Author: Dan McCuan (---.bak.rr.com)
In response to your questions.
The foam injected into the window jambs traps air in the membraine of the foam. Because the air particles are seperated from each other by the foam membraine, the thermal transfer between molicules is slowed down there fore you have a more efficient frame. Please note that the glass is still the most inefficient portion of a window. It depends on your climate to determine if the injected foam is worth the additional expense over a traditional vinyl frame. You might look at a window that is still hollow vinyl, but is very compartimented. (i.e. an Anlin window frame)
The old problems of window warping are nearly eliminated with the new vinyl windows in the market. Most top companies guarantee against window warping, and will repair/replace any window that has warpped.
Most refuitable companies will place steel into the jambs of a window for strength. Typically the only jambs that are steel reinforced are the center jambs, or the interlockers. All windows in California are required to pass grueling Government forced entry tests, and all windows that pass are AMMA certified.
Because vinly has no significant structural value, steel in incerted into the center jambs of higher quality windows inorder to make them stronger/safer.
Personally the steel is a nice additive, but not entirely necessary. If a theif wants in, he will get in. In my house I have non steel reinforced windows.