That’s the opposite of U-Value. If U-Value determines how much heat is going through your house, the R-Value determines how much heat is being absorbed in your window. The better glass you have, the more R-Value you have. In other words, if your window glass is hot, that’s not a good R-Value, which means that the window has no Low-E or Argon Gas. You want your window glass to stay cool even when it’s hot.