Window Flashing Technique

Window Flashing

Windows and doors represent a thermal short circuit in the wall assembly. These openings or holes in walls also represent a connection point in the wall assemblies that needs to effectively and redundantly manage air and water intrusion for the life of the building. Flashing system integration is one of the key factors in creating a continuous drainage plane. A drainage plane sits behind the wall cladding. After the cladding, the drainage plane creates the second line of defense against wind, rain and other moisture-related weather events.

New windows typically have an integral nailing flange that must be sealed and flashed to shed water. Over the years, getting this detail wrong has been the cause of many leaks and many lawsuits. While there is still ongoing debate about the best way to flash a window with an integral nailing flange; peel-and-stick flashing tape or adhesive membrane is a commonly accepted approach to ensure the continuity of the drainage plane when a weather resistive barrier (WRB) is part of the wall assembly. Adding an intentional airspace between the WRB and cladding materials; more commonly called a Rainscreen, is something we will discuss in other articles and it’s impact on providing an even greater safety margin.

In this article and animation the focus is on the installation of a system that relies primarily on the two layers of metal flashing above the window to effectively catch and drain or redirect water away from and out of the assembly. The additional layers act as a back-up or redundant system to effectively manage water and air intrusion. In my travels over the past years throughout Northern Europe and Canada it was apparent that simple tried and true techniques, such as sloped metal flashings can provide a long term foundation to water management techniques. Installing butyl adhesive flashings and high performance sealants on window and door installations provide great starting point, but are reliant on the long-term performance of the materials and installation consistency. Properly integrating metal flashing into the mix can provide another redundant layer of weather protection into the system.

window1Detail 1

Flashing Steps

As shown in this animation, the first step is to cut an opening in the WRB and fold the sides back into the framed opening. Next, make angled cuts at the top corners to create a flap and fold up, in order to install the metal drip cap directly onto the sheathing.

Add a monolithic pan flashing system at the rough opening sill to drain any water that may get into the window opening from a window leak or other sources out of the opening and back onto the drainage plane. The sill pan needs to cover the exposed face of the sill extending up the vertical jambs of the rough stud opening and extend over the WRB to ensure a continuous seal.

Next, apply a bed of sealant or caulk to the window flanges at the head and sides, leaving the bottom free to drain (Detail 1).  If sealant is preferred at the bottom flange be certain to leave 3” gaps in the sealant every two feet for drainage. Set the window into the opening plumb level and square with nails installed in the fastener holes provided.

Install the window over the WRB and sill pan, and cover the vertical flanges and the house wrap adjacent to the window unit with peel-and-stick butyl adhesive tape. These vertical strips must extend past the head of the window so the ends will be well-covered by metal flashing, butyl head tape and the flap of WRB that will go over the top flashing.

When you install any butyl flashing tape, lap the flashing layers shingle style – always installing upper flashing layers over lower ones. All flashing tapes should be applied to clean and dry surfaces and a great tip is to use adhesive primers in any situation where adverse weather conditions exist. All flashing tapes should also be rolled or pressure applied to them between 30 and 40 pounds per square inch for proper adhesion. This is  key to long-term, continuous protection of the adhesive flashing systems. Remember there is also sealant behind the window flanges and if all else fails the pan flashing will act as the last line of defense to properly drain the water from the window opening.

window2Detail 2

The bent metal flashing above the window has a few interesting features. It has a back leg of greater than 4 inches, a slope on the flashing to shed water with a drip edge on the bottom extended leg. It also was cut to the width of the window plus the vertical trim with end dams.(Detail 2) The end dams force water to run out and away from the siding and trim and not off the sides of the metal flashing. The top flashing is caulked in two places with a bed of sealant: 1).Along the horizontal edges that extends up the sheathing and 2). ‘L’ where the drip cap meets the head of the window unit. The drip cap is installed directly on the sheathing and sealed with a strip of butyl flashing tape. Drop the flap of WRB and seal with acrylic tape, but leave a few small 1” gaps in the tape to allow water to drain out in the event that it ever gets behind the WRB.

window3Detail 3

Install the top trim and then the metal drip cap. Seal the drip cap to the house wrap with acrylic tape. (Detail 3) Do not caulk any horizontal joints above or below the window or any flashing (between the trim and the flashing). This allows these areas to shed water to the layer below, while caulking will trap water instead of allowing it to drain. This window install method takes a little longer, but will help protect the home from the damage of water intrusion.

2016-10-18T21:52:36+00:00 Technical Articles|