Innerglass windows are "The Perfect Storm" for insulating historic homes and buildings. The standards that define a "historic" home or building are very specific. The original window units are part of the building’s historic integrity and replacement of these windows is strongly discouraged. The guidelines specifically say, "If the original windows are in place, repair and retrofit of these windows is preferred." That’s where Innerglass comes in. Because our windows compression-fit inside of the stops of the primary window, there is no need to disturb the integrity of the original windows. And because each of our windows are custom made to fit the exact measurements of each window, they are extremely efficient for windows that are no longer square, which is very often the case with historic homes and buildings.