I don't know what the layout is, but in the house we had growing up, my parents added on what they called a 'florida room' -- it had a normal roof, but louvered windows all the way around.
This was in central Maryland, which can get cold in the winter, and hot in the summer -- this place was miserable at both times. For the winters, we had an oil-filled electric heater which you'd go and turn on, and then wait 15 min or so for it to get bearable. For the winter, finally one year we put up mini-blinds to help in the summer ... it was better, but still not great. (and I passed out from heat exhaustion when putting them up ... there were a lot of windows).
As such, I would look at a few things --
- Greenhouse shading. It goes on the outside of the greenhouse. (it's supposed to be ~40% more effective outside vs. inside in keeping the temps down).
- Tall trees with few lower branches. You basically want to shade the room in the summer time, but prune the branches to allow the lower angled winter sun to get in.
- Vent fans. They make some that are solar powered -- so when the sun's hitting them, and it's warm, they'll run. You can also get some vents for greenhouses that'll open as it gets warmer, but the last one I installed didn't work too well.
- Thermal mass. It's usually suggested for greenhouses to deal with variations between day & night temperatures in areas, but if it's warming up too quickly in the day or cooling of to quickly at night, it might help. (large containers of water, painted black, is one of the suggestions for greenhouses)
You might also look into the concept of "Passive Solar Heating" if it's southern facing -- basically, you intentionally try to get it warm in the winter time, so you can then draw in air from there into the main house. My cousin has some fans on sensors so that they'll move air between rooms depending on the temperature differential and time of year.
Oh, and as for double glazed windows -- I'd check with a local garden center first -- they might have recommendations for the roof line; glass isn't always the best thing there, if you're in an area that's prone to hail storms. My greenhouse (from a kit; not attached to the house) has some corrugated plastic panels that act similar to double glazing, but can take a direct strike. (although, two of 'em caved in due to heavy snow load last year)