I am running an experiment with one of the BEST-TEST models in EnergyPlus (case 900). It is simply a box with two 6m2 windows on the south wall. I want to see the response of this structure when I hold outdoor conditions constant. I have modified a TMY file to hold the Tdb to 0C for all hours with no wind and no solar gains at all.

I have a heater inside the model maintaining indoor temperature 25oC for a few days, and then I force it off. I was hoping to see the model drift down to ambient conditions where the internal temperature slowly drops to meet the outdoor air temperature. However, the model cools down to 0C and then keeps on cooling! The model settles on a steady-state condition of -5C. I don't understand why it would cool below 0. There are no internal gains and no cooling equipment - it is simply a box.

What feature of the EnergyPlus heat transfer calculations could account for why the building reaches a steady state colder than the ambient temperature?