With windows being the largest holes in a buildings exterior, they will require both concealed and in some cases exposed flashing as well. Without flashing the windows, the wall cavity could become exposed to weather and moisture. Each type of window will have different guidelines to follow in terms of how to install their windows and maintain warranty, but the general premise is the same for all. The following is the general procedure for flanged windows being installed on a wall assembly with house wrap. The rough opening is prepared and the house wrap is cut to suit. This often consists of cutting an X in a manner that will allow the house wrap to wrap the jambs, sill, and head in preparation for the window.
The housewrap is then wrapped inward and attached to the rough jambs and sill. The housewrap at the header is left loose however as it will be placed overtop of the windows flanged piece at this location. Next self-adhering asphalt or butyl based flashing tape will be added, which will need a spray adhesive boost in cold weather. Asphalt-based tape may not be compatible with some types of vinyl. Also, butyl tape can be easier to mold and place. This tape limits moisture penetration through the rough jambs and provides a solid, smooth substrate for the window installation. Sometimes it is available with the window or a type of tape is specifically recommended by the window manufacturer.
A +/- 1/2″ caulk/adhesive bead is typically placed on the outer surface of the wall just outside the jambs and header to receive the window. No caulk is placed at the sill to avoid from trapping water within the pan flashing.
ASTM C-90 rated caulk is most commonly utilized for window applications. The window is then placed, leveled, and nailed with roofing nails. With the top of the window being the most vulnerable moisture penetrable location, ‘pan-flasing’ or a drip cap is often placed, especially if the window protrudes from the face of the wall. This cap my come with the window or can be made with aluminum coil stock. Then a second layer of flashing tape is installed over the window flanges, sandwhiching them everywhere but the sill where no second layer of tape flashing is required.
The flashing tape at the header will be installed directly on the wood sheathing or header wood, with the housewrap then being placed over top. One more layer of flashing tape is installed here sandwhiching the housewrap.
If the window flanges are integral the flashing tape can butt all the way up to the window. A non-integral flanged window is installed similarly but may need additional caulking to seal joints.