[a_square_peg]

This depends on your house - heat tends to leak out through the lowest path of resistance so if you have a well-insulated house with leaky window, it would certainly be the case but without considering what the rest of the house is like, it would be impossible to tell how much they would help.

[andi999]

Heat leaks out through all path of resistance not just the lowest. (The lowest might dominate though if it is orders of magnitude lower than the second lowest)