Check the rough opening
Measure the width of the rough opening at the top, middle, and bottom and the height at both sides and in the middle.
If the difference between the three width measurements or the three height measurements of the rough opening is more than 1 inch, cut tapered filler strips from stud stock and nail them to the sides of the opening that are out of level or out of plumb.
Make sure the outside dimensions of the window are at least ¾-inch narrower and ½-inch shorter than the smallest width and height measurements, respectively. If they're not, you'll have to either reframe the opening or order a new window.
Protect against water infiltration
Cut a 6-inch-wide strip of self-adhering waterproof membrane (or a 9- to 12-inch-wide strip of 15-pound builder's felt) 18 to 24 inches longer than the window's width. Center the membrane under the rough opening and adhere it to the existing builder's felt or house wrap. Make sure its top edge doesn't extend above the edge of the opening.
Cut two more strips of membrane (or felt) 1 foot longer than the height of the opening. Center and attach them along each side of the opening, overlapping the strip under the window.
Cut another strip of membrane (or felt) 1 foot longer than the window is wide; center and attach it across the top of the rough opening so that it overlaps the two side strips.
Install the window
Fold out the window unit's nailing fins so they are perpendicular to the sides of the window frame. Then set the window's sill into the bottom of the rough opening, and tip the frame into the opening until all the nailing fins are tight against the wall.
Have an assistant stand inside and tell you when the gaps between the sides of the window and the jack studs are equal on both sides. Tack the nailing fin to the sheathing at one upper corner with a 1 ½-inch roofing nail, but do not drive it all the way in.
Level the window
Place a 2-foot level on the windowsill, and note its high side. Then hold a 4-foot level against the window jamb on that side, and shift the sill left or right until the level shows the jamb is plumb. Tack a nail into the fin at the lower corner on the same side as the first nail.
Next, lay a 2-foot level on the sill, and adjust the free bottom corner up until the sill is leve. Tack the fin in this lower corner to the wall.
Check the window for square
Double-check that the window is square by measuring the frame diagonally from corner to corner. Measurements should be within 1/16 inch. If not, recheck the frame's side for plumb and the sill for level. You may have to pull out the last two temporary nails and adjust the frame.
When the sill is level and the frame square, drive home the nails at each corner. If the windowsill is level and corner-to-corner diagonal measurements are identical, the sides of the window are plumb.
Holding a tape measure horizontally, measure the width of the window at several places to make sure the side jambs aren't bowed. If they are, push the frame in or out at the center of the bow and nail the fin at that point. Then nail the sides, top, and bottom of the nailing fin or casing, driving one nail through every other pre-punched hole.
Seal the perimeter
Cut a strip of 6-inch-wide waterproof membrane 1 foot longer than the window is wide. Center it under the window and adhere it to the wall so it covers the bottom nailing fin.
Cut two more strips of membrane 1 foot longer than the height of the window and repeat the above process on both sides of the window. Make sure each strip's lower end overlaps the strip under the unit.
Flash the top
If the manufacturer has supplied snap-in metal flashing to cover the top of the window frame, apply a bead of caulk to the top edge of the window casing, then press the flashing in place.
If no flashing has been supplied, cut and bend a piece of metal flashing so it overhangs the front and sides of the casing by 1/4 inch and extends 3 inches up the wall. Fasten the top edge of the flashing to the wall with 1 ½-inch roofing nails and cover it with a strip of waterproof membrane long enough to cover the top ends of the two side strips.
Insulate against drafts
Fit the sash into the window frame.
Inside the house, apply a single thin bead of minimally expanding polyurethane foam to the gap between the window and the framing. Allow the bead to expand and cure for one hour before adding more. Repeat until the cavity is completely filled.
When using fiberglass insulation, cut unfaced batting a little larger than the space between the window and the framing, and push the batting in with a putty knife. To prevent air leaks, cover the gap with aluminum tape.
If the gap is too narrow for either foam or fiberglass insulation, seal it with a bead of caulk.
TIP: Do not fill the gap between jamb and framing with too much foam too quickly. Otherwise, the jamb could bow and thereby bind the sash.
