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Choosing Flexible Flashings - Continued

In general, butyl flashings cost about twice as much as rubberized-asphalt products (see "Flexible Flashing Costs," below). However, DuPont's FlexWrap is significantly more expensive than other butyl flashings; it costs about six times the price of the average rubberized-asphalt product. Manufacturers claim that butyl has several advantages over rubberized asphalt: longer-lasting stickiness, less staining, less high-temperature oozing, and a wider temperature range for installation.


Butyl rubber has a reputation for long-lived tackiness: One JLC editor has 21-year-old butyl glazing tape in his greenhouse that is still as pliable and tacky as the day it was installed. Jeff Winzeler, product manager for the roofing adhesive group at Ashland Chemical Co. in Columbus, Ohio — a manufacturer of EPDM and butyl tapes — says, "Compared to SBS-modified asphalt, butyl is a more high-performance adhesive, with the ability to adhere to difficult surfaces, and is much more weatherproof." Although butyl's bond is aggressive, it is slower acting than the bond of rubberized-asphalt products. Butyl manufacturers tout this as an advantage, because it allows readjustment of the flashing during installation.

Although rubberized asphalt can be formulated for low-temperature installation, butyl flashings, on average, can be applied at colder temperatures than most rubberized-asphalt flashings.

Butyl laminated with EPDM. Some butyl flashings are laminated to a top layer of EPDM to make a type of flexible flashing called cover tape or flashing tape (Figure 5). EPDM, a rubbery membrane used for roofing, is very resistant to weather exposure. Because EPDM flashings are relatively expensive — costing about six times as much as the average rubberized-asphalt flashing — they are rarely used anywhere except on roofs, where the ability to resist ultraviolet light is essential. Where a peel-and-stick flashing will be covered by siding or otherwise protected, weather resistance is not an issue and using an EPDM flashing would be overkill.


Figure 5. Butyl flashings with a top layer of EPDM are called cover tape or flashing tape and are commonly used to flash single-ply roofs. The EPDM layer protects the butyl from degradation by ultraviolet rays.

Moreover, EPDM flashings are so thick (usually about 70 mils) that they would be awkward to use under siding. Rubberized-asphalt flashings are typically much thinner — between 20 and 40 mils thick — and are therefore easier to fold and tuck.

Variations on a Theme

Although most peel-and-stick flashings have a top layer of polyethylene, some are topped with aluminum foil. A few manufacturers sell flexible flashings that are not self-adhering and require the use of fasteners (Figure 6).


Figure 6. Not all flexible flashings are self-adhering. Future Flash from MFM Building Products (left) and Moistop from Fortifiber (center) are nonstick flashings that are installed with fasteners. Fortifiber's Moistop E-Z Seal (right) is similar to regular Moistop, but includes a 3-inch-wide adhesive band along one side of the flashing.

Foil-faced flashings. Flashings that are topped with a thin layer (2 mils) of aluminum foil can be left exposed to the weather. These flashings, which include an adhesive layer of either butyl or rubberized asphalt, are promoted for a variety of uses, including repair of roof-top ducts, metal chimneys, gutters, and trucks.

Because the long-term durability of these flashings is open to question, their use is usually limited to temporary roof repairs. One manufacturer, Tyco Adhesives, recommends its aluminum-foil flashing, Polyken 626-20 Window Flashing Tape, for use at window perimeters. If siding installation is delayed, even for many months, the aluminum foil layer will still protect the flashing from deterioration.

Nonstick flashings. At least two manufacturers make nonstick flexible flashings designed to be attached with staples or nails. Although nonstick flashings may appear unsophisticated compared to peel-and-stick products, they have their advocates. Some building-science experts feel that using a nonstick flashing (or even plain asphalt-felt splines) is preferable to using peel-and-stick flashings, which may be more likely to trap moisture in wall assemblies.

MFM's Future Flash is a nonstick flashing made from rubberized asphalt sandwiched between two films, a bottom layer of polyethylene and a top layer of metalized polyester. According to the manufacturer, Future Flash behaves better in very hot temperatures than most rubberized-asphalt flashings, because the metalized polyester layer helps reflect sunlight.

Fortifiber's nonstick flashing, called Moistop, is a relatively thin, 12-mil flashing made from kraft paper laminated with two layers of polyethylene and one layer of fiberglass reinforcement. Moistop is inexpensive — about one-third the cost of the typical rubberized-asphalt product. Moistop shouldn't be used on windowsills, since the manufacturer warns that it is not intended for horizontal use. One disadvantage is that unlike Future Flash or other rubberized-asphalt flashings, Moistop can't seal around fastener holes. Moistop is also available in a version called E-Z Seal, which includes a narrow band of peel-and-stick adhesive along one side of the flashing.

Using Flexible Flashing

* Self-adhering flashings are particularly useful under windowsills and door thresholds, over deck ledger boards, and at horizontal projections and parapet walls that will be finished with stucco.

* To flash window and door perimeters, asphalt felt splines, nonstick products like Moistop, or self-adhering flashing can be used. Flashings should always be lapped to shed water.

* On wall sheathing, limit the use of self-adhering flashing to small areas in order to avoid creating a wrong-side vapor barrier.

Choosing the Right Flashing

Not surprisingly, manufacturers are eager to promote their flexible flashing products for a wide variety of applications. But not all manufacturers recommend the same applications, so it's important to read the installation instructions. Some manufacturers recommend using their products below grade or on roofs, while others specifically exclude those applications. In general, manufacturers of heavier 35-mil and 40-mil flashings are more likely to recommend roof or below-grade use than manufacturers of 20-mil products.

Thickness. Flexible flashings vary in thickness from 12 mils (Fortifiber's nonstick Moistop) to 79 mils (Illbruck Vapor Barrier Stucco Tape). Most self-adhering window and door flashings range in thickness from 20 mils to 40 mils. A thicker flashing may be more durable and better able to withstand abuse, but thinner flashing is easier to fold and conform to unusual shapes.

Hot locations. In very hot locations, butyl products are probably a better choice than rubberized asphalt, which can ooze at high temperatures. Oozing can occur when rubberized-asphalt flashing is installed under metal exposed to sunlight — for example, under metal roofing or on the nailing fins of south- or west-facing aluminum-clad windows. Grace Construction Products specifically prohibits the use of its Vycor Plus flashing in "hot desert areas in the Southwestern U.S." Similarly, Carlisle Coatings warns that its product, Window and Door Flashing, is "not recommended in areas where flashing will be subject to continuous exposure to sunlight or to temperatures in excess of 180°F."

Cold-weather installation. Trying to install a peel-and-stick flashing on a cold wall can be frustrating. Both rubberized asphalt and butyl become less sticky as the temperature drops, and below 40°F some products just won't stick. One manufacturer, Ridglass Manufacturing, ships different formulations of their Kwikwrap rubberized-asphalt flashing at different times of the year, with varying formulations to produce different levels of low-temperature stickiness. Unfortunately, there is no way to tell from the Kwikwrap label which product your local distributor has in stock.

The minimum application temperatures provided by flashing manufacturers vary from 10°F to 50°F (see "Flexible Flashing Specifications," page 4 of article). These recommendations should be taken as a guide, not a guarantee. An installer can push the minimum application temperature somewhat by storing the flashing in a warm location before use.

In consistently low temperatures, the best flexible flashing may be a nonstick flashing like Fortifiber Moistop or MFM Future Flash. Since these products are attached with fasteners, stickiness is not an issue. If you need a cold-weather self-adhering flashing, it's probably best to choose either a butyl product or Bakor Blueskin Weather Barrier, a rubberized-asphalt flashing that performs well at low temperatures. In a pinch, any flashing can be held up with roofing nails.

Compatibility problems. If you're using a flexible flashing anywhere near an asphalt product, it's best to choose a rubberized-asphalt flashing, because butyl flashings are incompatible with asphalt products. "There are oils that want to come out of the asphalt," says Jeff Winzeler. "The butyl will suck them up and lose its adhesive properties."

Tyco Adhesives' instructions for installing one of its butyl flashing products, Polyken 627-35, warns, "Avoid contact with residuary asphaltic products such as coatings and other roofing products." A Tyco representative confirmed that its butyl flashings shouldn't be in contact with asphalt roofing cement. Since Tyco promotes the product for use on roofs, where asphalt roofing cement is often found, installers must be vigilant to avoid compatibility problems.

The jury is still out on whether butyl tapes should be allowed contact with asphalt felt. "If you are talking about 15-pound felt, there is not a lot of asphalt, because felts are relatively dry," says Winzeler. "You'll probably have fewer issues with compatibility than with roofing cement. But until you test, you can't be sure." When Theresa Weston, a chemical engineer at DuPont, was asked whether DuPont's butyl tape, FlexWrap, is compatible with asphalt felt, she was noncommittal. "We're still testing it," she said.

Rubberized asphalt is incompatible with some types of flexible vinyl, especially vinyl flashings that come in a roll. It doesn't appear to have any compatibility problems with hard vinyl, like the vinyl used for window fins.

Watch out for staining. Rubberized asphalt, like other asphalt products, can stain some materials, especially vinyl. According to Bob Sims, customer service manager at Bakor, such staining, called plasticizer migration, occurs when oils in the asphalt dissolve plasticizers in the vinyl. Since rubberized-asphalt flashings shouldn’t be left exposed, staining is generally not a problem. The siding or other material used to cover the flashing usually hides any stains.