Don't be so quick to keep warm air away from windows. There are
reasons houses are designed with heat sources under windows.

1. It prevents condensation and the accompanying mold, mildew and
water damage.

2. It prevents a "drafty" feeling when you're next to windows by
ensuring the windows are warmer and thus giving off almost as much
radiant energy as interior walls.

Yes, it wastes some energy, but if you're that set on saving energy,
look to newer windows.... although I won't guarantee you'll save money
in the effort.