If the straw bales are "insulated with plaster" for some reason, wouldn't that also insulate them from heat gain in the daytime? If you are planning to store enough heat during the day using water, the recommended amount needed for adequate heating of a greenhouse at night is 4 gallons per sq. ft. For your 1500 sq. ft, that would be 6000 gal. There are many in-use greenhouses and homes that use subterranian heat storage for at least partial night-time heating. Full sun, winter days, with the recommended amount of water storage are usually necessary to achieve the goals. Additional considerations are getting the heated water from the greenhouse into the storage barrels during the day and out at night.

FWIW, I would be very surprised if you could use the milkhouse with the ouside walls covered up to 6' in dirt piled against them. Buildings are constructed to withstand blow-out, not cave-in. Basements are designed for cave-in, not blow-out. Of primary concern would be the buildings footings, but equally the thickness of the walls and whatever ceiling cross-bracing is in place. Consider the snow-load on the roof, as it won't be able to shed off as it did before if you get 2' of snow on the ground at the eaves. Assuming you piled dirt up to the top of the 6' wall, you will end up using nearly twice as much dirt as you would have to remove to create a 16'x16'x6' deep pit. So consider that. Cheers, Russ

Laura- I'm hoping to put a greenhouse on my remodel, too. It's been called an 'orangery' on another post, but it's just a fancy greenhouse. There will be one door from the mudroom and one window, from the pantry. This part of the home is the only place we're adding on...so we could use brick or some other material for the greenhouse wall, if that would help. I think sinking the greenhouse will be even more trouble, when the snow melts. You'd probably have to do french drains and other things to try to take the water around the greenhouse. I think your temperature could be more easily regulated (and more affordable) with some heavy shades for night and maybe even a little heat from the house...or a baseboard heat. I believe plants will still be fine at about 55 degrees (maybe even a little cooler). Good luck with your research and please let us know what you find. I'm still getting ideas and working out a few details, too :) Oh, and my greenhouse is just for plants...not a breakfast room or family room, which is often a concern on the forums.

A greenhouse without ventilation is a solar collector :) You need to get some vents in there. The best option for making a top vent is to add a cupola which will allow excess heat to escape without letting rain in. Plants will create humidity, empty greenhouses always have low RH%. Overhead misting is effective for increasing RH% and can help to lower temperatures but the ventilation issues need to be addressed before you do anything else.