Choosing Flexible Flashings Jun '01

CHOOSINGFlexible FlashingsIn recent years, builders have beenpaying more attention to theimportance of keeping water out ofwalls. News of construction-defectlawsuits in California, EIFS failures inby Martin HolladayNorth Carolina, and the “leaky condo”crisis in British Columbia have alldriven home the point that leakingbuildings can cause major headachesfor builders. One result of the focus onwaterproof walls is the growing use ofpeel-and-stick membranes and othertypes of flexible flashing.The term “flexible flashing” is used todescribe a broad category of nonmetallic flashings, including both peeland-stick and nonstick flashings.Manufacturers have not yet agreed on ageneric term for these products, whichare referred to as self-adhering bitumi-To ensure weathertight windowand door installations, keep aroll of flashing membrane handynous tapes, flashing tapes, waterproofing tapes, flexible window flashings,flashing membranes, and wall tapes.Only in the past few years have theseflashings become common on residential job sites. Flexible flashing is rapidlyreplacing traditional felt splines forsealing the perimeter of finned windows. Some flexible-flashing manufacturers promote the use of theseproducts at other locations, as well: tocover below-grade concrete cracks, atroof penetrations, under exterior doorJUNE JLC 2001sills, over deck ledger boards, at insideand outside corners of wall sheathing,under stucco shelves and parapets, andover sections of wall sheathing susceptible to splashback. But by far the mostcommon use of flexible flashing is atwindow and door perimeters.These new materials have some significant advantages over traditionalflashing materials. Unlike most metalflashings, for example, peel-and-stickflashings conform easily to unusualshapes. Most types of flexible flashing

can be folded to form a waterproof enddam on a rough windowsill, wheremaking the same shape with copperwould require soldering the flashing atthe corners. Manufacturers claim thatpeel-and-stick flashings, unlike metalflashing, can form a waterproof sealbetween the flashing and the substrate.These flashings are versatile and easy toinstall. But before slapping peel-and-stickover every exterior crack, you need to besure you’ve chosen the right product fora given application. It’s also important toknow about potential compatibilityFigure 1. Self-adhering rubberized-asphalt flashings are made of the same material as theeaves membranes used to prevent ice dam leaks.Figure 2. Rubberized-asphalt flashings, likeGrace’s Vycor Plus, stick well to unprimedplywood. Some manufacturers of rubberized-asphalt flashings warn that adhesionto OSB can be difficult unless the OSB isfirst primed.Figure 3. Since foil-topped flashings likePeel’N’Stick from Polyguard Products canbe left exposed to the weather for a longerperiod than polyethylene-topped flashings,they are a good choice when siding installation may be delayed.JUNE JLC 2001problems and to avoid accidentally creating a wrong-side vapor barrier.Rubberized AsphaltMost peel-and-stick flashings are madefrom rubberized asphalt, also known asmodified asphalt, modified bitumen, orrubberized bitumen. Rubberized-asphaltmembranes were originally developed toprotect roofs from ice dams. As buildersrecognized new uses for the product, several manufacturers began selling it innarrow rolls — typically between 4 and12 inches wide — for a variety of flashing applications (Figure 1).Rubberized asphalt used for flashingis made by modifying asphalt withstyrene butadiene styrene (SBS), whichmakes the asphalt more rubber-like.SBS-modified asphalt, being elastic, canaccommodate thermal expansion andcontraction in building components.Because of its “cold flow” characteristics, rubberized asphalt can also sealaround fastener penetrations.Sticky stuff. As long as the surface isclean and warm, rubberized asphaltsticks to a wide variety of substrates:dimensional lumber, plywood, steel,aluminum, hard vinyl, asphalt felt,and plastic housewrap (Figure 2). Somemanufacturers of rubberized-asphaltflashing advise that their products maynot stick well to concrete, masonry, orOSB unless these substrates are firstprimed.To make it possible to handle such asticky substance, one side of the rubberized asphalt is laminated to a thin sheet(usually about 8 mils) of cross-laminated high-density polyethylene, andthe other side is protected with a siliconized paper release sheet. Instead ofpolyethylene, some manufacturers laminate a thin layer of aluminum foil tothe top of their rubberized-asphaltflashings (Figure 3).Stickiness is a double-edged sword. Inwarm temperatures, when rubberizedasphalt is at its stickiest, it can beimpossible to readjust a flashing once ithas touched a surface.Keep it covered. Rubberized-asphaltflashings, except for those laminatedwith aluminum foil, should not be left

exposed to the weather. Eventually,ultraviolet light breaks down the polyethylene, exposing the modifiedasphalt, which then begins to oxidize.Most manufacturers recommend thattheir flashings be covered within 30days of installation, although one manufacturer, Protecto Wrap, says that itsBT20XL Building Tape can be leftexposed for up to 120 days.Butyl RubberSeveral manufacturers make peel-andstick flashings from butyl, also calledbutyl rubber (Figure 4). Butyl flashingsare usually black, resembling their rubberized-asphalt cousins. However, butylflashings lack the asphalt smell that distinguishes rubberized-asphalt products,and they feel more rubbery. Like rubberized-asphalt flashings, butyl flashingsare available with a top surface of eitherpolyethylene or aluminum foil. Thosewith a top surface of polyethyleneshould not be left permanently exposedto the weather. FlexWrap, a butyl flashing from DuPont, has a top layer of corrugated Tyvek that enables it to conformto curved shapes, like the heads of archtop windows.In general, butyl flashings cost abouttwice as much as rubberized-asphaltproducts (see “Flexible Flashing Costs”).However, DuPont’s FlexWrap is significantly more expensive than other butylflashings; it costs about six times theprice of the average rubberized-asphaltproduct. Manufacturers claim that butylhas several advantages over rubberizedasphalt: longer-lasting stickiness, lessstaining, less high-temperature oozing,and a wider temperature range forinstallation.Butyl rubber has a reputation for longlived tackiness: One JLC editor has 21year-old butyl glazing tape in hisgreenhouse that is still as pliable andtacky as the day it was installed. JeffWinzeler, product manager for the roofing adhesive group at Ashland ChemicalCo. 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 difficultsurfaces, and is much more weatherproof.” Although butyl’s bond is aggressive, it is slower acting than the bond ofrubberized-asphalt products. Butyl manufacturers tout this as an advantage,because it allows readjustment of theflashing during installation.Although rubberized asphalt can beformulated for low-temperature installation, butyl flashings, on average, canbe applied at colder temperatures thanmost rubberized-asphalt flashings.Butyl laminated with EPDM. Somebutyl flashings are laminated to a toplayer of EPDM to make a type of flexibleFigure 4. Self-adhering butyl flashings, like rubberized-asphalt flashings, can have a toplayer of either polyethylene or aluminum foil. Butyl flashing, although more expensivethan rubberized-asphalt flashings, can be installed over a wider temperature range.Flexible-Flashing Costs 2.50 2.00 1.50 1.00 .50NonstickflashingRubberizedasphalt withpolyethyleneRubberizedasphalt withaluminumfoilButylrubber withpolyethyleneor foilButyl rubberwith EPDM .00 .24 .39 .68 .84 2.28Average price per square foot of flexible-flashing materialsJUNE JLC 2001

Using Flexible Flashing Self-adhering flashings areparticularly useful underwindowsills and door thresholds, over deck ledger boards,and at horizontal projectionsand parapet walls that will befinished with stucco. To flash window and doorperimeters, asphalt felt splines,nonstick products like Moistop,or self-adhering flashing can beused. Flashings should alwaysbe lapped to shed water. On wall sheathing, limit the useof self-adhering flashing to smallareas in order to avoid creatinga wrong-side vapor barrier.flashing called cover tape or flashingtape (Figure 5). EPDM, a rubbery membrane used for roofing, is very resistantto weather exposure. Because EPDMflashings are relatively expensive —costing about six times as much as theaverage rubberized-asphalt flashing —they are rarely used anywhere except onroofs, where the ability to resist ultraviolet light is essential. Where a peel-andstick flashing will be covered by sidingor otherwise protected, weather resistance is not an issue and using an EPDMflashing would be overkill.Moreover, EPDM flashings are sothick (usually about 70 mils) that theywould be awkward to use under siding.Rubberized-asphalt flashings are typically much thinner — between 20 and40 mils thick — and are therefore easierto fold and tuck.Variations on a ThemeAlthough most peel-and-stick flashings have a top layer of polyethylene,some are topped with aluminum foil. Afew manufacturers sell flexible flashingsthat are not self-adhering and requirethe use of fasteners (Figure 6).Foil-faced flashings. Flashings thatare topped with a thin layer (2 mils) ofaluminum foil can be left exposed tothe weather. These flashings, whichFigure 5. Butyl flashings with a top layer of EPDM are called cover tape or flashing tapeand are commonly used to flash single-ply roofs. The EPDM layer protects the butyl fromdegradation by ultraviolet rays.JUNE JLC 2001include an adhesive layer of either butylor rubberized asphalt, are promoted fora variety of uses, including repair ofroof-top ducts, metal chimneys, gutters,and trucks.Because the long-term durability ofthese flashings is open to question,their use is usually limited to temporaryroof repairs. One manufacturer, TycoAdhesives, recommends its aluminumfoil flashing, Polyken 626-20 WindowFlashing Tape, for use at windowperimeters. If siding installation isdelayed, even for many months, thealuminum foil layer will still protect theflashing from deterioration.Nonstick flashings. At least two manufacturers make nonstick flexible flashings designed to be attached withstaples or nails. Although nonstickflashings may appear unsophisticatedcompared to peel-and-stick products,they have their advocates. Some building-science experts feel that using anonstick flashing (or even plainasphalt-felt splines) is preferable tousing peel-and-stick flashings, whichmay be more likely to trap moisture inwall assemblies.MFM’s Future Flash is a nonstickflashing made from rubberized asphaltsandwiched between two films, a bottom layer of polyethylene and a toplayer of metalized polyester. Accordingto the manufacturer, Future Flashbehaves better in very hot temperaturesthan most rubberized-asphalt flashings,because the metalized polyester layerhelps reflect sunlight.Fortifiber’s nonstick flashing, calledMoistop, is a relatively thin, 12-milflashing made from kraft paper laminated with two layers of polyethyleneand one layer of fiberglass reinforcement. Moistop is inexpensive — aboutone-third the cost of the typical rubberized-asphalt product. Moistop shouldn’tbe used on windowsills, since the manufacturer warns that it is not intendedfor horizontal use. One disadvantage isthat unlike Future Flash or other rubberized-asphalt flashings, Moistop can’tseal around fastener holes. Moistop isalso available in a version called E-ZSeal, which includes a narrow band of