Great windows are only great if installed well
A good window installation converts a hole in the wall into an integrated part of all three important barriers that make up a wall assembly: the air, thermal, and water barriers.
Because all windows leak at some point, rough openings need to be designed to handle water entry. The rough sill should be flashed with either a pre-formed manufactured sill pan or a site-built pan.
This detail shows a site-built sill pan installed in a wall sheathed with rigid foam. The outside face of the foam is the wall’s drainage planePath that water would take over the building envelope. Concealed drainage-plane materials, such as building paper or housewrap, are designed to shed water that penetrates the building’s cladding. Drainage planes are installed to overlap in shingle fashion (weatherlap) so that water flows downward and away from the building envelope..
For more on windows:
GBA Encyclopedia topic page
Video Series: Flashing a Window in a Foam-Sheathed Wall