Once you have all of your existing opening sizes written down, you need to go through and make deductions. On vinyl windows, you deduct 1/2” on the width and 1/4” on the height. Most window manufacturers build windows down to a 16th of an inch but for our purposes I would keep everything to 8th inch increments and when something falls between go with the smaller measurement.
The reason we only deduct 1/4” off the height is that that vinyl windows require the sill (the bottom of the window) to have full support across the entire length of the window. Therefore we set the new window on the track of the old window so that it has support across the entire window. The width we want 1/4” on either side for adjustments and the same on the height but the bottom we want to rest right on the sill of the old window.
In some cases the old window wasn’t level across the sill. Therefore you will have to make some adjustments to the old window frame. Usually you can just bang down the high spots to make it level. Don’t get too crazy with the sledgehammer though, you don’t want to ruin the integrity of the existing water seal, but don’t be surprised if you have to bang down a few sills or bowed frames here and there, so take this into account when taking your measurements.
Some people are unhappy with the unequal reveals that can come about as a result of setting the new window on the existing windows sill. See the picture below.
Notice how the bottom of the window will have a smaller frame size than the rest of the window? This is problem A. Problem B is that many manufacturers don’t like their window sitting on just a small thin piece of aluminum, especially if it protrudes high above the other parts of the window frame, they are afraid this isn’t enough support for them. Well there is a solution to both problem A and problem B and that is to install sill blocking before installing your windows. We’ll discuss that in the next section.