Is it worth it to tint existing windows?
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Anything that reduces the amount of UV coming through the glass.
Replacement windows or sashes made with Low E (Low emissivity) glass actually have a zinc coating on the inside surfaces of both panes of glass as shown below (surfaces 2&3):
This reduces heat (UV) from coming through the glass in the summer and maintains heat within the home during winter. The reason the zinc or Low E coating is applied to the inside surfaces of the glass is because it is barely noticeable to the human eye, but it can be scratched off.
If your home was built within the last 30 years, odds are you can purchase replacement window sashes with more energy efficient glass at a very reasonable price. My last home in Virginia was built in 1985 (2-story, 3 bed, 2400 sf) and I replaced all the sashes in the house with Low E and argon gas for less than $3k.