We in the Maritimes live in a pretty wet place. For that reason, it’s vital to properly seal wall openings properly during construction.
First off: it’s absolutely vital to note that it is not sufficient to nail a flanged window in place, and use red tape to seal it. Not only is this not Code-compliant, it is very poor practice. The Atlantic Home Warranty organization reports that leaks around windows are a major cause for claims made - and that's due to poor installation.
That’s quite a grim thing to ponder given that proper use of some of the many modern flashing materials virtually eliminates any risk of water intrusion.
Here are some basic steps in properly installing a window.
1) trim the housewrap. Most installers will cut an “x” in the window opening, then pull the material into the home, staple to stud frame, and trim.
2) install a flashing pan. If, by chance, there is any water intrusion, it will most usually collect at the bottom of the window. For this reason, best practice is to seal this off with a flashing tape product by creating what’s called a “flashing pan,” that seals the bottom of the sill, as well as the sides to about eight or so inches. The idea is that any water collecting in the bottom of the window cannot penetrate the sill. What’s flashing tape? There are several brands of flashing tape on the market, but in general they are strips of semi-flexible synthetic material, six to eight inches in width that can be creating a flexible, entirely waterproof seal.)
The best practice is to ensure the finished sill pan slopes to the exterior. This can be done by placing the flashing pan over a slender piece of wood ripped from dimensional lumber – about 1/8” - to create a slope, using a double layer of tape, or even laying down a line of string: whatever keeps any water from going into the home.
3) Install the window. Most windows these days have nailing flanges, making installation quite easy.
4) Seal the flanges to the housewrap. This is done by using flashing tape, again, first on the sides, and then over the top, so the top layer overlaps both sides. This seals the window flange, and ensures that any water has no point of entry.