When JLC published its primer on flexible flashings a decade ago ("Choosing Flexible Flashings," Jun/01), these non-metallic, self-adhered membranes were just becoming common on jobsites. Today, they're everywhere. They have all but replaced felt splines for the edges of finned windows and are now the standard for sealing most other roof and wall penetrations. Ease of use is a big reason: All of these products work well on flat, two-dimensional surfaces, and accordion-style membranes such as DuPont's FlexWrap or Typar's Flashing Flex will conform to three intersecting surfaces, such as the corner of a window opening.
Manufacturers have made it a priority to teach contractors how to use these products correctly, and their websites offer photos, videos, and animations on proper installation techniques. This magazine has published several articles on the subject, and the annual JLC Live show always includes demonstrations of how to flash windows and doors properly. But despite all this effort, experts still see lots of mistakes and misunderstandings in the field—about the materials and key installation details. This article addresses some of those common misconceptions.
The 2001 article went into detail on the chemistry behind flexible flashing products as well as the choices available. The manufacturers we interviewed confirmed that those basic formulations haven't changed much since that article. So we will quickly review the three main product types and then look at a few installation issues that warrant close attention.
Three Product types
Flexible flashing for residential work is available in thicknesses from 12 to 70 mils, in widths from 3 to 36 inches, and with several types of facings. Despite this variety, the products can be broadly categorized into three main types: rubberized asphalt, butyl, and acrylic. Manufacturers offer some different formulations and enhancements (for instance, one tape combines butyl and acrylic), but the general performance expectations are the same within each category.
Rubberized asphalt (RA).This is the most common and least expensive type of peel-and-stick eaves flashing. One face is laminated to a high-density polyethylene backing while the other is covered with release tape. The asphalt is chemically modified to make it more elastic, allowing it to expand and contract with the substrate. RA is a great all-around flashing material and it sticks to a variety of substrates. It's also self-healing, which means it's supposed to seal around fasteners driven through it.
One drawback to RA is its narrow range of application temperatures when compared with other flashing types. Depending on the product, it can stiffen in cold temperatures, making it less likely to adhere properly. Some manufacturers advise warming the product before using it when temperatures fall below 55°F, but installers tell us that problems occur mostly below 45°F. Flashing can be warmed with a heat gun or by putting it in an ice chest with an incandescent bulb (be careful not to overheat the tape or set it on fire).
At very high temperatures, such as you might find on walls exposed to constant summer sun (180°F temperatures are not unusual for walls in the South and Southwest), RA flashings can soften and lose adhesion. James Katsaros, R&D team leader at DuPont, told me that solvents from the asphalt have been known to leach out and stain the siding.
Long exposure to UV light has also been known to cause adhesion problems for RA flashing. Because of this, asphalt-based flashings should not be left uncovered for more than 30 days after application. But that said, Bill Rose, senior research architect at the University of Illinois and author of put two layers of RA flashing on a small flat roof 20 years ago and so far it hasn't leaked. That's not to recommend uncovered RA flashing, but rather to show the durability of high-quality products.
Butyl rubber. Compared with RA, butyl flashings are considered to be of higher quality, and their prices reflect that. Butyl is a synthetic rubber used in many applications, including spacers for insulated glass. Butyl flashings are made by laminating the butyl rubber compound to a polyolefin-based film. Unlike RA flashing, butyl products contain no solvents or VOCs (volatile organic compounds), and they do not off-gas. They are supposed to be self-healing like RA, they stick better to a wider variety of materials, and they have better long-term stability. Butyl tapes can also be installed at lower temperatures than RA. DuPont's Flashing Tape, for instance, can be laid over a bare substrate at temperatures as low as 25°F; any colder and you'll need to use an adhesive primer.
Acrylic. These products, which consist of an acrylic adhesive and various backings, are thinner than rubberized asphalt or butyl membranes. Acrylic also makes for a better adhesive bond with many (but not all) materials, so it is sometimes used as the adhesive layer for butyl or RA.
The best acrylic tapes use a VOC-free solid adhesive that makes a molecular bond to the substrate. According to BuildingGreen.com, "Solid acrylic adhesives can form the strongest adhesive bonds at a wide range of temperatures and even achieve adhesion to damp or wet substrates." ProClima's Tescon and Huber's Zip System tape are examples: ProClima claims that its product can be applied in temperatures down to 15°F, while a Huber contact said that the company's tape has an application temperature range of 20°F to 100°F.
Some acrylic tapes are pressure-activated. ProClima's Tescon Vana tape, for instance, feels as sticky to the touch as butyl or RA, but if you don't pressurize it, it won't bond. In an improvised test, I needed to apply firm pressure (I used a roller) to the tape in order for it to strongly grip the substrate.
Acrylic flashings are warranted for longer UV exposure than are RA products. ProClima has a 90-day warranty, while Huber offers a warranty for nine months of exposure.
Ensuring Compatibility
The big issue with flexible flashings has always been compatibility. Manufacturers have tweaked their formulas to address this, leading some contractors to believe that you can use any flashing anywhere. Not so. "Compatibility is still the No.1 issue," says Train2Build.com's Bill Robinson, who teaches builders about how to prevent moisture problems. There are two parts to flashing compatibility: how well it adheres to the substrate (the sheathing, the housewrap, or window fins), and whether it has an undesirable chemical reaction with that substrate.
Contractors report sometimes having problems with some butyl flashing sticking to asphalt products; manufacturers, however, say that this is not an issue, and certainly not with felt paper.
Putting acrylic tape on an asphalt substrate is theoretically OK if the surface isn't too oily, but that's not always possible to determine in the field. Even felt paper can be a problem. I put rubberized asphalt, butyl, and acrylic tape on 15-pound felt and rolled them to make a bond. After waiting a half-hour, we tried to pull the tapes off and found that the acrylic tape came off relatively easily, while the other two put up a real fight. We could remove only about half of each piece of asphalt and butyl tape.
Some substrates are problematic for all flashings. These include masonry and gypsum sheathing. "Their surfaces are loosely bonded, so the flashing products have nothing to stick to," says DuPont's Katsaros. Most flashings also have a tough time grabbing onto the rough side of OSB. The solution in all cases is to use an adhesive primer. Primers come in brush-on and spray-on forms, and they do maximize adhesion. While they're more common on commercial jobs than on residential ones, there should be a can of primer on every jobsite.
