| Windows that have been retrofit into old double hung frames usually have an interior stop on the sides and top that can be removed. Once the interior stop is removed, and the old replacement window is unscrewed and the sealant has been cut, it will pull out to the inside. Without doing all that, you should be able to accurately measure the replacement window that sits behind the interior stops.
You could obviously figure this out without removing any of the interior trim, but it wouldn't hurt to remove a few pieces to reveal the edges of your replacement window... you'll be taking it off anyway when you go to tear the old window out, so it makes sense to at least check one window now.
Replacement windows are usually ordered 1/4" smaller than the opening is wide, and 1/4" shorter than the entire opening is tall. If you want to verify the thickness of your stools (in old houses they were closer to 1 1/8" thick) you can just open one of your old double hungs and check the distance from the top of the stool to the point at which the sloped sill starts. That's the point at which you would actually measure the height of the opening if the Thermal Guard window was removed. So if you measure from the top of the window stool to the top of the window jamb, the thickness of the window stool would have to be added in when you measure the height. Some people get the window shorter than needed, and then use a head expander. I think head expanders are pointless provided the retrofit opening is square and you measure accurately in the first place.
You can also doublecheck these measurements on the outside of the house if there is any question as to the size. You just have to remember that the opening gets taller on the exterior side of the window due to the angle of the sloped sill.  |