Is your cousin going to live in the sunroom?
If not, can you close the door to the sunroom? Simply open it during the day to heat the house and close it at night so the house doesn't lose heat through all those windows.
Assuming you have other radiators through out the house for your hydronic heating system, you could use the warm floor in the sun room as a heat source, and not use your "real" heating source. Just let the circulator pump run and it will help balance the house temperature during the day, and remove the sunroom from the system at night. You'll sort of have a really expensive solar hot water heater that is also a sunroom :D
Ian

Daox: You can do fun things with color. Reds absorb the least thermal gain (that's why it's red); greens, blues, grays, and of course black are best. It's a nasty job, but you might consider changing the reddish floor tiles to a more thermal color. You could use the opportunity to lay down a radiant barrier underneath. Likewise the back wall, if the sun gets low in winter.

Cellular blinds have an R value of around R-2 or 3. Not great -- but uninsulated glass is around R-1, so it still helps. We have single-cell blinds on sloped glazing, and it's a terrible thing if we forget to close them at night! Some (expensive) blinds have a silvered lining, which can be hugely effective at stopping re-emission of heat. Coated windows likewise. But for a cheap solution, stitch up some winter drapes: a nice heavy fabric on the inside, a layer of silver bubble foam, and some polyester ticking on the window side. In conjunction with your cellular shades, they will reduce losses by convection and radiation.

Ah, radiation. It moves in straight lines and abhors partitions or hallways. My house is designed spoke-wise in an attempt to get the passive solar into each room, but older homes aren't usually that open. Your big double doors should carry heat into the house fairly well, though transom windows and vents near the floor set up a nice convection loop. You could go for some tromb wall thing, but I'd rather use free-standing solar hot water panels and active solar. Or put some black or blue 55 gallon drums of water in the sunroom in winter.:D

Nice-looking house!

Thanks for the tips on the blinds Bob. I'll have to look into it a bit further. I haven't taken the time to do anything with the sunroom yet.

The book: 'Design for a limited planet' should be a must for you here.

I doubt you would ever regret finding a copy, but rather it is a real treasure of information.

A hot tub or indoor fish pond could be your heat storage? You could pump its water under your bedroom or living area... And collect heat via a long bench seat below the windows along the long wall...filled with drums of water??

Ideally taken to extreme... modify the whole wall/glazing/eveoverhang/colour relationship...summertime opening right up to a courtyard or paved area would be nice

A blower fan ducting warm air from near the ceiling to other parts of the home, would also capitalize on potentially wasted heat thru the lean to roof

The heavy drapes idea is a great way to help keep the heat from escaping. This winter I put blankets up over large windows and the patio door that had only blinds before. It seems to be making quite a difference. This winter has had alote more heating degree days than last year, 4671 vs. 3989 so far. We have used a few % less propane, and a couple face cords less wood.

You could add some skylights for more direct gain, but you'd need to make insulated panels that close them off when the sun isn't out.

Our sunroom is very similar--about 14 by 24 feet on the south side of a hundred year old farmhouse. We have more glass on the south wall, and more roof overhang. Our south glass is shaded much of the summer so it doesn't overheat much, and is usable all summer. In the winter we have insulated homemade shades or curtains the cover much of the glass, and with almost no added heat it stays above freezing even if it is 30 below outside. We have a number of black 15 gallon plastic drums in the room for heat storage, but don't really know how much good they do.

Ooooh, plastic drums, I like that.

Actually, my cousin's family is moving out this weekend, so I will be reclaiming the space. This winter it'll hopefully be turned into a full blown heat source. I really want to add as much thermal mass as I can do the room and plastic drums are an idea (one that the wife doesn't love, but would accept if I could do it and make it not look bad). The next idea is adding lots of plants to the room. The dirt and water in them should hold heat to some extent. Additional ideas are quite welcome!

This sounds like a great candidate for a solar-thermal radiant floor set-up to me. Wish my "sunporch" was starting out this functional (facing east, non-functioning windows, uninsulated slab floor, prob no ceiling insulation):(.
That hydronic radiant floor system is begging to be linked up to a DIY panel or two on the roof (like the "Build-It-Solar" ones...). Part of the plumbing job is already done. Maybe trimming that tree would let you put one on the east end of the sunroom roof?

As far as thermal mass:
I'm not clear why you'd want to put it outside the insulated space (below the floor). ?
It isn't really a passive-solar designed room, so: Unless the room gets uncomfortably hot during the day in the heating season, with the house door open, or you want to use the room at night without heating it, why add thermal mass? You could just circulate the heat into the house during the day & close it off at night. Would it really stay warm enough to add heat to the house at night even with loads of mass?

The extra thermal mass wouldn't be outside the insulated space. It would be in the room somewhere.

I'm not sure how much heat it brings in during winter, but its a fair amount. That is by far the warmest room in the house if the blinds are kept open.

Now I'm really jealous. My little sunroom (7.5x12) is seriously cold in winter & too hot in summer. I do have plans for insulation, windows & doors, but I don't think I'll ever get much heat from it do to poor orientation.