There are several types of glass or “glazings” that can be found in a residential window. A major factor is the age of the window. Older windows, those about 40 years or more old, are usually made of a single thickness of one piece of glass called single glazed. Typically the glass is 1/8” in thickness and is cut to shape with a sharp tool that has a small wheel that is run along a straight edge causing a small scratch or scoring in the face of the glass. The glass is then snapped on this weak point and the rough edge is usually polished so as not to be sharp. Kerosene is often used as a lubricant during the scoring process.
Windows that are made up of many smaller sections in a grid pattern each have a piece of glass cut to size to fit into the grids. These are called true divided lite windows. ( Lite is the correct spelling) These types of windows are usually found in colonial style homes in a kind of window called double-hung. These are the kind of windows where the movable sections, called sashes, move up and down in a track.
The name or designation of the window is determined by the number of glass panes or “ lites”. For example, if there is only one lite on the top sash and one lite on the bottom sash we would call that a one over one ( 1/1). If there were 6 lites on top over 6 lites on the bottom it would be a 6/6. Simple.
Here’s where it gets a little complicated. In an effort to make windows more energy efficient, the idea of making windows with multiple layers of glass was born. The most common trade name for this type of glass is Thermopane. The problem with a single pane of glass is that the exterior side is in direct contact with the elements and with the cold air that at times can be present. Being a solid relatively thin material, glass is not a great insulator so it doesn’t do a great job in keeping the cold out or in keeping your expensive heat in.
So by adding a second piece, not in direct contact with the other piece but spaced apart, this greatly increases the insulation or “R” value of the window. An R-value is the term used to measure the Resistance to heat loss a certain material has. The higher the number the better. The small gap between the 2 glass panes acts as a thermal break which is sort of like short-circuiting the path that the heat loss is taking. Here is a picture showing a cutaway of a typical insulated window:
However, it’s a little more complicated than that. Simply taking two pieces of glass and spacing them apart will not work for a couple of reasons. The air that would be trapped between the pieces of glass would act as a conductor and partially defeat the objective of creating a thermal break. The second issue is that moisture would collect between the panes of glass and greatly affect the visibility of the window. So all true thermopane windows are manufactured in such a way that there is a vacuum, or lack of air, between the two glass panes.
There is a seal that goes around the perimeter of the two panes that maintains this vacuum and prevents atmospheric pressure from forcing air back in between the glass sheets. The seal usually lasts for many years but there is a saying in physics, nature abhors a vacuum. This means that the seal is constantly being “ attacked” by air pressure trying to fill that void and sometimes the seal gives in. The problem is that air has a level of moisture contained within it. We usually commonly refer to it as humidity.
This moisture is what is causing the cloudiness or foggy look in your windowpane. Unfortunately, the usual solution is to replace the pane but I have seen a company recently make a claim that they can repair this condition on-site at your house. I have no idea if this actually works but I guess it’s something to look into. One other thing I should point out. If you have one of those 6/6 double-hung windows and only one pane is effected there is something you need to know.
While older single pane multi lite windows were always made with small separate pieces of glass, thermopane might not. It is very expensive to make 6 small pieces of thermopane as opposed to one larger piece. So some, probably most, multi lite (6/6 for example) thermopane windows actually are made with one large piece that then has a wood or plastic grill applied on the top of the glass giving the illusion of separate pieces. At a minimum, they apply it to the interior side but some have an opposing piece applied to the exterior side enhancing the illusion.
Taking it to the next level, most recent windows of this type will also have a metal grid ( think tic-tac-toe) placed between the glass panes in the vacuum in a shape that coordinates with the grills. This really gives the illusion of separate pieces of glass when in reality it’s only one. This type of window construction is called “Simulated Divided Lite”. However, certain high-end window manufacturers do use separate individual pieces of thermopane known as “True Divided Lite”. It’s helpful to know which one you have before calling out the repairman. If you look between the panes of glass and see a continuous aluminum spacer or grid, then you probably have Simulated Divided Lite and will need to replace the whole piece and not just the one smaller pane. Good luck!