The water, vapor, air and thermal control layers have to be wrapped into the rough opening and terminated into the window. The two primary means of achieving this are through the use of tapes, such as the Dow WeatherMate Flashing, or through the use of liquid applied flashing, such as the Prosoco Fast Flash. The objective is to prevent any water or air from penetrating into the house around the window, even with wind driven rains or when the windows themselves leak.
When using flashing tape, a clean dry solid surface is a must. The flashing tape does not stick to old dried out lumber found in historic homes. The tape sticks easily to the surface of the insulation, but not to it’s cut edges. The flashing has to bridge between the face of the insulation and the rough opening. The tape has to extend past the inside finish edge of the window so that any leaks through the window itself drains out to the exterior and down the outside of the insulation. The bottom of the rough opening should be pitched. A piece of beveled cedar siding works well for this. Using the thin edge will keep the profile low.
Apply the flashing along the bottom of the rough opening, staying an inch or two from the corner. Then use a piece of flexible flashing, overlapping the first piece by two inches and run it up the side four to six inches. Apply straight flashing up each vertical side, overlapping the corner piece by at least two inches, and terminate it two inches from the top corner. Put a piece of flexible flashing in each of the top corners, overlapping the straight flashing on the sides by two inches. Apply a piece of straight flashing on the top of the rough opening, overlapping the edges by two inches. Apply a piece of Dow Weathermate Construction Tape over the top edge of the flashing tape and onto the face of the insulation. This will keep the leading edge of the flashing tape from peeling away over time. The only joint of the flashing assembly that is counter flashed, and thus a potential leakage path, is where the top flashing pieces terminate on the front surface of the insulation. The piece of sheathing tape provides extra protection of this joint for long term durability.
Sealing the fenestration using a fluid applied membrane:
Voids should be sealed up with a backer rod. Place a bead from the caulk gun along crack or joint. Position multiple beads of Fast Flash around the four sides of the rough opening. Using a plastic putty knife, smooth the Fast Flash into a uniform layer, overlapping the front of the insulation two inches and extending to the back of the rough opening past where the inside face of the window will be. Let the Fast Flash setup before installing the window, to prevent smearing the window with it.
I recommend only using liquid applied products, such as the Prosoco Fast Flash and Air Dam to seal the rough openings and windows. They are easy to install and provide a seamless drainage plane that can be wrapped into the fenestration and join the window and the external drainage plane of the insulation. The Prosoco Air Dam provides a reliable interior air and water seal that windblown rain will not penetrate.
We found that tape applied drainage planes are hard to install, and they are also expensive. They require multiple steps that must be completed in the correct order. They do not stick to old wood, dirty surfaces, or friable surfaces.
We found that the Prosoco products were easy to use and took much less time and skill to properly install. The only issue was that they were a bit messy to install, and any that gets onto clothing will not wash out. Plan on letting the Fast Flash set for a day before installing the window to prevent accidental smearing.