Choosing Flexible Flashings Jun '01

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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

peel-and-stick adhesive along one sideof the flashing.Choosing the Right FlashingNot surprisingly, manufacturers areeager to promote their flexible flashingproducts for a wide variety of applications. But not all manufacturers recommend the same applications, so it’simportant to read the installationinstructions. Some manufacturers recommend using their products belowgrade or on roofs, while others specifically exclude those applications. In general, manufacturers of heavier 35-miland 40-mil flashings are more likely torecommend roof or below-grade usethan manufacturers of 20-mil products.Thickness. Flexible flashings vary inthickness from 12 mils (Fortifiber’s nonstick Moistop) to 79 mils (IllbruckVapor Barrier Stucco Tape). Most selfadhering window and door flashingsrange in thickness from 20 mils to 40mils. A thicker flashing may be moredurable and better able to withstandabuse, but thinner flashing is easier tofold and conform to unusual shapes.Hot locations. In very hot locations,butyl products are probably a betterchoice than rubberized asphalt, whichcan ooze at high temperatures. Oozingcan occur when rubberized-asphaltflashing is installed under metalexposed to sunlight — for example,under metal roofing or on the nailingfins of south- or west-facing aluminumclad windows. Grace ConstructionProducts specifically prohibits the useof its Vycor Plus flashing in “hotdesert areas in the Southwestern U.S.”Similarly, Carlisle Coatings warns thatits product, Window and Door Flashing,is “not recommended in areas whereflashing will be subject to continuousexposure to sunlight or to temperaturesin excess of 180ºF.”Cold-weather installation. Trying toinstall a peel-and-stick flashing on acold wall can be frustrating. Bothrubberized asphalt and butyl becomeless sticky as the temperature drops,and below 40ºF some productsjust won’t stick. One manufacturer,Ridglass Manufacturing, ships differentformulations of their Kwikwrap rubberized-asphalt flashing at different timesof the year, with varying formulationsto produce different levels of low-temperature stickiness. Unfortunately, thereis no way to tell from the Kwikwraplabel which product your local distributor has in stock.The minimum application temperatures provided by flashing manufacturers vary from 10ºF to 50ºF (see “FlexibleFlashing Specifications,” next page).These recommendations should betaken as a guide, not a guarantee. Aninstaller can push the minimum application temperature somewhat by storing the flashing in a warm locationbefore use.In consistently low temperatures, thebest flexible flashing may be a nonstickflashing like Fortifiber Moistop or MFMFuture Flash. Since these products areattached with fasteners, stickiness is notan issue. If you need a cold-weather selfadhering flashing, it’s probably best tochoose either a butyl product or BakorBlueskin Weather Barrier, a rubberizedasphalt flashing that performs well atlow temperatures. In a pinch, any flashing can be held up with roofing nails.Compatibility problems. If you’reusing a flexible flashing anywhere nearan asphalt product, it’s best to choose arubberized-asphalt flashing, becausebutyl flashings are incompatible withasphalt products. “There are oils thatwant to come out of the asphalt,” saysJeff Winzeler. “The butyl will suck themup and lose its adhesive properties.”Tyco Adhesives’ instructions forinstalling one of its butyl flashing products, Polyken 627-35, warns, “Avoid contact with residuary asphaltic productssuch as coatings and other roofing products.” A Tyco representative confirmedthat its butyl flashings shouldn’t be incontact with asphalt roofing cement.Since Tyco promotes the product for useon roofs, where asphalt roofing cementis often found, installers must be vigilantto avoid compatibility problems.The jury is still out on whether butyltapes should be allowed contact withasphalt felt. “If you are talking about15-pound felt, there is not a lot ofasphalt, because felts are relatively dry,”says Winzeler. “You’ll probably havefewer issues with compatibility thanwith roofing cement. But until you test,you can’t be sure.” When TheresaWeston, a chemical engineer at DuPont,was asked whether DuPont’s butyl tape,FlexWrap, is compatible with asphaltfelt, she was noncommittal. “We’re stilltesting it,” she said.Rubberized asphalt is incompatibleFigure 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-inchwide adhesive band along one side of the flashing.JUNE JLC 2001

with some types of flexible vinyl, especially vinyl flashings that come in aroll. It doesn’t appear to have any compatibility problems with hard vinyl,like the vinyl used for window fins.Watch out for staining. Rubberizedasphalt, like other asphalt products, canstain some materials, especially vinyl.According to Bob Sims, customer service manager at Bakor, such staining,called plasticizer migration, occurswhen oils in the asphalt dissolve plasticizers in the vinyl. Since rubberizedasphalt flashings shouldn’t be leftexposed, staining is generally not aproblem. The siding or other materialused to cover the flashing usually hidesany stains.Installing Flexible FlashingOn most job sites, peel-and-stickflashings are installed without a lot offuss. Typically, the flashing is cut tolength, the release paper is removed,and the flashing is pressed in place byhand (Figure 7, page 54). But the easyway may not be the right way. Somemanufacturers recommend that substrates should be primed beforeinstalling their peel-and-stick flashing,and that pressure should be appliedwith a roller, not the palm of the hand.Is a primer necessary? Self-stickingflashings often adhere better to aprimed surface than an unprimed surface. Manufacturers that recommendpriming generally focus on concreteFlexible Flashing SpecificationsPrice perSquare FootThicknessAvailable Widths(inches)Min. ApplicationTemperaturePriming of Substrate Required?Roller RequiredMaximumfor Installation? Exposure TimeRubberized Asphalt/PolyethyleneBakor Blueskin Weather Barrier 0.8025 mils4, 6, 910º FRecommendedYes42 daysCarlisle CCW-705 Window and Door Flashing 0.4440 mils4, 6, 9, 1225º FOnly for concrete, masonry,and some exterior gypsumNo30 daysn/a40 mils12, 18, 24, 3625º FPriming may be necessaryYes25 and 40 mils4, 6, 9, 12, 3640º FNoNoDur-O-Wal Polytite PolyBarrierFortifiber FortiFlash 0.29Grace Vycor Plus 0.6125 mils4, 6, 9, 1225º FOnly for concrete and masonryNo30 daysn/a40 mils6, 9, 1240º FOnly for concrete and masonryNo30 daysn/aGrace Vycor Weather Barrier StripsMaster Wall Weather Stop Flashing TapeCover as soonas possiblen/an/a45º FYesYes42 daysMFM Sub Seal 0.4045 mils4, 6, 9, 12, 18, 3650º FOnly for concrete, masonry, and OSBYes45 daysMFM Window Wrap (polyester top) 0.3025 mils3, 4, 6, 9, 12, 1850º FOnly for "some wood compositionpanels as well as dirty,dusty or weathered surfaces"Recommended90 daysNEI Advanced Composite Homeseal 0.3230 mils4, 640º FOnly for concrete and masonryYes30 daysPolyguard WindowSeal 0.2420 mils4, 6, 9, 12, 18, 3645º FOnly for OSBNo30 daysProtecto Wrap BT20XL Building Tape 0.2420 mils4, 6, 9, 1245º FOnly for concrete, masonry,Dens Glas Gold, and some OSBsYes120 daysRidglass Kwikwrap 0.3940 mils4, 6, 9, 12, 3640º FNoRecommended30 daysSandell Presto-Seal 0.4240 mils6, 12, 18, 24, 3625º FYesYesn/aTamko Moisture Wrap 0.4540 mils4, 6, 9, 1230º FRecommended but not required forconcrete and masonry20 and 40 mils4, 6, 36 0.2140 mils4, 6, 9, 12, 16Illbruck Weather Barrier Tape 0.7341 milsMFM Peel & Seal 0.7350 milsPolyguard Peel ’N’ Stick 0.70Ridglass Kwiksilver 0.56Tremco Sealants Window/Door WrapW.R. Meadows Sealtight Air Shieldn/aNo limitationRecommendedn/aOnly for OSBYesn/a25º FYesNon/a2, 3, 4, 641º FOnly for porous or wet surfacesYesNo limitation3, 4, 6, 9, 12, 18, 3655 FOnly for OSBYesNo limitation45 mils4, 6, 9, 12, 18, 3645º FNoYesNo limitation60 mils2, 4, 6, 940º FOnly for concrete andsome gypsum sheathingsNoNo limitationRubberized Asphalt/Aluminum FoilJUNE JLC 2001

and masonry as the mos

paying more attention to the importance of keeping water out of walls. News of construction-defect . North Carolina, and the “leaky condo” . builders. One result of the focus on waterproof walls is the growing use of peel-and-stick membranes and other types of flexible flashing. The term “flexible flashing” is used to describe a .

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