Get some of that clear plastic sheet window covering at the hardware - the kind that you use a hairdryer to set in place (goes from frame side to side inside, creates air space between window and itself and is completely see thru). Then again if you have any cats (like mine) who are likely to paw at it, and you have lots of money, consider replacing the windows with doubles.

since it is a sun room,I assume it is all windows. Windows have little insulating value. Are the windows single or double pane. If they are single pane they have even less insulating value. you are basically loosing all your heat out the windows. The only thing you can do is either add a supplemental heat source or put insulation over the windows. You have to remember a window has a R value of about R-3.

I'd put in a oil-filled space heater (DeLonghi-style) in the winter, or if you want permanent heat, add electric baseboards. The heat loss from three walls of windows exceeds the capacity of the single vent (Heat pump or furnace?) to heat it.
Casey

The sunroom has inadequte heating. You need to determine the cause. Old windows and/or not enough btu's. Or both.
Try posting again with some details.
Ron

I'm installing a Mitsubishi mini-split heat pump in a screened in porch that I am remodeling into a four season room.

Just curious as to how you get much air circulation with no return air duct. For a quick fix, I would install electric baseboard heaters under the windows and ceiling fans.

My parents put on a small addition and put a direct vented wall hung gas heater in the room. It uses no electric and generates the thermostat voltage from the pilot. It is direct vented and gets combustion air from the outside. I would use it over a non vented unit. You still need some way to insulate the glass,probably honeycomb blinds or homemade insulating shades.

I have the same/similar gas heater as rjoh. It works great.