These should read "thermal if flush mounted within the frame", as if you mount them outside the window frame, the ends are open to the room, wich looks weird, and may insulate a little with superimposed air, but is not sealed, thus likely mixes with the room air. I don't notice a huge heating/cooling benefit to them. However, they are hoplding up well to a six year old pulling them up and down frequently without breaking, so moderate value.