Flashing Membranes

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Flashing membranes are now widely used to prevent water leakage around windows and doors and other trouble spots on walls. Sold as flashing tape, flexible flashing, and peel-and-stick window flashing, these narrow rolls of rubberized asphalt membrane can solve many flashing problems if installed correctly. Non-adhesive versions of the same material are also available.

This is the same peel-and-stick membrane used for over 40 years to prevent roof leaks at the eaves and other roofing trouble spots. The original membrane flashings,  introduced by  Grace (now GTP) in 1978, were called “eaves membranes” and were 36 in. wide, designed to prevent leakage from ice dams along the lower edge of the roof. In the 1990s, manufacturers started offering similar materials in narrower rolls, typically 4 to 12 inches, for use on wall (see Photo).

The narrow rolls of membrane were introduced as a solution to water leakage around window and door openings and other trouble spots. Window flashing methods that worked reliably in older homes did not work well with the advent of plastic sheathing wraps, flange-style windows, rigid foam insulation, and a host of modern building techniques. The problems were hidden from sight for a few years, but came to light dramatically with a number of high-profile lawsuits by condo associations and owners of commercial buildings.

Sadly, owners of single-family homes often had to take it on the chin and pay out of pocket to replace or repair windows, trim, and the surrounding structure long before their normal service life. Trying to file a warranty claim with a contractor or window manufacturer typically lead to a classic finger-pointing exercise where each party blames the other and you’re left holding the bag.

While the peel-and-stick bituminous membranes are not a cure-all, they have proved a useful tool in the ongoing battle against water intrusion in modern homes. They simplify the task of creating a continuous barrier to water entry around doors, windows, decks, and other problem areas like the intersection of walls with roofs, decks, other building components. Unlike metal flashings, the membranes can be easily shaped or folded to protect areas that in the old days might have used custom lead or copper flashing with solder joints – not an economical option for most projects today.

Another potentially big advantage is that they can form a waterproof bond to a wide variety of materials including metal, vinyl window flanges, wood, OSB, and even plastic sheathing wraps (see Photo).

In theory, this solves a multitude of flashing problems around windows. In addition, the materials self-seal around nail holes, a property that has made them especially useful on roofs.

But do they stick to everything? In practice, most experienced contractors do not rely on this bond to last forever – especially to OSB, wood, and plastic sheathing wraps, materials that are notoriously difficult to bond to. Some peel-and-stick manufacturers provide specific requirements for priming of OSB, masonry, and other materials before bonding the flashing. And there may be cautions in the fine print about application conditions such as temperature, moisture levels, dirt on surfaces, and the pressure the installer needs to apply for a good bond. Cold temperatures and wet or damp surfaces are a problem for most tapes. When in doubt, wait until it is dryer and warmer and use a primer on OSB or plywood.

Running a hand pressure roller over the tape will provide a better bond than simple hand pressure. The best approach, in all cases, is to not rely on the bond lasting forever, but to apply all flashing materials lapped to shed water – following the shingle principle.

Types of Flashing Tapes

Most membrane flashings come in rolls from 4 to 12 inches wide. The body is a rubberized asphalt material, faced on the outer surface with reinforced polyethylene or foil. The adhesive side is protected by a peel-away release paper. Some products, such as Vycor, have a cord to pull that lets you peel one half of the release paper at a time – very handy with wider rolls because the stuff is extremely sticky and may be hard or impossible to reposition.

The foil-faced products can be left exposed to the weather, whereas the plastic-faced flashing membranes should not be exposed to sunlight and weather for more than 30 days (longer for some brands) since UV radiation will degrade the material.

Modified Bitumen vs. Butyl. Most flashing tapes are made from SBS-modified bitumen, the same rubber­ized asphalt used in eaves flashing on roofs. These have a distinctive asphalt odor.

Most manufacturers also offer a more ex­pensive butyl-rubber flashing with a butyl-based adhesive. Butyl flashings are easier to apply in cold weather and are more stable at high temperatures, making them less likely to ooze out and stain materials.  Butyl flashings also bond better to difficult substrates than bituminous membranes. Also, because the butyl adhesive sets more slowly, the installer can peel off and adjust the tape dur­ing installation, a nice feature when you need it.

The higher performance of butyl comes with a higher price tag —  typically about twice the price of bituminous flashing tapes.

Moldable Flashing. Dupont introduced the first moldable flashing tape, called Tyvek FlexWrap, with a wrinkled facing that allows the material to be easily molded to irregular shapes. Also called “flexible flashing,” this proved to be a popular solution for curved head flashings above round-top windows and the “pan flashing” that lines the framing at the bottom of windows and doors (see Photo).

Other manufacturers, such as James Hardie (HardieWrap Flex Flashing) have followed suit with similar moldable products. Despite higher material costs, labor savings and simpler installation make these flashing tapes appealing for tricky applications.

Installation of Flashing Membranes

To obtain good results with flashing membranes and maintain warranty coverage, it is critical to follow the manufacturer’s installation instructions. These vary from product to product, but generally they address the same issues: installation techniques, application temperatures, priming of certain materials such as masonry and OSB, and compatibility with substrates (what your sticking to) as well as surrounding materials that come into contact with the membrane.

Cold and Hot Temperatures. In general, the rubberized asphalt (modi­fied-bitumen) products do not work well in cold weather. Most become less sticky at around 50°F and will not stick well below about 40°F. Unless you are working with a rubberized-asphalt membrane formulated for low-temperature usage, a butyl-based membrane is a better choice in cold weather.

High temperatures can also be a problem for rubberized-asphalt membranes. At high temperatures and pressures, for example, when squeezed under a dark-colored metal flashing exposed to direct sun, standard modified bitumen will soften and begin to flow. The flashing will deteriorate and leave dark stains where it oozes out of the joint in the roofing or siding. In hot climates, roof temperatures often reach 150°F or higher.

In general, butyl membranes are more stable at higher temperatures, but also have upper limits. Unless specially formu­lated for high temperatures and labeled “hi-temp,” flashing tapes begin to soften at somewhere between 120°F and 180°F. Some high-temperature formulations made for commercial applications can tolerate  temperatures over 200°F, but are generally not as sticky and may be difficult to locate. If the manufacturer does not publish the upper-limit “service temperature” contact them for this information.

Substrates. Each manufacturer specifies which products are safe to stick to and which require priming. Solid wood, plywood, vinyl window flanges, and metal are usually fine as long as they are free of oil and dirt or dust. Most manufacturers recommend that concrete and masonry be primed, and some require the priming of OSB and gypsum sheathing (used mostly in commercial building) as well. Some manufacturers recommend that all substrates be primed for best performance of flashing membranes, especially in cold weather

Windows and door details. Flashing around windows and exterior doors is number one use of flashing tapes. There are different opinions on which is the best detail, but all follow the general approach shown here.

Some manufacturers of flashing tapes claim that their products will bond satisfactorily to both plastic sheathing wrap and asphalt felt (tar paper) as long as they are clean, but few guarantee this in their product literature.Do not expect a good bond to dirty plastic housewrap that has been exposed to the weather for a month or to any dirty job-site material. Even under ideal conditions, however, (rarely the case on building sites) many builders know that the bond can fail over time. The best approach is to lap materials, following the shingle principle.

Follow the shingle principle. Given all the factors that can affect the longevity of an adhesive bond, it’s best not to rely on on a taped joint to keep water out of your home’s exterior. Every flashing detail, adhesive or not, should follow the age-old “shingle principle.” In this approach the upper material is always lapped over the lower material so water will naturally flow down and away from the building structure, even if the adhesive bond fails. This is how materials like roof shingles, cedar shingles, and horizontal sidings work – they shed water naturally. Peel-and-stick flashings still simplify many flashing joints, but they are not magic.

Compatibility. Modified-asphalt flashing membranes should never be in direct contact with flexible vinyl flashings or vinyl siding. The as­phalt compound will draw the plasticizers out of the vinyl, causing the asphalt to soften and flow and the vinyl to become brittle. The rigid polyvinyl chloride (PVC) used in window flanges, however, is generally not a problem.

It’s also best to not apply caulks and sealants in direct contact with the modified-asphalt membranes, unless they are specifically formulated for that use. Like soft vinyl, the chemicals in sealants, including silicones and polyurethanes, may react with the asphalt, causing it to flow and stain the adjacent materials, such as window flanges.

Butyl-based flashings (as opposed to the more common modified-asphalt flashings) are compatible with hard and soft vinyl and most caulks and sealants, but they should never be in­stalled in contact with any asphalt-based products such as roofing cement or bituminous flashing membranes. These may degrade the butyl and undermine its ability to seal. In these applications, modified-asphalt membranes are a better choice.

If you plan to stick the flashing tape to asphalt-saturated felt (tar paper), modified asphalt membranes are the natural choice, although some manufacturers of butyl tape approve of this detail. The question is whether there is enough asphalt in the felt paper to cause problems with the butyl adhesive. It’s best to check with the butyl tape manufacturer before proceeding.

Applying Pressure. You can’t just slap on the flashing membrane and expect it to stick forever. All adhesive membranes must be pressed firmly into place to ensure full contact and a good bond. Most manufacturers recommend using a hard rubber roller for best results.   .

Caution: Cold-Side Vapor Barrier

Three-foot-wide sections of flashing membrane have been installed on roofs for decades with few problems. In snow country, some builders routinely cover the entire roof with a flashing membrane membrane, especially on low slopes. Some builders use a similar wide band of membrane along the bottom of walls to protect against splashback of rain and snow buildup.

On walls, however, overuse of membranes can lead to serious moisture problems (see Photo). In cold climates waterproofing membranes on the wall exterior create a cold-side vapor barrier. Any water that gets into the wall by leakage or condensation will be very slow to dry out, potentially leading to wood decay in the wall structure.

In most cases, a section of membrane up to 3 feet wide is not likely to cause problems. However, if the wall has an imperious vapor barrier, such as polyethylene or foil, on the interior, I would avoid any large areas of membrane on the exterior.