It’s best to review manufacturer instructions before installing vinyl windows. However, the following procedures are generally accepted practices to prevent leaks and callbacks:
To start, be sure to size the rough opening according to the manufacturer’s instructions. Most specify an opening 1/2 inch larger than the window frame. Then center the window, left to right, leaving a 1/4 inch gap on either side. It’s also best to install the window on a continuous level base, avoiding shimming the sill. Still, some installers double up the rough sill by using stacked 2x4s or 2x6s. This permits easy shimming between the two sills.
Next, install a flexible flashing around the perimeter of the window opening. Made from a variety of materials like polyethylene or rubberized asphalt, flexible flashing generally comes in 9 inch rolls. It’s important that the flashing at the side of the rough opening extend over the flashing at the bottom. Also, lap the flashing in the proper sequence to ensure it conveys moisture away from the sheathing.
When fastening the nailing fins, do so according to the manufacturer’s instructions. Be sure not to puncture the nailing fin with extra holes when installing housewrap, exterior trim, wire lath, or siding. Either way, fasteners should be held back from the nailing fin.
To allow for expansion, leave a 1/4 inch gap between the siding and the window. If you’re installing in cold weather, the gap might need to be a little larger, since the window will be at its smallest dimension. The opposite is true in hot weather. In general, it’s best to avoid cold weather installation. A hammer can easily shatter the brittle vinyl nailing fins in frigid temperatures. If the temperature is below 20°F, and you must install the window, use flat-head screws instead of nails.