Termopane windows or double windows?

[Walter R.]

I have a large single-pane window in my den (side-by-side slider) that transmits road noise.

I thought of replacing it with a new dual pane (thermopane) window to reduce noise transmission.This means ripping out the old window & frame.

Another route to go is to place a duplicate inside window on the inside sill of the existing window, like an interior storm window.

This site claims that a double-window is superior in suppressing noise than a single thermopane window.

Is this only salesmanship? Any opinions?

[Joseph Meehan]

Yes, but it is less effective at keeping you cool in the summer and warm in the winter. Of course just about anything will be better than most sliders.