Not insulated, and not connected to the rest of the house, it will be the outdoor ambient temperature. It would need insulation and a heat source to remain above 40ð. You would also need a very strong light source, or the plants will die from lack of sunlight.

Not insulated, and not connected to the rest of the house, it will be the outdoor ambient temperature. It would need insulation and a heat source to remain above 40ð. You would also need a very strong light source, or the plants will die from lack of sunlight.

If it were me, I'd buy a $50 oil-filled electric heater with a thermostat. I have a similar climate to yours and I store plants in my greenhouse during the winter. I have a heater that is set to come on at about 37-38 degrees F. I don't end up using much energy, and I don't have to worry about a cold snap zapping my plants. I agree with GreenDesigns that you need a light source if you have plants that are not completely dormant, and putting in some insulation will be a cheap way to make sure you don't use a lot of electricity unnecessarily.

As cold as outside, or even COLDER because of the lack of sunshine. Unless you put in a heater, of course.

If it is on a slab, it is coupled to the ground and that will moderate the temperature to some extent. What the end temperature will be is hard to predict because it depends on heat transfer rated from the ground, what the ground temp is, what the air temp is and transfer rated to the exterior. Insulation can't hurt. If it is not air-sealed from the attic, that would be your first step.