I've been installing replacement windows in my house and see the instructions to drill through the outer inside face of the trim to put screws laterally into the studs and sill and top header. The windows come with caps to cover the holes after the screws are put in. These windows come with nailing flange with holes every 4 or so inches and that's what I've used to secure the window. Before installing the windows I've added 2" of EPS foam-board insulation and on top of that 5/8 sheet of OSB. The nailing flange is screwed (4" screws) into the OSB (and finding the studs and sill). Is there a problem with not using lateral screws from inside the window trim to the studs? Seems cleaner not doing this, the window isin't going anywhere with all the nailing-flange screws. The lateral screws likely wouldn't hit wood – instead they'd likely hit the EPS insulation.
Installing windows – screws laterally into studs vs nailing flange
windows
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Yes put a nail into every hole. I would recommend galvanized nails that are 1 to 2 inches in length. If there is nothing behind the flange but plywood, anything over an inch and the nail will just be sticking through the plywood.
I assume you will hit some framing behind the playwood, so a little longer and you will hit that. I looked over my new vinyl windows in my sunroom and there are no attachment points on the inside of the window.
I don't recall doing anything on the inside of the window when installing. The most important point I remember was with the flashing on the outside. Start at the bottom, then the sides, then the top so that water can be properly channelled away. There is a video below in the comments showing an installation. I remember it was pretty straight forward, just made sure the windows are level.
Just get replacement windows instead of new construction and use tapcons to fasten the windows through the sides. Caulk outside good to keep out water. Foundation should also be graded away from the window to keep out water.
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Best Answer
You are probably correct that the nailing flange will hold the windows in place.
However, if there is ever a warranty claim, there's a very high likelihood that they'd deny it if you don't put the screws through face trim as they instruct. Heck, it's possible they'd try to deny the warranty because you've used screws (as I likely would) instead of nails through the nailing flange.
i.e. Failure to follow installation directions to the letter = failure to honor warranty.
Whether you choose to follow the instructions or take your chances on the warranty is your decision.