Peel and stick flashing tape or membrane:
This article describes the selection and installation of peel-and-stick flashing membranes used on building exteriors to seal housewrap joints and to seal against air or water leaks around windows, doors, or other openings. Our photo at page top shows Typar® flashing tape installed above a new window. A better practice laps the housewrap over rather than under the stick-on flashing tape, a better detail but one omitted by many builders.
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What is the best way to flash around windows & doors and how should flashing tape be used at these openings? This article shows best practices, common practices, and some not-so-good practices using peel-and stick flashing tape around windows, doors, and other building wall penetrations.
This article series discusses best practices construction details for building exteriors, including water and air barriers, building flashing products & installation, wood siding material choices & installation, vinyl siding, stucco exteriors, building trim, exterior caulks and sealants, exterior building adhesives, and choices and application of exterior finishes on buildings: paints, stains.
Portions of this article are adapted/paraphrased with permission from Best Practices Guide to Residential Construction by Steven Bliss, courtesy of Wiley & Sons.
Typically ranging in width from 4 to 12 inches, these peel and- stick membranes greatly simplify the task of creating a continuous barrier to water entry around doors, windows, decks, and other problem areas. Flashing tapes are faced with reinforced polyethylene or foil on the outer surface and a peel-away paper on the adhesive surface.
The foil faced products may be left exposed to the weather permanently, whereas the plastic-faced tapes should not be exposed to sunlight and weather for more than 30 days (longer for some brands) since UV radiation will degrade the facing.
Most flashing membranes are made from modified bitumen, the same rubberized asphalt used in eaves flashing. Some use a more expensive butyl rubber core, which stays more flexible in cold weather and is more stable at high temperatures. Butyl products also bond better to difficult substrates than modified bitumen and can be peeled off and adjusted during installation.
A unique butyl-based flashing tape from DuPont, called Tyvek FlexWrap®, has a wrinkled facing that allows it to be molded easily to irregular shapes such as the head flashing of round-top windows. It can also be bent to create a pan flashing at window sills without any cutting and folding at the corners. Despite the higher material costs, labor savings make this product appealing for tricky applications.
These products offer several distinct advantages over metal flashings: They are easily bent or molded for an accurate fit, can accommodate settlement and shrinkage movement, are self-sealing around nail holes, and bond well to a variety of materials, including metal, wood, plywood, and vinyl window flanges.
Flashing tapes provide long-lasting waterproof protection if installed correctly. Oriented-strand board (OSB), concrete, and other masonry materials, however, can be problematic for some of the rubberized-asphalt flashings and may require priming for a good bond. Consult with the product’s specifications for compatible surfaces and priming requirements.
To obtain the best results with these products and be protected by the manufacturer’s warranty, it is advisable to follow the manufacturer’s recommendations. These vary from product to product, but generally they address the same issues: application temperature, priming, installation techniques, and compatibility with surrounding materials.
In general, the rubberized asphalt (modified- bitumen) products start to lose stickiness at around 50°F and will not bond much below 40°F. Unless you are working with a rubberized-asphalt product specifically formulated for low-temperature applications, a butyl-based product is a better choice in cold weather.
Very high temperatures can also be problematic for rubberized-asphalt membranes. When subjected to high temperatures and pressure, for example, when squeezed under a dark-colored metal flashing exposed to direct sun, the material will soften and begin to flow. Unless formulated for high temperatures and labeled “hi-temp,” most modified bitumen will begin to soften between 185°F and 210°F. High-temperature formulations can tolerate up to around 240°F, but are generally not as sticky.
Each manufacturer specifies which products are safe to bond 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 dust. Some manufacturers of rubberized-asphalt tapes recommend that all materials be primed for best performance, particularly in cold weather.
Most require that concrete and masonry be primed, and some require the priming of OSB and gypsum sheathing as well.
Many published details show asphalt-rubber flashing tapes bonded to asphalt felt and plastic housewraps. While these are rarely listed as suitable substrates in product literature, manufacturers of flashing tapes claim that their products will bond satisfactorily to both these materials as long as they are clean.
Do not expect a good bond to dirty housewrap that has been exposed to the weather for a month or to any dirty job-site material. For that reason, it is always best to detail flashings and to layer materials so that they shed water even if the adhesive bond fails.
Rubberized-asphalt flashings should not be in direct contact with flexible vinyl flashings. 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. Rubberized-asphalt flashings should also not come into contact with any caulks or sealants unless specifically formulated for that use. Like soft vinyl, sealants may react with the asphalt, causing it to flow and stain the adjacent materials, such as window flanges.
Butyl-based flashings are compatible with most construction 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, rubberized-asphalt is a much better choice.
Watch out: FortiFlash, for example, is not compatible with EPDM [rubber roofing] or flexible (plasticized) Polyvinyl Chloride (PVC) based products. FortiFlash and Moistop E#-Z Seal are not compatible with some sealants. Consult with sealant manufacturer for compatibility information.
Flashing tapes must be pressed firmly into place to ensure full contact and a good bond. Some manufacturers recommend using a hard rubber roller for best results.
While most flashing tapes are used around doors and windows, they can be put to good use wherever water penetration is an issue. Other applications include band joists, deck ledgers, inside and outside corners, and any areas subject to frequent wetting.
On wall areas adjacent to a deck or abutting a roof, for example, where splashback or snow buildup is likely to wet the siding, sections of membrane up to 36 inches wide can protect wall assemblies. Make sure to lap all layers of flashing, sheathing wrap, and adhesive membrane so that water is directed to the outside of the building, even if the adhesive bond fails.
Watch out: In cold climates, covering an entire wall section with waterproof membrane will create a cold-side vapor barrier, potentially leading to serious moisture problems and wood decay within the wall cavity. A section of membrane up to 3 feet wide, however, is unlikely to cause problems.
Can someone please clarify this issue? Most everywhere I've read about window flashing tape, it states to never apply it to the bottom of the window on the outside so water can exit to the house wrap instead of under the house wrap.
My siding crew taped the bottom of the window like shown in this article but I'm less worried about it because there's an open flap of house wrap below the window where water can eventually exit. Also, I insisted on a rain screen and the house wrap should let water escape so it should get stuck inside. Either way, I don't think taping the bottom is hardly ever recommended.
[Click to enlarge any image] This photo of flashing tape installed below a window and over the window's bottom flange or fin does not show the very best installation practice. Photos by D Friedman & Eric Galow, Galow Homes.
You're quite correct. The photo showing peel and stick flashing tape applied over the window bottom flange and onto the building WRP or housewrap is not a recommended detail and might trap water in the window rough opening. Please see complete details at WINDOW & DOOR FLASHING TAPE DETAILS - separate article
Fortifiber Building Systems Group
1-800-773-4777 for sales and technical assistance.
Fortifiber flashing tape manual, [PDF] retrieved 2016/09/13, original source: http://www.fortifiber.com/pdf/install_guides/IG_window_flashing_method_a1.pdf
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