Vinyl windows are designed that the frames are very good insulators. They can be enhanced with various types of fill in the hollows inside the extruded vinyl, some for more insulating value, some for more structural integrity. Well designed vinyl frames perform at least equal to the best wood windows and far superior to any of the other alternative framing materials. But the crux of creating a high performance energy saving window is the glass system. Sometimes the difficulty is in choosing which type of glass is best for you. Cost is certainly a factor, but some of the other factors are the part of the country you live in or the amount of security you desire. Some manufacturers offer as many as eight different glass systems to allow you the freedom of choice you deserve.
The key part of high performance glass is the type of Low E (Low Emissivity) glass that is incorporated into the unit. The number and type of the microscopic coating layers makes a dramatic difference in the energy performance that can be obtained. There are some excellent double glass systems, some very appropriate for specific parts of the country. But the popularity of triple glass is now a fact. Triple glass has been around since the 1970's but the original glass packs made back then were problematic. They experienced a much higher failure rate than double glass, probably because there were double the seals to the glass surface required.
But modern insulating glass is far superior to what was around in "the early days" of replacement windows. Low E glass first became available somewhere in the 1980's but the original coatings had significantly less performance values than the coatings available today. Every few years there seems to be a new better coating offering even more energy performance. Now to the facts, if the windows in your home were manufactured before about 1990, you cannot possibly have truly high performance windows as the current technology didn't exist then. In just the last ten years the performance levels have about doubled from what was available on the market in 1990. So give some thought about when your windows were made, and look into what is currently available to save you energy, be more green, and make your home more comfortable and more attractive.