I was just looking at some inside storm windows..
Larson Manufacturing
They look very easy to install and may even be available in Low-E glass..
I think maybe I'll call Lowes and see if they can order some for me.
With those on the inside, I would have 3 panes of glass and a lot lower heat losses.
Of course, there is always
one more step..
Larson Gold Series storm windows are premium, top-of-the-line storm windows. - Larson Manufacturing
Exterior Storm windows! Dang! 4 panes of glass is three layers of trapped air insulation!!
That would really hold in the heat..