The windows generally drop in the rough opening from the outside and are held in place from the inside with a clamp ring. Screws spaced about 4 inches apart screw through the clamp ring into a track running around the inserted window, hence "the clamp". You prevent leaks
by bedding the exterior frame with butyl caulk. Avoid using silicone. If you can't make it tight, say because your clamp ring was meant for walls 2 inches thick, you must insert some wood shims, or buy a different clamp ring.
- If no clamp ring is included you'll have to try ordering one from the manufacturer, like I had to, so watch out for that very big expense.
- Manufacturers will not make you a custom clamp ring for "emergency exit windows."
If the window is flat and the egg wall is curved, you got a problem to solve. Length of the sheet metal screw needed will depend upon the depth of the clamp ring and the wall thickness. Drilling into that track allows use of longer screws when necessary. If this doesn't help, find my post, Installing Windows.