Water intrusion around window and door openings can cause structural degradation and fastener corrosion that weakens the window or door frame or even the wall itself. Water intrusion can also lead to preventable building damage such as water damage to interior finished surfaces, furnishings, and mold growth.
Like the roof, walls and openings in walls require redundant barriers to shed water regardless of the type of exterior cladding and trim used—or the direction, frequency, and duration of the wind and rain. Lapped flashing will direct the flow of water that naturally penetrates behind some cladding, like vinyl, downward and away from the window or door. Flashing at the side and head of an opening is usually installed over sheathing and a weather resistant barrier (WRB) after the window is installed. The window flanges also act like flashing. Pan flashing, installed at the window sill or door threshold, goes in before the component (i.e., the window or door). Manufacturers will provide recommended flashing instructions for their product(s).
Caulks, adhesives, and sealants are another way to seal abutting joints, like trim and window frames, from water penetration. Durability of these depends on the elasticity of the sealant, the size of the gap, and the movement of the abutting components. Salt and sunshine (ultra-violet rays) can deteriorate caulk and prematurely degrade the seal. Because of this, sealants are best employed as the secondary barrier and should be regularly inspected and maintained, where accessible. Window and door manufacturers will often specify that a bead of caulk be placed behind integral flanges and molding in prehung door assemblies as an additional precaution against water intrusion.
All window and door replacement scopes of work should include new flashing at the opening. The materials and work may require an additional $8 - $15 per window.