist Joe Lstiburek urges caution. "A problem with these membranes is that they can peel away," says Lstiburek. "Don't rely on the adhesive property for waterproofing."

In an informal JLC test, 21 different peel-and-stick flashings were bonded to wood for 14 hours. About half of them failed to make a waterproof seal. Although further curing might have resulted in a waterproof bond, the test shows the need for caution when depending on an adhesive alone to seal out water.

Wrong-side vapor barrier. Peel-and-stick membranes should be used sparingly on wall sheathing, since they can create a wrong-side vapor barrier. "If you put a big hunk of peel-and-stick on the sheathing, interior moisture can condense behind it, causing rot," says Lstiburek. "It doesn't happen very often, but it happens occasionally. You've got to be careful not to get slap-happy with the stuff."

Several builders in British Columbia report finding sheathing rot behind peel-and-stick membranes, especially at window heads. But most investigators say that an important contributing factor in these cases was the use of damp framing lumber, and there don't appear to be any reports of such problems in other parts of North America. In fact, manufacturers of peel-and-stick membranes confidently recommend their use at window heads. "Clearly, vapor is an issue," says Rick Scruggs, technical service specialist at Grace Construction Products, a manufacturer of rubberized-asphalt flashing. "We wouldn't like to see you cover the whole wall, unless there are provisions for the vapor to get out. But if just a narrow strip of membrane is used around a window, the vapor can escape from other areas."

Many peel-and-stick manufacturers recommend the use of their products under siding at areas subject to splashback. However, because of the wrong-side vapor barrier problem, such an application is controversial. "The use of self-adhering membrane at splashback areas concerns me greatly," says Bob Switzer, chair of the Canadian Home Builders Association of British Columbia. "You are far better off finding ways to prevent the splash, like replacing the soil with lava rock."

Yet many builders confidently use peel-and-stick to protect sheathing from splashback. "We use it at the splashback area all the time," says McDaniel. "I think you are much more likely to get water into a structure with bad flashing details than vapor problems, at least in our climate."

Over or under the housewrap? One debate that won't be settled soon is whether peel-and-stick should be applied directly to the sheathing or is best applied to the housewrap or felt. There are strong advocates for both positions.

Some manufacturers recommend that their peel-and-stick membrane should be applied directly to the sheathing. "In general, the membrane should be adhered directly to the wall sheathing, and not to a layer of felt or housewrap. What's the value of the membrane if it can't be fully adhered to the substrate to prevent water from getting behind it?" says Scruggs. McDaniel agrees. "Installing housewrap and then slapping windows in and then attaching peel-and-stick to the housewrap doesn't do anything," she says.

By contrast, builders who are worried about a wrong-side vapor barrier prefer to see a layer of building paper or housewrap between the sheathing and the peel-and-stick. "Rarely do I apply a piece of peel-and-stick directly to sheathing," says Randy Faustmann, president of Rainforest Envelope Protection Services, a consulting firm in Langley, B.C. "Usually, it is installed over the building paper. I think that having the layer of paper between the peel-and-stick and the sheathing allows a little bit more drying than it would without it."

No matter how you assemble your sandwich of flexible flashing, nailing fins, and building paper, everyone agrees on one point: Lap all the layers to shed water. "You have to lap your layers, because at some point the glue's going to give," says McDaniel. "Physics is going to win over chemistry."

Martin Holladayis an associate editor at The Journal of Light Construction.