This method also doesn't require that the casings are changed. However, it may be slightly less thermally efficient and you will lose a 2-3 inches of glass size in a typical opening. This is not generally noticeable in a larger opening but can be an issue in a smaller opening such as a bathroom. Also, if there is substantial rot in the existing frames this method is not advisable.
Complete Frame Replacement (Sometimes referred to as Brick to Brick)
In this method the entire window assembly is removed right back to the stud opening and is replaced with a complete new window, including the frame, sash, sealed unit, exterior brick mold and casing. While initially more expensive than a retrofit installation it will result in more energy efficient installation. The space between the stud and the new window is completely sealed with low expansion foam. This ensures minimal heat loss and air leakage around the perimeter of the window between the stud and the new frame. New casings are required giving the opportunity to upgrade to larger more attractive casings. Quite often complete frame removal is preferable to a retro fit installation but not always. In an older home with large wood moldings, plaster walls, brick and block wall it may be advantageous to leave the existing frames in, that is why it is important that your initial consultation is with a person with several years of INSTALATION experience and not just a guy who was selling used cars last week. We are Certified and trained installers but we don't believe that certification is enough and one specified approach doesn't work for every home. It takes experience to tailor the installation specifically to your home to optimize the results both aesthetically and functionally.