Spacers are made up of some type of physical material that literally separates the panes of glass.
If it's double pane you have 1 spacer but if it's triple pane you'll have 2 spacers.
A bad "spacer" is one of the top 3 culprits that can lead to seal failure sooner than later. Like most other components used on windows in the U.S. the majority of the spacers being used aren't really built for longevity or durability.
If you look at the pictures below it becomes very clear "spacers often can and do fail".
The best spacer on the market currently I believe (based on my research) is the 1 piece "Endur" stainless steel from Cardinal Glass. (meaning it's 1 continuous piece) as opposed to 4 "cut" pieces.
That's not to say that any window manufacturer buying glass from Cardinal is getting the same product (that's a whole other subject) I'll be doing a video on.
A better spacing (system) will also be "dual sealed" as opposed to a "single seal".
The problem with 4 cut pieces is you have to butt joint them at the corners of the glass, then put sealants at each corner, and because it's 4 separate pieces it doesn't really structurally support the glass; but 1 continuous piece does.
The problem with using aluminum or tin instead of stainless steel is "aluminum" as a material conducts more than 1000 btu's per hour per square ft. so it transfers heat very quickly (and often) through the glass package around the edges of the glass.
In addition the majority of the aluminum spacers being used are also 4 separate pieces. (This means more heat loss in the colder months and more heat gain in the warmer months).
The foam spacers you see in the pictures obviously have common issues of buckling and sagging (if they didn't they why do I have these pictures)?
Some companies using the foam spacers make 1 continuous loop where-as I've seen other companies using 4 separate pieces instead of 1.
