Last week, I explained how to remove your old steel casement windows as you prepare each opening for the new Vinyl windows. In most of the country, you are limited to a replacement style frame, which is a new construction frame with the nail fin removed. Remember, when you removed the old casement window, you left the perimeter frame in place. So, you have a lip protruding into the opening that is approximately 1/2" wide. You need to order your replacement style frame to fit inside of this old frame.
Measure the width from lip to lip and deduct 1/4 to 3/8". Measure the height and deduct 1/4". When you install the new window, rest the new frame on the bottom lip of the old frame. Leave the front of the new frame about 1/8" further outside than the old frame lip. Drive a screw in the top center to hold it in place. Then, make sure the window is perfectly upright before installing a screw in the bottom center. Now you can secure the rest of the window with screws.
At this point you should have a replacement window that is approximately 5/8" away from the left and right wall, 1/2" from the bottom, and 3/4" from the top. You now need to insulate and trim all four sides. You should get a trim that will adhere to the face of the window frame on the outside, then go over to the wall from which the lip is protruding on all four sides. However, before applying the trim, run a bead of caulk about 1/4" thick on the face of the steel lip on all 4 sides. When you apply the trim, it will stick to the face of the vinyl frame and the back of the trim will seal to the face of the steel lips. Go inside and fill the space with fiberglass insulation. You can get a roll of insulation at the hardware store. You can also get the trim there. I sell a vinyl flat trim in 2 different widths that can be reduced in 1/4" increments. I sell an interior and exterior grade trim, so you will need to purchase both types to do the interior and exterior. Be sure to seal where the trim meets the wall on all four sides inside and out.
If you live in the west, you have a second option. You can get a retrofit frame with a 2" lip on the outside. The purpose of the lip is to stop against the exterior of the house as you insert the window into the opening. Many times, steel casement windows will be recessed into the opening, so having the flush fin go on the stucco is not an option. In that case, you want the thin fin so you can cut it to fit into the recessed opening and seal against the steel lip on all 4 sides. If you are able to use the retrofit style frame, you eliminate the step of trimming out the exterior. You still must trim out the interior. If you have any questions about anything in this article, you can send me an email.