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 further outside than the old frame lip. How far out depends on you, and how much inside sill space you want. A quality vinyl replacement window will measure 3 1/4" deep. 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, fill the space with fiberglass insulation. You can get a roll of insulation at the hardware store. You can also get the trim there, but if you choose a wood trim, you will have to paint it. Also, wood is susceptible to weather, so you might be repainting somewhere down the road. I sell a vinyl flat trim in several different widths, that has an adhesive backing to stick to the face of the window. Since it's an exterior grade trim, it will never need maintenance. You do the same procedure when trimming out the inside. 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. If you have a stucco exterior with wood trim around the opening, the retrofit lip will cover the wood and rest on the stucco. However, you have to make sure the new window frame goes further into the room than the lip of the old frame. If the new frame does not go beyond the old frame, you won't be able to trim out the interior to hide the old frame. A quality retrofit window frame measures 3" deep from the back of the retrofit lip. The distance from the stucco surface to the old frame lip must be less than 3", otherwise you have to use the replacement style frame as I described above. 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.
John Rocco has been installing replacement windows since 1978. To learn more, visit How To Install Windows