. You're essentially looking at making a thermal mass that simply swamps out the level of "losses" to the outside world. Thermally 100% correct I'd say . . if it's truly big enough, and can exchange it's heat with the surroundings efficiently enough. No matter HOW efficient the place could be; there will be losses . . . and at a time of year when you want them the least . . winter . . when they are also least available to replace. I'd say s'can the flat roof . . . put enough pitch on it to shed rain . . and . . perhaps more importantly; snow . . like metal roof. The weight can get incredible. Insulate the space between inside roof and the "attic" . . . will help cool in the summer, and avoid icicles in the winter. You are barking up the right tree with regards to surface area vs volume; but I believe that bees have it right: Believe that a hexagon, or septagon, or whatever they have; actually gives the most volume per lineal outside material. May not lend itself to livable space terribly well . . although bees seem to do fine with it . . Running tubing throughout the concrete mass I think would be a good idea . . . just don't pour all at once lest ye collapse forms as the weight / pressures can get quite high as the mass gets larger. Pressurize the tubing during the pour !! lest ye collapse it. As far as pumps / controls etc for circulating fluid; why not use passive thermo-syphon ? ? Only ONE moving part . . . a check valve. No motors / pumps / controls / sensors etc. Heat during the winter; and oodles of heated water during the summer months. Follow the KISS principle: Keep It Simple Stupid . . less crap to go wrong . . . easy to protect against freezing etc . . . Circulate well water through the mass in the summer . . get free cooling ( and likely some condensation ) while pre-heating your domestic hot water. Perhaps some cavities / "tunnels" through the mass . . which could have air blown through them to help exchange along better by virtue of more surface area ? ? ? Make the mass be part of the "architectural" design . . not just put there to serve the main purpose. Incorporate masonry stove etc into it . . as PART of the living space . . . shelving . . cubbyholes . . a small sitting area . . "couch" . . all as part of it ? ? While thinking of cheap AND efficient . . ever look into straw bale construction ? ? ? I've read some about it . . and it is certainly cheap . . and if PROPERLY done ( to exclude water infiltration ) will last a VERY long time. Low tech, cheap, works well. How's 'bout earth-bermed or "underground" construction . . . a few feet down and the temperature doesn't vary nearly so much . . . a much more constant "outside" temperature to your structure ? ? ? Don't forget that building very tight . . . . also means lack of fresh air . . . you'd have to have ERV or such to get that . . . and there are some losses with it. An interesting idea . . . curious what others may say . . and what you end up doing with it . . . . Bob

With reasonable electric rates (which I assume would be cheaper to operate a heat pump with than an 80% gas furnace with your gas rates), and a mild winter climate, a heat pump would be a good option in my opinion. You can keep the gas furnace for backup, though it may not see much use, and doing so would not allow you to leverage the tax credit since you'd need a variable-speed fan which boosts the efficiency for higher SEER equipment. What you would potentially install is a heat pump and a fan coil inside, and that would take care of the heating and cooling and could keep you the tax credit if you choose the correct components. A contractor should come to your home and discuss all this with you and hopefully do a heat loss/heat gain calculation to determine the correct size equipment. I'll attach the link below where you can see what is a rated match (just a tip: keep the model numbers as simple as possible; for example, a Trane may be proposed as 4TWX5036A1000A, but in the database it only reads 4TWX5036A1 -- if you are having trouble finding results). The first link is for air conditioners, and the second for heat pumps. Your 58STA is probably not in the database because it isn't variable speed. Just so you see how it works, look at heat pumps and type in 4TWX5036 for outside and 4TEE3F for inside, and you'll see possible indoor fan coil matchups and their ratings. Here is a link that might be useful: AHRI Directory

"Ice damns" are basically just a term for when the roof gets warm enough to melt some snow in some areas. The water then can refreeze when it hits a cooler spot like at the overhang. That basically forms a dam that can hold water on the roof. Roofs are designed to let water run off them, not to hold water for any length of time. If the problem persists, the water can work its way up under shingles etc and leak into the house. Obviously, that can be a huge problem. The way to avoid this is to avoid transferring too much heat to the roof during winter. A big open and vented attic accomplishes that in most instances. If you are going to enclose the attic, you will need an effective way of keeping the heat from rising up through the new room and to the roof. The "low tech" way to do that is to make a mini-attic above your new attic room. It doesn't have to be tall, just enough room to keep air flowing. For that to work, you need vents to get air between the roof and the ceiling of your new attic room. The heat needs a way to escape instead of through the roof. The "high tech" way to do it is completely shut off all air flow going up to the roof and insulate the heck out of it with expanding spray foam. That is a less reliable way to do it though because you are just counting on the insulation to keep heat from getting out of your room. If the insulation is compromised for any reason, you'll end up with hot spots on the roof.

My 2 story house was built in 1940 with a boiler and cast iron radiators for heat, and no A/C. We later added A/C in the attic, so we how have a gas boiler with radiant in the winter, and A/C to provide cooling in the summer. I am in climate zone 4 with moderate to high humidity in the summer. Cold air is heavier and goes down easily, so the attic-mounted A/C works well to cool both floors. The cast iron radiators and boiler provide very even heat in the winter, with only about a 1 degree temperature variation between the two floors. It wouldn't make sense for a new build to buy two systems, but it is definitely a very comfortable heating and cooling combination. Bruce