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