Keeping my sunroom warmer (making interior storm windows)

[Daox]

Last year was our first year using the sunroom. We love the room and all of the 13 windows, but that does come with a price once the sun sets. The windows are newer and of decent quality. Still, I believe they probably have an R value of 2-3 where the wall has R20 insulation. So, they loose tons of heat at night.

I'm really looking for a semi-permanent way of reducing heat loss through these windows. Something I can put up in fall and take down in summer. I don't want to use those disposable films as this will be a requirement year after year.

My innitial idea (and only one really) is to remove the screen in winter and replace it with a piece of lexan or plexiglass. They would clip in with the pins that hold the screen in currently (spring loaded goes into the window frame). I would add a gasket to the inside of the plastic to seal it. This will add another air pocket just like the disposable film does and help insulate further. However, it will be reusable every year.

I'm open to any ideas, so toss em out there.

This is the room before we moved into it. I'll get more detailed/updated pics later on.