We did one....north wall is built into the mountain, south wall is earth bermed up to the level of the table inside, and also doubles as a strawberry bed. Glazing is triple wall 16mm polycarbonate. Far left end doubles as my electrical room for solar gear. Never gets below freezing with no heat, even down to zero nights.
Wife over winters her flowers, and uses it to start veggies for the garden in late winter. We have raised some tomatoes in winter, but really need heat for that to keep it 60 or so and above. I have some solar water heating panels and stainless steel 55gal drums to store hot water in ready to install, but it's one of those "round to it" projects I haven't found time to do yet.
